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WelcometoSwift

AboutSwift

Swiftisafantasticwaytowritesoftware,whetherit’sforphones,desktops,servers,oranythingelsethatrunscode.It’sasafe,fast,andinteractiveprogramminglanguagethatcombinesthebestinmodernlanguagethinkingwithwisdomfromthewiderAppleengineeringcultureandthediversecontributionsfromitsopen-sourcecommunity.Thecompilerisoptimizedforperformanceandthelanguageisoptimizedfordevelopment,withoutcompromisingoneither.

Swiftisfriendlytonewprogrammers.It’sanindustrial-qualityprogramminglanguagethat’sasexpressiveandenjoyableasascriptinglanguage.WritingSwiftcodeinaplaygroundletsyouexperimentwithcodeandseetheresultsimmediately,withouttheoverheadofbuildingandrunninganapp.

Swiftdefinesawaylargeclassesofcommonprogrammingerrorsbyadoptingmodernprogrammingpatterns:

Variablesarealwaysinitializedbeforeuse.

Arrayindicesarecheckedforout-of-boundserrors.

Integersarecheckedforoverflow.

Optionalsensurethatnilvaluesarehandledexplicitly.

Memoryismanagedautomatically.

Errorhandlingallowscontrolledrecoveryfromunexpectedfailures.

Swiftcodeiscompiledandoptimizedtogetthemostoutofmodernhardware.Thesyntaxandstandardlibraryhavebeendesignedbasedontheguidingprinciplethattheobviouswaytowriteyourcodeshouldalsoperformthebest.ItscombinationofsafetyandspeedmakeSwiftanexcellentchoiceforeverythingfrom“Hello,world!”toanentireoperatingsystem.

Swiftcombinespowerfultypeinferenceandpatternmatchingwithamodern,lightweightsyntax,allowingcomplexideastobeexpressedinaclearand

concisemanner.Asaresult,codeisnotjusteasiertowrite,buteasiertoreadandmaintainaswell.

Swifthasbeenyearsinthemaking,anditcontinuestoevolvewithnewfeaturesandcapabilities.OurgoalsforSwiftareambitious.Wecan’twaittoseewhatyoucreatewithit.

VersionCompatibility

ThisbookdescribesSwift5.1,thedefaultversionofSwiftthat’sincludedinXcode11.YoucanuseXcode11tobuildtargetsthatarewrittenineitherSwift5.1,Swift4.2,orSwift4.

WhenyouuseXcode11tobuildSwift4andSwift4.2code,mostSwift5.1functionalityisavailable.Thatsaid,thefollowingchangesareavailableonlytocodethatusesSwift5.1orlater:

FunctionsthatreturnanopaquetyperequiretheSwift5.1runtime.

Thetry?expressiondoesn’tintroduceanextralevelofoptionalitytoexpressionsthatalreadyreturnoptionals.

Largeintegerliteralinitializationexpressionsareinferredtobeofthecorrectintegertype.Forexample,UInt64(0xffff_ffff_ffff_ffff)evaluatestothecorrectvalueratherthanoverflowing.

AtargetwritteninSwift5.1candependonatargetthat’swritteninSwift4.2orSwift4,andviceversa.Thismeans,ifyouhavealargeprojectthat’sdividedintomultipleframeworks,youcanmigrateyourcodefromSwift4toSwift5.1oneframeworkatatime.

ASwiftTour

Traditionsuggeststhatthefirstprograminanewlanguageshouldprintthewords“Hello,world!”onthescreen.InSwift,thiscanbedoneinasingleline:

1 print("Hello,world!")

2 //Prints"Hello,world!"

IfyouhavewrittencodeinCorObjective-C,thissyntaxlooksfamiliartoyou—inSwift,thislineofcodeisacompleteprogram.Youdon’tneedtoimportaseparatelibraryforfunctionalitylikeinput/outputorstringhandling.Codewrittenatglobalscopeisusedastheentrypointfortheprogram,soyoudon’tneedamain()function.Youalsodon’tneedtowritesemicolonsattheendofeverystatement.

ThistourgivesyouenoughinformationtostartwritingcodeinSwiftbyshowingyouhowtoaccomplishavarietyofprogrammingtasks.Don’tworryifyoudon’tunderstandsomething—everythingintroducedinthistourisexplainedindetailintherestofthisbook.

NOTE

OnaMacwithXcodeinstalled,oronaniPadwithSwiftPlaygrounds,youcanopenthischapterasaplayground.Playgroundsallowyoutoeditthecodelistingsandseetheresultimmediately.

DownloadPlayground

SimpleValues

Uselettomakeaconstantandvartomakeavariable.Thevalueofaconstantdoesn’tneedtobeknownatcompiletime,butyoumustassignitavalueexactlyonce.Thismeansyoucanuseconstantstonameavaluethatyoudetermineoncebutuseinmanyplaces.

1 varmyVariable=42

2 myVariable=50

3 letmyConstant=42

Aconstantorvariablemusthavethesametypeasthevalueyouwanttoassigntoit.However,youdon’talwayshavetowritethetypeexplicitly.Providingavaluewhenyoucreateaconstantorvariableletsthecompilerinferitstype.Intheexampleabove,thecompilerinfersthatmyVariableisanintegerbecauseitsinitialvalueisaninteger.

Iftheinitialvaluedoesn’tprovideenoughinformation(orifthereisnoinitialvalue),specifythetypebywritingitafterthevariable,separatedbyacolon.

1 letimplicitInteger=70

2 letimplicitDouble=70.0

3 letexplicitDouble:Double=70

EXPER IMENT

CreateaconstantwithanexplicittypeofFloatandavalueof4.

Valuesareneverimplicitlyconvertedtoanothertype.Ifyouneedtoconvertavaluetoadifferenttype,explicitlymakeaninstanceofthedesiredtype.

1 letlabel="Thewidthis"

2 letwidth=94

3 letwidthLabel=label+String(width)

EXPER IMENT

TryremovingtheconversiontoStringfromthelastline.Whaterrordoyouget?

There’sanevensimplerwaytoincludevaluesinstrings:Writethevalueinparentheses,andwriteabackslash(\)beforetheparentheses.Forexample:

1 letapples=3

2 letoranges=5

3 letappleSummary="Ihave\(apples)apples."

4 letfruitSummary="Ihave\(apples+oranges)piecesof

fruit."

EXPER IMENT

Use\()toincludeafloating-pointcalculationinastringandtoincludesomeone’snameinagreeting.

Usethreedoublequotationmarks(""")forstringsthattakeupmultiplelines.Indentationatthestartofeachquotedlineisremoved,aslongasitmatchestheindentationoftheclosingquotationmarks.Forexample:

1 letquotation="""

2 Isaid"Ihave\(apples)apples."

3 AndthenIsaid"Ihave\(apples+oranges)piecesoffruit."

4 """

Createarraysanddictionariesusingbrackets([]),andaccesstheirelementsbywritingtheindexorkeyinbrackets.Acommaisallowedafterthelastelement.

1 varshoppingList=["catfish","water","tulips"]

2 shoppingList[1]="bottleofwater"

3

4 varoccupations=[

5 "Malcolm":"Captain",

6 "Kaylee":"Mechanic",

7 ]

8 occupations["Jayne"]="PublicRelations"

Arraysautomaticallygrowasyouaddelements.

1 shoppingList.append("bluepaint")

2 print(shoppingList)

Tocreateanemptyarrayordictionary,usetheinitializersyntax.

1 letemptyArray=[String]()

2 letemptyDictionary=[String:Float]()

Iftypeinformationcanbeinferred,youcanwriteanemptyarrayas[]andanemptydictionaryas[:]—forexample,whenyousetanewvalueforavariableorpassanargumenttoafunction.

1 shoppingList=[]

2 occupations=[:]

ControlFlow

Useifandswitchtomakeconditionals,andusefor-in,while,andrepeat-whiletomakeloops.Parenthesesaroundtheconditionorloopvariableareoptional.Bracesaroundthebodyarerequired.

1 letindividualScores=[75,43,103,87,12]

2 varteamScore=0

3 forscoreinindividualScores{

4 ifscore>50{

5 teamScore+=3

6 }else{

7 teamScore+=1

8 }

9 }

10 print(teamScore)

11 //Prints"11"

Inanifstatement,theconditionalmustbeaBooleanexpression—thismeansthatcodesuchasifscore{...}isanerror,notanimplicitcomparisontozero.

Youcanuseifandlettogethertoworkwithvaluesthatmightbemissing.Thesevaluesarerepresentedasoptionals.Anoptionalvalueeithercontainsavalueorcontainsniltoindicatethatavalueismissing.Writeaquestionmark(?)afterthetypeofavaluetomarkthevalueasoptional.

1 varoptionalString:String?="Hello"

2 print(optionalString==nil)

3 //Prints"false"

4

5 varoptionalName:String?="JohnAppleseed"

6 vargreeting="Hello!"

7 ifletname=optionalName{

8 greeting="Hello,\(name)"

9 }

EXPER IMENT

ChangeoptionalNametonil.Whatgreetingdoyouget?AddanelseclausethatsetsadifferentgreetingifoptionalNameisnil.

Iftheoptionalvalueisnil,theconditionalisfalseandthecodeinbracesisskipped.Otherwise,theoptionalvalueisunwrappedandassignedtotheconstantafterlet,whichmakestheunwrappedvalueavailableinsidetheblockofcode.

Anotherwaytohandleoptionalvaluesistoprovideadefaultvalueusingthe??operator.Iftheoptionalvalueismissing,thedefaultvalueisusedinstead.

1 letnickName:String?=nil

2 letfullName:String="JohnAppleseed"

3 letinformalGreeting="Hi\(nickName??fullName)"

Switchessupportanykindofdataandawidevarietyofcomparisonoperations—theyaren’tlimitedtointegersandtestsforequality.

1 letvegetable="redpepper"

2 switchvegetable{

3 case"celery":

4 print("Addsomeraisinsandmakeantsonalog.")

5 case"cucumber","watercress":

6 print("Thatwouldmakeagoodteasandwich.")

7 caseletxwherex.hasSuffix("pepper"):

8 print("Isitaspicy\(x)?")

9 default:

10 print("Everythingtastesgoodinsoup.")

11 }

12 //Prints"Isitaspicyredpepper?"

EXPER IMENT

Tryremovingthedefaultcase.Whaterrordoyouget?

Noticehowletcanbeusedinapatterntoassignthevaluethatmatchedthepatterntoaconstant.

Afterexecutingthecodeinsidetheswitchcasethatmatched,theprogramexitsfromtheswitchstatement.Executiondoesn’tcontinuetothenextcase,sothereisnoneedtoexplicitlybreakoutoftheswitchattheendofeachcase’scode.

Youusefor-intoiterateoveritemsinadictionarybyprovidingapairofnamestouseforeachkey-valuepair.Dictionariesareanunorderedcollection,sotheirkeysandvaluesareiteratedoverinanarbitraryorder.

1 letinterestingNumbers=[

2 "Prime":[2,3,5,7,11,13],

3 "Fibonacci":[1,1,2,3,5,8],

4 "Square":[1,4,9,16,25],

5 ]

6 varlargest=0

7 for(kind,numbers)ininterestingNumbers{

8 fornumberinnumbers{

9 ifnumber>largest{

10 largest=number

11 }

12 }

13 }

14 print(largest)

15 //Prints"25"

EXPER IMENT

Addanothervariabletokeeptrackofwhichkindofnumberwasthelargest,aswellaswhatthatlargestnumberwas.

Usewhiletorepeatablockofcodeuntilaconditionchanges.Theconditionofaloopcanbeattheendinstead,ensuringthattheloopisrunatleastonce.

1 varn=2

2 whilen<100{

3 n*=2

4 }

5 print(n)

6 //Prints"128"

7

8 varm=2

9 repeat{

10 m*=2

11 }whilem<100

12 print(m)

13 //Prints"128"

Youcankeepanindexinaloopbyusing..<tomakearangeofindexes.

1 vartotal=0

2 foriin0..<4{

3 total+=i

4 }

5 print(total)

6 //Prints"6"

Use..<tomakearangethatomitsitsuppervalue,anduse...tomakearangethatincludesbothvalues.

FunctionsandClosures

Usefunctodeclareafunction.Callafunctionbyfollowingitsnamewithalistofargumentsinparentheses.Use->toseparatetheparameternamesandtypesfromthefunction’sreturntype.

1 funcgreet(person:String,day:String)->String{

2 return"Hello\(person),todayis\(day)."

3 }

4 greet(person:"Bob",day:"Tuesday")

EXPER IMENT

Removethedayparameter.Addaparametertoincludetoday’slunchspecialinthegreeting.

Bydefault,functionsusetheirparameternamesaslabelsfortheirarguments.Writeacustomargumentlabelbeforetheparametername,orwrite_tousenoargumentlabel.

1 funcgreet(_person:String,onday:String)->String{

2 return"Hello\(person),todayis\(day)."

3 }

4 greet("John",on:"Wednesday")

Useatupletomakeacompoundvalue—forexample,toreturnmultiplevaluesfromafunction.Theelementsofatuplecanbereferredtoeitherbynameorbynumber.

1 funccalculateStatistics(scores:[Int])->(min:Int,max:Int,

sum:Int){

2 varmin=scores[0]

3 varmax=scores[0]

4 varsum=0

5

6 forscoreinscores{

7 ifscore>max{

8 max=score

9 }elseifscore<min{

10 min=score

11 }

12 sum+=score

13 }

14

15 return(min,max,sum)

16 }

17 letstatistics=calculateStatistics(scores:[5,3,100,3,

9])

18 print(statistics.sum)

19 //Prints"120"

20 print(statistics.2)

21 //Prints"120"

Functionscanbenested.Nestedfunctionshaveaccesstovariablesthatweredeclaredintheouterfunction.Youcanusenestedfunctionstoorganizethecodeinafunctionthatislongorcomplex.

1 funcreturnFifteen()->Int{

2 vary=10

3 funcadd(){

4 y+=5

5 }

6 add()

7 returny

8 }

9 returnFifteen()

Functionsareafirst-classtype.Thismeansthatafunctioncanreturnanotherfunctionasitsvalue.

1 funcmakeIncrementer()->((Int)->Int){

2 funcaddOne(number:Int)->Int{

3 return1+number

4 }

5 returnaddOne

6 }

7 varincrement=makeIncrementer()

8 increment(7)

Afunctioncantakeanotherfunctionasoneofitsarguments.

1 funchasAnyMatches(list:[Int],condition:(Int)->Bool)->

Bool{

2 foriteminlist{

3 ifcondition(item){

4 returntrue

5 }

6 }

7 returnfalse

8 }

9 funclessThanTen(number:Int)->Bool{

10 returnnumber<10

11 }

12 varnumbers=[20,19,7,12]

13 hasAnyMatches(list:numbers,condition:lessThanTen)

Functionsareactuallyaspecialcaseofclosures:blocksofcodethatcanbecalledlater.Thecodeinaclosurehasaccesstothingslikevariablesandfunctionsthatwereavailableinthescopewheretheclosurewascreated,eveniftheclosureisinadifferentscopewhenitisexecuted—yousawanexampleofthisalreadywithnestedfunctions.Youcanwriteaclosurewithoutanamebysurroundingcodewithbraces({}).Useintoseparatetheargumentsandreturntypefromthebody.

1 numbers.map({(number:Int)->Intin

2 letresult=3*number

3 returnresult

4 })

EXPER IMENT

Rewritetheclosuretoreturnzeroforalloddnumbers.

Youhaveseveraloptionsforwritingclosuresmoreconcisely.Whenaclosure’stypeisalreadyknown,suchasthecallbackforadelegate,youcanomitthetypeofitsparameters,itsreturntype,orboth.Singlestatementclosuresimplicitlyreturnthevalueoftheironlystatement.

1 letmappedNumbers=numbers.map({numberin3*number})

2 print(mappedNumbers)

3 //Prints"[60,57,21,36]"

Youcanrefertoparametersbynumberinsteadofbyname—thisapproachisespeciallyusefulinveryshortclosures.Aclosurepassedasthelastargumenttoafunctioncanappearimmediatelyaftertheparentheses.Whenaclosureistheonlyargumenttoafunction,youcanomittheparenthesesentirely.

1 letsortedNumbers=numbers.sorted{$0>$1}

2 print(sortedNumbers)

3 //Prints"[20,19,12,7]"

ObjectsandClasses

Useclassfollowedbytheclass’snametocreateaclass.Apropertydeclarationinaclassiswrittenthesamewayasaconstantorvariabledeclaration,exceptthatitisinthecontextofaclass.Likewise,methodandfunctiondeclarationsarewrittenthesameway.

1 classShape{

2 varnumberOfSides=0

3 funcsimpleDescription()->String{

4 return"Ashapewith\(numberOfSides)sides."

5 }

6 }

EXPER IMENT

Addaconstantpropertywithlet,andaddanothermethodthattakesanargument.

Createaninstanceofaclassbyputtingparenthesesaftertheclassname.Usedotsyntaxtoaccessthepropertiesandmethodsoftheinstance.

1 varshape=Shape()

2 shape.numberOfSides=7

3 varshapeDescription=shape.simpleDescription()

ThisversionoftheShapeclassismissingsomethingimportant:aninitializertosetuptheclasswhenaninstanceiscreated.Useinittocreateone.

1 classNamedShape{

2 varnumberOfSides:Int=0

3 varname:String

4

5 init(name:String){

6 self.name=name

7 }

8

9 funcsimpleDescription()->String{

10 return"Ashapewith\(numberOfSides)sides."

11 }

12 }

Noticehowselfisusedtodistinguishthenamepropertyfromthenameargumenttotheinitializer.Theargumentstotheinitializerarepassedlikeafunctioncallwhenyoucreateaninstanceoftheclass.Everypropertyneedsavalueassigned—eitherinitsdeclaration(aswithnumberOfSides)orintheinitializer(aswithname).

Usedeinittocreateadeinitializerifyouneedtoperformsomecleanupbefore

theobjectisdeallocated.

Subclassesincludetheirsuperclassnameaftertheirclassname,separatedbyacolon.Thereisnorequirementforclassestosubclassanystandardrootclass,soyoucanincludeoromitasuperclassasneeded.

Methodsonasubclassthatoverridethesuperclass’simplementationaremarkedwithoverride—overridingamethodbyaccident,withoutoverride,isdetectedbythecompilerasanerror.Thecompileralsodetectsmethodswithoverridethatdon’tactuallyoverrideanymethodinthesuperclass.

1 classSquare:NamedShape{

2 varsideLength:Double

3

4 init(sideLength:Double,name:String){

5 self.sideLength=sideLength

6 super.init(name:name)

7 numberOfSides=4

8 }

9

10 funcarea()->Double{

11 returnsideLength*sideLength

12 }

13

14 overridefuncsimpleDescription()->String{

15 return"Asquarewithsidesoflength\(sideLength)."

16 }

17 }

18 lettest=Square(sideLength:5.2,name:"mytestsquare")

19 test.area()

20 test.simpleDescription()

EXPER IMENT

MakeanothersubclassofNamedShapecalledCirclethattakesaradiusandanameasargumentstoitsinitializer.Implementanarea()andasimpleDescription()methodontheCircleclass.

Inadditiontosimplepropertiesthatarestored,propertiescanhaveagetterandasetter.

1 classEquilateralTriangle:NamedShape{

2 varsideLength:Double=0.0

3

4 init(sideLength:Double,name:String){

5 self.sideLength=sideLength

6 super.init(name:name)

7 numberOfSides=3

8 }

9

10 varperimeter:Double{

11 get{

12 return3.0*sideLength

13 }

14 set{

15 sideLength=newValue/3.0

16 }

17 }

18

19 overridefuncsimpleDescription()->String{

20 return"Anequilateraltrianglewithsidesoflength\

(sideLength)."

21 }

22 }

23 vartriangle=EquilateralTriangle(sideLength:3.1,name:"a

triangle")

24 print(triangle.perimeter)

25 //Prints"9.3"

26 triangle.perimeter=9.9

27 print(triangle.sideLength)

28 //Prints"3.3000000000000003"

Inthesetterforperimeter,thenewvaluehastheimplicitnamenewValue.Youcanprovideanexplicitnameinparenthesesafterset.

NoticethattheinitializerfortheEquilateralTriangleclasshasthreedifferentsteps:

1. Settingthevalueofpropertiesthatthesubclassdeclares.

2. Callingthesuperclass’sinitializer.

3. Changingthevalueofpropertiesdefinedbythesuperclass.Anyadditionalsetupworkthatusesmethods,getters,orsetterscanalsobedoneatthispoint.

Ifyoudon’tneedtocomputethepropertybutstillneedtoprovidecodethatisrunbeforeandaftersettinganewvalue,usewillSetanddidSet.Thecodeyouprovideisrunanytimethevaluechangesoutsideofaninitializer.Forexample,theclassbelowensuresthatthesidelengthofitstriangleisalwaysthesameasthesidelengthofitssquare.

1 classTriangleAndSquare{

2 vartriangle:EquilateralTriangle{

3 willSet{

4 square.sideLength=newValue.sideLength

5 }

6 }

7 varsquare:Square{

8 willSet{

9 triangle.sideLength=newValue.sideLength

10 }

11 }

12 init(size:Double,name:String){

13 square=Square(sideLength:size,name:name)

14 triangle=EquilateralTriangle(sideLength:size,name:

name)

15 }

16 }

17 vartriangleAndSquare=TriangleAndSquare(size:10,name:

"anothertestshape")

18 print(triangleAndSquare.square.sideLength)

19 //Prints"10.0"

20 print(triangleAndSquare.triangle.sideLength)

21 //Prints"10.0"

22 triangleAndSquare.square=Square(sideLength:50,name:

"largersquare")

23 print(triangleAndSquare.triangle.sideLength)

24 //Prints"50.0"

Whenworkingwithoptionalvalues,youcanwrite?beforeoperationslikemethods,properties,andsubscripting.Ifthevaluebeforethe?isnil,everythingafterthe?isignoredandthevalueofthewholeexpressionisnil.Otherwise,theoptionalvalueisunwrapped,andeverythingafterthe?actsontheunwrappedvalue.Inbothcases,thevalueofthewholeexpressionisanoptionalvalue.

1 letoptionalSquare:Square?=Square(sideLength:2.5,name:

"optionalsquare")

2 letsideLength=optionalSquare?.sideLength

EnumerationsandStructures

Useenumtocreateanenumeration.Likeclassesandallothernamedtypes,enumerationscanhavemethodsassociatedwiththem.

1 enumRank:Int{

2 caseace=1

3 casetwo,three,four,five,six,seven,eight,nine,ten

4 casejack,queen,king

5

6 funcsimpleDescription()->String{

7 switchself{

8 case.ace:

9 return"ace"

10 case.jack:

11 return"jack"

12 case.queen:

13 return"queen"

14 case.king:

15 return"king"

16 default:

17 returnString(self.rawValue)

18 }

19 }

20 }

21 letace=Rank.ace

22 letaceRawValue=ace.rawValue

EXPER IMENT

WriteafunctionthatcomparestwoRankvaluesbycomparingtheirrawvalues.

Bydefault,Swiftassignstherawvaluesstartingatzeroandincrementingbyoneeachtime,butyoucanchangethisbehaviorbyexplicitlyspecifyingvalues.Intheexampleabove,Aceisexplicitlygivenarawvalueof1,andtherestoftherawvaluesareassignedinorder.Youcanalsousestringsorfloating-pointnumbersastherawtypeofanenumeration.UsetherawValuepropertytoaccesstherawvalueofanenumerationcase.

Usetheinit?(rawValue:)initializertomakeaninstanceofanenumerationfromarawvalue.ItreturnseithertheenumerationcasematchingtherawvalueornilifthereisnomatchingRank.

1 ifletconvertedRank=Rank(rawValue:3){

2 letthreeDescription=convertedRank.simpleDescription()

3 }

Thecasevaluesofanenumerationareactualvalues,notjustanotherwayofwritingtheirrawvalues.Infact,incaseswherethereisn’tameaningfulrawvalue,youdon’thavetoprovideone.

1 enumSuit{

2 casespades,hearts,diamonds,clubs

3

4 funcsimpleDescription()->String{

5 switchself{

6 case.spades:

7 return"spades"

8 case.hearts:

9 return"hearts"

10 case.diamonds:

11 return"diamonds"

12 case.clubs:

13 return"clubs"

14 }

15 }

16 }

17 lethearts=Suit.hearts

18 letheartsDescription=hearts.simpleDescription()

EXPER IMENT

Addacolor()methodtoSuitthatreturns“black”forspadesandclubs,andreturns“red”forheartsanddiamonds.

Noticethetwowaysthattheheartscaseoftheenumerationisreferredtoabove:Whenassigningavaluetotheheartsconstant,theenumerationcaseSuit.heartsisreferredtobyitsfullnamebecausetheconstantdoesn’thaveanexplicittypespecified.Insidetheswitch,theenumerationcaseisreferredtobytheabbreviatedform.heartsbecausethevalueofselfisalreadyknowntobeasuit.Youcanusetheabbreviatedformanytimethevalue’stypeisalreadyknown.

Ifanenumerationhasrawvalues,thosevaluesaredeterminedaspartofthedeclaration,whichmeanseveryinstanceofaparticularenumerationcasealwayshasthesamerawvalue.Anotherchoiceforenumerationcasesistohavevaluesassociatedwiththecase—thesevaluesaredeterminedwhenyoumaketheinstance,andtheycanbedifferentforeachinstanceofanenumerationcase.Youcanthinkoftheassociatedvaluesasbehavinglikestoredpropertiesoftheenumerationcaseinstance.Forexample,considerthecaseofrequestingthesunriseandsunsettimesfromaserver.Theservereitherrespondswiththerequestedinformation,oritrespondswithadescriptionofwhatwentwrong.

1 enumServerResponse{

2 caseresult(String,String)

3 casefailure(String)

4 }

5

6 letsuccess=ServerResponse.result("6:00am","8:09pm")

7 letfailure=ServerResponse.failure("Outofcheese.")

8

9 switchsuccess{

10 caselet.result(sunrise,sunset):

11 print("Sunriseisat\(sunrise)andsunsetisat\

(sunset).")

12 caselet.failure(message):

13 print("Failure...\(message)")

14 }

15 //Prints"Sunriseisat6:00amandsunsetisat8:09pm."

EXPER IMENT

AddathirdcasetoServerResponseandtotheswitch.

NoticehowthesunriseandsunsettimesareextractedfromtheServerResponsevalueaspartofmatchingthevalueagainsttheswitchcases.

Usestructtocreateastructure.Structuressupportmanyofthesamebehaviorsasclasses,includingmethodsandinitializers.Oneofthemostimportantdifferencesbetweenstructuresandclassesisthatstructuresarealwayscopiedwhentheyarepassedaroundinyourcode,butclassesarepassedbyreference.

1 structCard{

2 varrank:Rank

3 varsuit:Suit

4 funcsimpleDescription()->String{

5 return"The\(rank.simpleDescription())of\

(suit.simpleDescription())"

6 }

7 }

8 letthreeOfSpades=Card(rank:.three,suit:.spades)

9 letthreeOfSpadesDescription=

threeOfSpades.simpleDescription()

EXPER IMENT

Writeafunctionthatreturnsanarraycontainingafulldeckofcards,withonecardofeachcombinationofrankandsuit.

ProtocolsandExtensions

Useprotocoltodeclareaprotocol.

1 protocolExampleProtocol{

2 varsimpleDescription:String{get}

3 mutatingfuncadjust()

4 }

Classes,enumerations,andstructscanalladoptprotocols.

1 classSimpleClass:ExampleProtocol{

2 varsimpleDescription:String="Averysimpleclass."

3 varanotherProperty:Int=69105

4 funcadjust(){

5 simpleDescription+="Now100%adjusted."

6 }

7 }

8 vara=SimpleClass()

9 a.adjust()

10 letaDescription=a.simpleDescription

11

12 structSimpleStructure:ExampleProtocol{

13 varsimpleDescription:String="Asimplestructure"

14 mutatingfuncadjust(){

15 simpleDescription+="(adjusted)"

16 }

17 }

18 varb=SimpleStructure()

19 b.adjust()

20 letbDescription=b.simpleDescription

EXPER IMENT

AddanotherrequirementtoExampleProtocol.WhatchangesdoyouneedtomaketoSimpleClassandSimpleStructuresothattheystillconformtotheprotocol?

NoticetheuseofthemutatingkeywordinthedeclarationofSimpleStructuretomarkamethodthatmodifiesthestructure.ThedeclarationofSimpleClassdoesn’tneedanyofitsmethodsmarkedasmutatingbecausemethodsonaclasscanalwaysmodifytheclass.

Useextensiontoaddfunctionalitytoanexistingtype,suchasnewmethodsandcomputedproperties.Youcanuseanextensiontoaddprotocolconformancetoatypethatisdeclaredelsewhere,oreventoatypethatyouimportedfromalibraryorframework.

1 extensionInt:ExampleProtocol{

2 varsimpleDescription:String{

3 return"Thenumber\(self)"

4 }

5 mutatingfuncadjust(){

6 self+=42

7 }

8 }

9 print(7.simpleDescription)

10 //Prints"Thenumber7"

EXPER IMENT

WriteanextensionfortheDoubletypethataddsanabsoluteValueproperty.

Youcanuseaprotocolnamejustlikeanyothernamedtype—forexample,tocreateacollectionofobjectsthathavedifferenttypesbutthatallconformtoasingleprotocol.Whenyouworkwithvalueswhosetypeisaprotocoltype,methodsoutsidetheprotocoldefinitionarenotavailable.

1 letprotocolValue:ExampleProtocol=a

2 print(protocolValue.simpleDescription)

3 //Prints"Averysimpleclass.Now100%adjusted."

4 //print(protocolValue.anotherProperty)//Uncommenttosee

theerror

EventhoughthevariableprotocolValuehasaruntimetypeofSimpleClass,thecompilertreatsitasthegiventypeofExampleProtocol.Thismeansthatyoucan’taccidentallyaccessmethodsorpropertiesthattheclassimplementsinadditiontoitsprotocolconformance.

ErrorHandling

YourepresenterrorsusinganytypethatadoptstheErrorprotocol.

1 enumPrinterError:Error{

2 caseoutOfPaper

3 casenoToner

4 caseonFire

5 }

Usethrowtothrowanerrorandthrowstomarkafunctionthatcanthrowanerror.Ifyouthrowanerrorinafunction,thefunctionreturnsimmediatelyandthecodethatcalledthefunctionhandlestheerror.

1 funcsend(job:Int,toPrinterprinterName:String)throws->

String{

2 ifprinterName=="NeverHasToner"{

3 throwPrinterError.noToner

4 }

5 return"Jobsent"

6 }

Thereareseveralwaystohandleerrors.Onewayistousedo-catch.Insidethedoblock,youmarkcodethatcanthrowanerrorbywritingtryinfrontofit.Insidethecatchblock,theerrorisautomaticallygiventhenameerrorunlessyougiveitadifferentname.

1 do{

2 letprinterResponse=trysend(job:1040,toPrinter:"Bi

Sheng")

3 print(printerResponse)

4 }catch{

5 print(error)

6 }

7 //Prints"Jobsent"

EXPER IMENT

Changetheprinternameto"NeverHasToner",sothatthesend(job:toPrinter:)functionthrowsanerror.

Youcanprovidemultiplecatchblocksthathandlespecificerrors.Youwriteapatternaftercatchjustasyoudoaftercaseinaswitch.

1 do{

2 letprinterResponse=trysend(job:1440,toPrinter:

"Gutenberg")

3 print(printerResponse)

4 }catchPrinterError.onFire{

5 print("I'lljustputthisoverhere,withtherestofthe

fire.")

6 }catchletprinterErrorasPrinterError{

7 print("Printererror:\(printerError).")

8 }catch{

9 print(error)

10 }

11 //Prints"Jobsent"

EXPER IMENT

Addcodetothrowanerrorinsidethedoblock.Whatkindoferrordoyouneedtothrowsothattheerrorishandledbythefirstcatchblock?Whataboutthesecondandthirdblocks?

Anotherwaytohandleerrorsistousetry?toconverttheresulttoanoptional.Ifthefunctionthrowsanerror,thespecificerrorisdiscardedandtheresultisnil.Otherwise,theresultisanoptionalcontainingthevaluethatthefunctionreturned.

1 letprinterSuccess=try?send(job:1884,toPrinter:

"Mergenthaler")

2 letprinterFailure=try?send(job:1885,toPrinter:"NeverHas

Toner")

Usedefertowriteablockofcodethatisexecutedafterallothercodeinthefunction,justbeforethefunctionreturns.Thecodeisexecutedregardlessofwhetherthefunctionthrowsanerror.Youcanusedefertowritesetupandcleanupcodenexttoeachother,eventhoughtheyneedtobeexecutedatdifferenttimes.

1 varfridgeIsOpen=false

2 letfridgeContent=["milk","eggs","leftovers"]

3

4 funcfridgeContains(_food:String)->Bool{

5 fridgeIsOpen=true

6 defer{

7 fridgeIsOpen=false

8 }

9

10 letresult=fridgeContent.contains(food)

11 returnresult

12 }

13 fridgeContains("banana")

14 print(fridgeIsOpen)

15 //Prints"false"

Generics

Writeanameinsideanglebracketstomakeagenericfunctionortype.

1 funcmakeArray<Item>(repeatingitem:Item,numberOfTimes:Int)

->[Item]{

2 varresult=[Item]()

3 for_in0..<numberOfTimes{

4 result.append(item)

5 }

6 returnresult

7 }

8 makeArray(repeating:"knock",numberOfTimes:4)

Youcanmakegenericformsoffunctionsandmethods,aswellasclasses,

enumerations,andstructures.

1 //ReimplementtheSwiftstandardlibrary'soptionaltype

2 enumOptionalValue<Wrapped>{

3 casenone

4 casesome(Wrapped)

5 }

6 varpossibleInteger:OptionalValue<Int>=.none

7 possibleInteger=.some(100)

Usewhererightbeforethebodytospecifyalistofrequirements—forexample,torequirethetypetoimplementaprotocol,torequiretwotypestobethesame,ortorequireaclasstohaveaparticularsuperclass.

1 funcanyCommonElements<T:Sequence,U:Sequence>(_lhs:T,_

rhs:U)->Bool

2 whereT.Element:Equatable,T.Element==U.Element

3 {

4 forlhsIteminlhs{

5 forrhsIteminrhs{

6 iflhsItem==rhsItem{

7 returntrue

8 }

9 }

10 }

11 returnfalse

12 }

13 anyCommonElements([1,2,3],[3])

EXPER IMENT

ModifytheanyCommonElements(_:_:)functiontomakeafunctionthatreturnsanarrayoftheelementsthatanytwosequenceshaveincommon.

Writing<T:Equatable>isthesameaswriting<T>...whereT:Equatable.

LanguageGuide

TheBasics

SwiftisanewprogramminglanguageforiOS,macOS,watchOS,andtvOSappdevelopment.Nonetheless,manypartsofSwiftwillbefamiliarfromyourexperienceofdevelopinginCandObjective-C.

SwiftprovidesitsownversionsofallfundamentalCandObjective-Ctypes,includingIntforintegers,DoubleandFloatforfloating-pointvalues,BoolforBooleanvalues,andStringfortextualdata.Swiftalsoprovidespowerfulversionsofthethreeprimarycollectiontypes,Array,Set,andDictionary,asdescribedinCollectionTypes.

LikeC,Swiftusesvariablestostoreandrefertovaluesbyanidentifyingname.Swiftalsomakesextensiveuseofvariableswhosevaluescan’tbechanged.Theseareknownasconstants,andaremuchmorepowerfulthanconstantsinC.ConstantsareusedthroughoutSwifttomakecodesaferandclearerinintentwhenyouworkwithvaluesthatdon’tneedtochange.

Inadditiontofamiliartypes,SwiftintroducesadvancedtypesnotfoundinObjective-C,suchastuples.Tuplesenableyoutocreateandpassaroundgroupingsofvalues.Youcanuseatupletoreturnmultiplevaluesfromafunctionasasinglecompoundvalue.

Swiftalsointroducesoptionaltypes,whichhandletheabsenceofavalue.Optionalssayeither“thereisavalue,anditequalsx”or“thereisn’tavalueatall”.UsingoptionalsissimilartousingnilwithpointersinObjective-C,buttheyworkforanytype,notjustclasses.NotonlyareoptionalssaferandmoreexpressivethannilpointersinObjective-C,they’reattheheartofmanyofSwift’smostpowerfulfeatures.

Swiftisatype-safelanguage,whichmeansthelanguagehelpsyoutobeclearaboutthetypesofvaluesyourcodecanworkwith.IfpartofyourcoderequiresaString,typesafetypreventsyoufrompassingitanIntbymistake.Likewise,typesafetypreventsyoufromaccidentallypassinganoptionalStringtoapieceofcodethatrequiresanon-optionalString.Typesafetyhelpsyoucatchandfixerrorsasearlyaspossibleinthedevelopmentprocess.

ConstantsandVariables

Constantsandvariablesassociateaname(suchasmaximumNumberOfLoginAttemptsorwelcomeMessage)withavalueofaparticulartype(suchasthenumber10orthestring"Hello").Thevalueofaconstantcan’tbechangedonceit’sset,whereasavariablecanbesettoadifferentvalueinthefuture.

DeclaringConstantsandVariablesConstantsandvariablesmustbedeclaredbeforethey’reused.Youdeclareconstantswiththeletkeywordandvariableswiththevarkeyword.Here’sanexampleofhowconstantsandvariablescanbeusedtotrackthenumberofloginattemptsauserhasmade:

1 letmaximumNumberOfLoginAttempts=10

2 varcurrentLoginAttempt=0

Thiscodecanbereadas:

“DeclareanewconstantcalledmaximumNumberOfLoginAttempts,andgiveitavalueof10.Then,declareanewvariablecalledcurrentLoginAttempt,andgiveitaninitialvalueof0.”

Inthisexample,themaximumnumberofallowedloginattemptsisdeclaredasaconstant,becausethemaximumvalueneverchanges.Thecurrentloginattemptcounterisdeclaredasavariable,becausethisvaluemustbeincrementedaftereachfailedloginattempt.

Youcandeclaremultipleconstantsormultiplevariablesonasingleline,separatedbycommas:

varx=0.0,y=0.0,z=0.0

NOTE

Ifastoredvalueinyourcodewon’tchange,alwaysdeclareitasaconstantwiththeletkeyword.

Usevariablesonlyforstoringvaluesthatneedtobeabletochange.

TypeAnnotationsYoucanprovideatypeannotationwhenyoudeclareaconstantorvariable,tobeclearaboutthekindofvaluestheconstantorvariablecanstore.Writeatypeannotationbyplacingacolonaftertheconstantorvariablename,followedbyaspace,followedbythenameofthetypetouse.

ThisexampleprovidesatypeannotationforavariablecalledwelcomeMessage,toindicatethatthevariablecanstoreStringvalues:

varwelcomeMessage:String

Thecoloninthedeclarationmeans“…oftype…,”sothecodeabovecanbereadas:

“DeclareavariablecalledwelcomeMessagethatisoftypeString.”

Thephrase“oftypeString”means“canstoreanyStringvalue.”Thinkofitasmeaning“thetypeofthing”(or“thekindofthing”)thatcanbestored.

ThewelcomeMessagevariablecannowbesettoanystringvaluewithouterror:

welcomeMessage="Hello"

Youcandefinemultiplerelatedvariablesofthesametypeonasingleline,separatedbycommas,withasingletypeannotationafterthefinalvariablename:

varred,green,blue:Double

NOTE

It’srarethatyouneedtowritetypeannotationsinpractice.Ifyouprovideaninitialvalueforaconstantorvariableatthepointthatit’sdefined,Swiftcanalmostalwaysinferthetypetobeusedforthatconstantorvariable,asdescribedinTypeSafetyandTypeInference.InthewelcomeMessageexampleabove,noinitialvalueisprovided,andsothetypeofthewelcomeMessagevariableis

specifiedwithatypeannotationratherthanbeinginferredfromaninitialvalue.

NamingConstantsandVariablesConstantandvariablenamescancontainalmostanycharacter,includingUnicodecharacters:

1 letπ=3.14159

2 let =""

3 let ="dogcow"

Constantandvariablenamescan’tcontainwhitespacecharacters,mathematicalsymbols,arrows,private-useUnicodescalarvalues,orline-andbox-drawingcharacters.Norcantheybeginwithanumber,althoughnumbersmaybeincludedelsewherewithinthename.

Onceyou’vedeclaredaconstantorvariableofacertaintype,youcan’tdeclareitagainwiththesamename,orchangeittostorevaluesofadifferenttype.Norcanyouchangeaconstantintoavariableoravariableintoaconstant.

NOTE

IfyouneedtogiveaconstantorvariablethesamenameasareservedSwiftkeyword,surroundthekeywordwithbackticks(`)whenusingitasaname.However,avoidusingkeywordsasnamesunlessyouhaveabsolutelynochoice.

Youcanchangethevalueofanexistingvariabletoanothervalueofacompatibletype.Inthisexample,thevalueoffriendlyWelcomeischangedfrom"Hello!"to"Bonjour!":

1 varfriendlyWelcome="Hello!"

2 friendlyWelcome="Bonjour!"

3 //friendlyWelcomeisnow"Bonjour!"

Unlikeavariable,thevalueofaconstantcan’tbechangedafterit’sset.

Attemptingtodosoisreportedasanerrorwhenyourcodeiscompiled:

1 letlanguageName="Swift"

2 languageName="Swift++"

3 //Thisisacompile-timeerror:languageNamecannotbe

changed.

PrintingConstantsandVariablesYoucanprintthecurrentvalueofaconstantorvariablewiththeprint(_:separator:terminator:)function:

1 print(friendlyWelcome)

2 //Prints"Bonjour!"

Theprint(_:separator:terminator:)functionisaglobalfunctionthatprintsoneormorevaluestoanappropriateoutput.InXcode,forexample,theprint(_:separator:terminator:)functionprintsitsoutputinXcode’s“console”pane.Theseparatorandterminatorparameterhavedefaultvalues,soyoucanomitthemwhenyoucallthisfunction.Bydefault,thefunctionterminatesthelineitprintsbyaddingalinebreak.Toprintavaluewithoutalinebreakafterit,passanemptystringastheterminator—forexample,print(someValue,terminator:"").Forinformationaboutparameterswithdefaultvalues,seeDefaultParameterValues.

Swiftusesstringinterpolationtoincludethenameofaconstantorvariableasaplaceholderinalongerstring,andtopromptSwifttoreplaceitwiththecurrentvalueofthatconstantorvariable.Wrapthenameinparenthesesandescapeitwithabackslashbeforetheopeningparenthesis:

1 print("ThecurrentvalueoffriendlyWelcomeis\

(friendlyWelcome)")

2 //Prints"ThecurrentvalueoffriendlyWelcomeisBonjour!"

NOTE

AlloptionsyoucanusewithstringinterpolationaredescribedinStringInterpolation.

Comments

Usecommentstoincludenonexecutabletextinyourcode,asanoteorremindertoyourself.CommentsareignoredbytheSwiftcompilerwhenyourcodeiscompiled.

CommentsinSwiftareverysimilartocommentsinC.Single-linecommentsbeginwithtwoforward-slashes(//):

//Thisisacomment.

Multilinecommentsstartwithaforward-slashfollowedbyanasterisk(/*)andendwithanasteriskfollowedbyaforward-slash(*/):

1 /*Thisisalsoacomment

2 butiswrittenovermultiplelines.*/

UnlikemultilinecommentsinC,multilinecommentsinSwiftcanbenestedinsideothermultilinecomments.Youwritenestedcommentsbystartingamultilinecommentblockandthenstartingasecondmultilinecommentwithinthefirstblock.Thesecondblockisthenclosed,followedbythefirstblock:

1 /*Thisisthestartofthefirstmultilinecomment.

2 /*Thisisthesecond,nestedmultilinecomment.*/

3 Thisistheendofthefirstmultilinecomment.*/

Nestedmultilinecommentsenableyoutocommentoutlargeblocksofcodequicklyandeasily,evenifthecodealreadycontainsmultilinecomments.

Semicolons

Unlikemanyotherlanguages,Swiftdoesn’trequireyoutowriteasemicolon(;)aftereachstatementinyourcode,althoughyoucandosoifyouwish.However,semicolonsarerequiredifyouwanttowritemultipleseparatestatementsonasingleline:

1 letcat=" ";print(cat)

2 //Prints" "

Integers

Integersarewholenumberswithnofractionalcomponent,suchas42and-23.Integersareeithersigned(positive,zero,ornegative)orunsigned(positiveorzero).

Swiftprovidessignedandunsignedintegersin8,16,32,and64bitforms.TheseintegersfollowanamingconventionsimilartoC,inthatan8-bitunsignedintegerisoftypeUInt8,anda32-bitsignedintegerisoftypeInt32.LikealltypesinSwift,theseintegertypeshavecapitalizednames.

IntegerBoundsYoucanaccesstheminimumandmaximumvaluesofeachintegertypewithitsminandmaxproperties:

1 letminValue=UInt8.min//minValueisequalto0,andisof

typeUInt8

2 letmaxValue=UInt8.max//maxValueisequalto255,andis

oftypeUInt8

Thevaluesofthesepropertiesareoftheappropriate-sizednumbertype(suchasUInt8intheexampleabove)andcanthereforebeusedinexpressionsalongside

othervaluesofthesametype.

IntInmostcases,youdon’tneedtopickaspecificsizeofintegertouseinyourcode.Swiftprovidesanadditionalintegertype,Int,whichhasthesamesizeasthecurrentplatform’snativewordsize:

Ona32-bitplatform,IntisthesamesizeasInt32.

Ona64-bitplatform,IntisthesamesizeasInt64.

Unlessyouneedtoworkwithaspecificsizeofinteger,alwaysuseIntforintegervaluesinyourcode.Thisaidscodeconsistencyandinteroperability.Evenon32-bitplatforms,Intcanstoreanyvaluebetween-2,147,483,648and2,147,483,647,andislargeenoughformanyintegerranges.

UIntSwiftalsoprovidesanunsignedintegertype,UInt,whichhasthesamesizeasthecurrentplatform’snativewordsize:

Ona32-bitplatform,UIntisthesamesizeasUInt32.

Ona64-bitplatform,UIntisthesamesizeasUInt64.

NOTE

UseUIntonlywhenyouspecificallyneedanunsignedintegertypewiththesamesizeastheplatform’snativewordsize.Ifthisisn’tthecase,Intispreferred,evenwhenthevaluestobestoredareknowntobenonnegative.AconsistentuseofIntforintegervaluesaidscodeinteroperability,avoidstheneedtoconvertbetweendifferentnumbertypes,andmatchesintegertypeinference,asdescribedinTypeSafetyandTypeInference.

Floating-PointNumbers

Floating-pointnumbersarenumberswithafractionalcomponent,suchas3.14159,0.1,and-273.15.

Floating-pointtypescanrepresentamuchwiderrangeofvaluesthanintegertypes,andcanstorenumbersthataremuchlargerorsmallerthancanbestoredinanInt.Swiftprovidestwosignedfloating-pointnumbertypes:

Doublerepresentsa64-bitfloating-pointnumber.

Floatrepresentsa32-bitfloating-pointnumber.

NOTE

Doublehasaprecisionofatleast15decimaldigits,whereastheprecisionofFloatcanbeaslittleas6decimaldigits.Theappropriatefloating-pointtypetousedependsonthenatureandrangeofvaluesyouneedtoworkwithinyourcode.Insituationswhereeithertypewouldbeappropriate,Doubleispreferred.

TypeSafetyandTypeInference

Swiftisatype-safelanguage.Atypesafelanguageencouragesyoutobeclearaboutthetypesofvaluesyourcodecanworkwith.IfpartofyourcoderequiresaString,youcan’tpassitanIntbymistake.

BecauseSwiftistypesafe,itperformstypecheckswhencompilingyourcodeandflagsanymismatchedtypesaserrors.Thisenablesyoutocatchandfixerrorsasearlyaspossibleinthedevelopmentprocess.

Type-checkinghelpsyouavoiderrorswhenyou’reworkingwithdifferenttypesofvalues.However,thisdoesn’tmeanthatyouhavetospecifythetypeofeveryconstantandvariablethatyoudeclare.Ifyoudon’tspecifythetypeofvalueyouneed,Swiftusestypeinferencetoworkouttheappropriatetype.Typeinferenceenablesacompilertodeducethetypeofaparticularexpressionautomaticallywhenitcompilesyourcode,simplybyexaminingthevaluesyouprovide.

Becauseoftypeinference,SwiftrequiresfarfewertypedeclarationsthanlanguagessuchasCorObjective-C.Constantsandvariablesarestillexplicitly

typed,butmuchoftheworkofspecifyingtheirtypeisdoneforyou.

Typeinferenceisparticularlyusefulwhenyoudeclareaconstantorvariablewithaninitialvalue.Thisisoftendonebyassigningaliteralvalue(orliteral)totheconstantorvariableatthepointthatyoudeclareit.(Aliteralvalueisavaluethatappearsdirectlyinyoursourcecode,suchas42and3.14159intheexamplesbelow.)

Forexample,ifyouassignaliteralvalueof42toanewconstantwithoutsayingwhattypeitis,SwiftinfersthatyouwanttheconstanttobeanInt,becauseyouhaveinitializeditwithanumberthatlookslikeaninteger:

1 letmeaningOfLife=42

2 //meaningOfLifeisinferredtobeoftypeInt

Likewise,ifyoudon’tspecifyatypeforafloating-pointliteral,SwiftinfersthatyouwanttocreateaDouble:

1 letpi=3.14159

2 //piisinferredtobeoftypeDouble

SwiftalwayschoosesDouble(ratherthanFloat)wheninferringthetypeoffloating-pointnumbers.

Ifyoucombineintegerandfloating-pointliteralsinanexpression,atypeofDoublewillbeinferredfromthecontext:

1 letanotherPi=3+0.14159

2 //anotherPiisalsoinferredtobeoftypeDouble

Theliteralvalueof3hasnoexplicittypeinandofitself,andsoanappropriateoutputtypeofDoubleisinferredfromthepresenceofafloating-pointliteralaspartoftheaddition.

NumericLiterals

Integerliteralscanbewrittenas:

Adecimalnumber,withnoprefix

Abinarynumber,witha0bprefix

Anoctalnumber,witha0oprefix

Ahexadecimalnumber,witha0xprefix

Alloftheseintegerliteralshaveadecimalvalueof17:

1 letdecimalInteger=17

2 letbinaryInteger=0b10001//17inbinarynotation

3 letoctalInteger=0o21//17inoctalnotation

4 lethexadecimalInteger=0x11//17inhexadecimalnotation

Floating-pointliteralscanbedecimal(withnoprefix),orhexadecimal(witha0xprefix).Theymustalwayshaveanumber(orhexadecimalnumber)onbothsidesofthedecimalpoint.Decimalfloatscanalsohaveanoptionalexponent,indicatedbyanuppercaseorlowercasee;hexadecimalfloatsmusthaveanexponent,indicatedbyanuppercaseorlowercasep.

Fordecimalnumberswithanexponentofexp,thebasenumberismultipliedby10exp:

1.25e2means1.25x102,or125.0.

1.25e-2means1.25x10-2,or0.0125.

Forhexadecimalnumberswithanexponentofexp,thebasenumberismultipliedby2exp:

0xFp2means15x22,or60.0.

0xFp-2means15x2-2,or3.75.

Allofthesefloating-pointliteralshaveadecimalvalueof12.1875:

1 letdecimalDouble=12.1875

2 letexponentDouble=1.21875e1

3 lethexadecimalDouble=0xC.3p0

Numericliteralscancontainextraformattingtomakethemeasiertoread.Bothintegersandfloatscanbepaddedwithextrazerosandcancontainunderscorestohelpwithreadability.Neithertypeofformattingaffectstheunderlyingvalueoftheliteral:

1 letpaddedDouble=000123.456

2 letoneMillion=1_000_000

3 letjustOverOneMillion=1_000_000.000_000_1

NumericTypeConversion

UsetheInttypeforallgeneral-purposeintegerconstantsandvariablesinyourcode,evenifthey’reknowntobenonnegative.Usingthedefaultintegertypeineverydaysituationsmeansthatintegerconstantsandvariablesareimmediatelyinteroperableinyourcodeandwillmatchtheinferredtypeforintegerliteralvalues.

Useotherintegertypesonlywhenthey’respecificallyneededforthetaskathand,becauseofexplicitlysizeddatafromanexternalsource,orforperformance,memoryusage,orothernecessaryoptimization.Usingexplicitlysizedtypesinthesesituationshelpstocatchanyaccidentalvalueoverflowsandimplicitlydocumentsthenatureofthedatabeingused.

IntegerConversion

Therangeofnumbersthatcanbestoredinanintegerconstantorvariableisdifferentforeachnumerictype.AnInt8constantorvariablecanstorenumbersbetween-128and127,whereasaUInt8constantorvariablecanstorenumbersbetween0and255.Anumberthatwon’tfitintoaconstantorvariableofasizedintegertypeisreportedasanerrorwhenyourcodeiscompiled:

1 letcannotBeNegative:UInt8=-1

2 //UInt8cannotstorenegativenumbers,andsothiswillreport

anerror

3 lettooBig:Int8=Int8.max+1

4 //Int8cannotstoreanumberlargerthanitsmaximumvalue,

5 //andsothiswillalsoreportanerror

Becauseeachnumerictypecanstoreadifferentrangeofvalues,youmustoptintonumerictypeconversiononacase-by-casebasis.Thisopt-inapproachpreventshiddenconversionerrorsandhelpsmaketypeconversionintentionsexplicitinyourcode.

Toconvertonespecificnumbertypetoanother,youinitializeanewnumberofthedesiredtypewiththeexistingvalue.Intheexamplebelow,theconstanttwoThousandisoftypeUInt16,whereastheconstantoneisoftypeUInt8.Theycan’tbeaddedtogetherdirectly,becausethey’renotofthesametype.Instead,thisexamplecallsUInt16(one)tocreateanewUInt16initializedwiththevalueofone,andusesthisvalueinplaceoftheoriginal:

1 lettwoThousand:UInt16=2_000

2 letone:UInt8=1

3 lettwoThousandAndOne=twoThousand+UInt16(one)

BecausebothsidesoftheadditionarenowoftypeUInt16,theadditionisallowed.Theoutputconstant(twoThousandAndOne)isinferredtobeoftypeUInt16,becauseit’sthesumoftwoUInt16values.

SomeType(ofInitialValue)isthedefaultwaytocalltheinitializerofaSwifttypeandpassinaninitialvalue.Behindthescenes,UInt16hasaninitializerthat

acceptsaUInt8value,andsothisinitializerisusedtomakeanewUInt16fromanexistingUInt8.Youcan’tpassinanytypehere,however—ithastobeatypeforwhichUInt16providesaninitializer.Extendingexistingtypestoprovideinitializersthatacceptnewtypes(includingyourowntypedefinitions)iscoveredinExtensions.

IntegerandFloating-PointConversionConversionsbetweenintegerandfloating-pointnumerictypesmustbemadeexplicit:

1 letthree=3

2 letpointOneFourOneFiveNine=0.14159

3 letpi=Double(three)+pointOneFourOneFiveNine

4 //piequals3.14159,andisinferredtobeoftypeDouble

Here,thevalueoftheconstantthreeisusedtocreateanewvalueoftypeDouble,sothatbothsidesoftheadditionareofthesametype.Withoutthisconversioninplace,theadditionwouldnotbeallowed.

Floating-pointtointegerconversionmustalsobemadeexplicit.AnintegertypecanbeinitializedwithaDoubleorFloatvalue:

1 letintegerPi=Int(pi)

2 //integerPiequals3,andisinferredtobeoftypeInt

Floating-pointvaluesarealwaystruncatedwhenusedtoinitializeanewintegervalueinthisway.Thismeansthat4.75becomes4,and-3.9becomes-3.

NOTE

Therulesforcombiningnumericconstantsandvariablesaredifferentfromtherulesfornumericliterals.Theliteralvalue3canbeaddeddirectlytotheliteralvalue0.14159,becausenumberliteralsdon’thaveanexplicittypeinandofthemselves.Theirtypeisinferredonlyatthepointthatthey’reevaluatedbythecompiler.

TypeAliases

Typealiasesdefineanalternativenameforanexistingtype.Youdefinetypealiaseswiththetypealiaskeyword.

Typealiasesareusefulwhenyouwanttorefertoanexistingtypebyanamethatiscontextuallymoreappropriate,suchaswhenworkingwithdataofaspecificsizefromanexternalsource:

typealiasAudioSample=UInt16

Onceyoudefineatypealias,youcanusethealiasanywhereyoumightusetheoriginalname:

1 varmaxAmplitudeFound=AudioSample.min

2 //maxAmplitudeFoundisnow0

Here,AudioSampleisdefinedasanaliasforUInt16.Becauseit’sanalias,thecalltoAudioSample.minactuallycallsUInt16.min,whichprovidesaninitialvalueof0forthemaxAmplitudeFoundvariable.

Booleans

SwifthasabasicBooleantype,calledBool.Booleanvaluesarereferredtoaslogical,becausetheycanonlyeverbetrueorfalse.SwiftprovidestwoBooleanconstantvalues,trueandfalse:

1 letorangesAreOrange=true

2 letturnipsAreDelicious=false

ThetypesoforangesAreOrangeandturnipsAreDelicioushavebeeninferredasBoolfromthefactthattheywereinitializedwithBooleanliteralvalues.AswithIntandDoubleabove,youdon’tneedtodeclareconstantsorvariablesasBoolifyousetthemtotrueorfalseassoonasyoucreatethem.Typeinferencehelps

makeSwiftcodemoreconciseandreadablewhenitinitializesconstantsorvariableswithothervalueswhosetypeisalreadyknown.

Booleanvaluesareparticularlyusefulwhenyouworkwithconditionalstatementssuchastheifstatement:

1 ifturnipsAreDelicious{

2 print("Mmm,tastyturnips!")

3 }else{

4 print("Eww,turnipsarehorrible.")

5 }

6 //Prints"Eww,turnipsarehorrible."

ConditionalstatementssuchastheifstatementarecoveredinmoredetailinControlFlow.

Swift’stypesafetypreventsnon-BooleanvaluesfrombeingsubstitutedforBool.Thefollowingexamplereportsacompile-timeerror:

1 leti=1

2 ifi{

3 //thisexamplewillnotcompile,andwillreportanerror

4 }

However,thealternativeexamplebelowisvalid:

1 leti=1

2 ifi==1{

3 //thisexamplewillcompilesuccessfully

4 }

Theresultofthei==1comparisonisoftypeBool,andsothissecondexamplepassesthetype-check.Comparisonslikei==1arediscussedinBasicOperators.

AswithotherexamplesoftypesafetyinSwift,thisapproachavoidsaccidentalerrorsandensuresthattheintentionofaparticularsectionofcodeisalwaysclear.

Tuples

Tuplesgroupmultiplevaluesintoasinglecompoundvalue.Thevalueswithinatuplecanbeofanytypeanddon’thavetobeofthesametypeaseachother.

Inthisexample,(404,"NotFound")isatuplethatdescribesanHTTPstatuscode.AnHTTPstatuscodeisaspecialvaluereturnedbyawebserverwheneveryourequestawebpage.Astatuscodeof404NotFoundisreturnedifyourequestawebpagethatdoesn’texist.

1 lethttp404Error=(404,"NotFound")

2 //http404Errorisoftype(Int,String),andequals(404,"Not

Found")

The(404,"NotFound")tuplegroupstogetheranIntandaStringtogivetheHTTPstatuscodetwoseparatevalues:anumberandahuman-readabledescription.Itcanbedescribedas“atupleoftype(Int,String)”.

Youcancreatetuplesfromanypermutationoftypes,andtheycancontainasmanydifferenttypesasyoulike.There’snothingstoppingyoufromhavingatupleoftype(Int,Int,Int),or(String,Bool),orindeedanyotherpermutationyourequire.

Youcandecomposeatuple’scontentsintoseparateconstantsorvariables,whichyouthenaccessasusual:

1 let(statusCode,statusMessage)=http404Error

2 print("Thestatuscodeis\(statusCode)")

3 //Prints"Thestatuscodeis404"

4 print("Thestatusmessageis\(statusMessage)")

5 //Prints"ThestatusmessageisNotFound"

Ifyouonlyneedsomeofthetuple’svalues,ignorepartsofthetuplewithanunderscore(_)whenyoudecomposethetuple:

1 let(justTheStatusCode,_)=http404Error

2 print("Thestatuscodeis\(justTheStatusCode)")

3 //Prints"Thestatuscodeis404"

Alternatively,accesstheindividualelementvaluesinatupleusingindexnumbersstartingatzero:

1 print("Thestatuscodeis\(http404Error.0)")

2 //Prints"Thestatuscodeis404"

3 print("Thestatusmessageis\(http404Error.1)")

4 //Prints"ThestatusmessageisNotFound"

Youcannametheindividualelementsinatuplewhenthetupleisdefined:

lethttp200Status=(statusCode:200,description:"OK")

Ifyounametheelementsinatuple,youcanusetheelementnamestoaccessthevaluesofthoseelements:

1 print("Thestatuscodeis\(http200Status.statusCode)")

2 //Prints"Thestatuscodeis200"

3 print("Thestatusmessageis\(http200Status.description)")

4 //Prints"ThestatusmessageisOK"

Tuplesareparticularlyusefulasthereturnvaluesoffunctions.Afunctionthattriestoretrieveawebpagemightreturnthe(Int,String)tupletypetodescribethesuccessorfailureofthepageretrieval.Byreturningatuplewithtwodistinctvalues,eachofadifferenttype,thefunctionprovidesmoreusefulinformationaboutitsoutcomethanifitcouldonlyreturnasinglevalueofasingletype.For

moreinformation,seeFunctionswithMultipleReturnValues.

NOTE

Tuplesareusefulforsimplegroupsofrelatedvalues.They’renotsuitedtothecreationofcomplexdatastructures.Ifyourdatastructureislikelytobemorecomplex,modelitasaclassorstructure,ratherthanasatuple.Formoreinformation,seeStructuresandClasses.

Optionals

Youuseoptionalsinsituationswhereavaluemaybeabsent.Anoptionalrepresentstwopossibilities:Eitherthereisavalue,andyoucanunwraptheoptionaltoaccessthatvalue,orthereisn’tavalueatall.

NOTE

Theconceptofoptionalsdoesn’texistinCorObjective-C.ThenearestthinginObjective-Cistheabilitytoreturnnilfromamethodthatwouldotherwisereturnanobject,withnilmeaning“theabsenceofavalidobject.”However,thisonlyworksforobjects—itdoesn’tworkforstructures,basicCtypes,orenumerationvalues.Forthesetypes,Objective-Cmethodstypicallyreturnaspecialvalue(suchasNSNotFound)toindicatetheabsenceofavalue.Thisapproachassumesthatthemethod’scallerknowsthere’saspecialvaluetotestagainstandrememberstocheckforit.Swift’soptionalsletyouindicatetheabsenceofavalueforanytypeatall,withouttheneedforspecialconstants.

Here’sanexampleofhowoptionalscanbeusedtocopewiththeabsenceofavalue.Swift’sInttypehasaninitializerwhichtriestoconvertaStringvalueintoanIntvalue.However,noteverystringcanbeconvertedintoaninteger.Thestring"123"canbeconvertedintothenumericvalue123,butthestring"hello,world"doesn’thaveanobviousnumericvaluetoconvertto.

TheexamplebelowusestheinitializertotrytoconvertaStringintoanInt:

1 letpossibleNumber="123"

2 letconvertedNumber=Int(possibleNumber)

3 //convertedNumberisinferredtobeoftype"Int?",or

"optionalInt"

Becausetheinitializermightfail,itreturnsanoptionalInt,ratherthananInt.AnoptionalIntiswrittenasInt?,notInt.Thequestionmarkindicatesthatthevalueitcontainsisoptional,meaningthatitmightcontainsomeIntvalue,oritmightcontainnovalueatall.(Itcan’tcontainanythingelse,suchasaBoolvalueoraStringvalue.It’seitheranInt,orit’snothingatall.)

nilYousetanoptionalvariabletoavaluelessstatebyassigningitthespecialvaluenil:

1 varserverResponseCode:Int?=404

2 //serverResponseCodecontainsanactualIntvalueof404

3 serverResponseCode=nil

4 //serverResponseCodenowcontainsnovalue

NOTE

Youcan’tusenilwithnon-optionalconstantsandvariables.Ifaconstantorvariableinyourcodeneedstoworkwiththeabsenceofavalueundercertainconditions,alwaysdeclareitasanoptionalvalueoftheappropriatetype.

Ifyoudefineanoptionalvariablewithoutprovidingadefaultvalue,thevariableisautomaticallysettonilforyou:

1 varsurveyAnswer:String?

2 //surveyAnswerisautomaticallysettonil

NOTE

Swift’snilisn’tthesameasnilinObjective-C.InObjective-C,nilisapointertoanonexistentobject.InSwift,nilisn’tapointer—it’stheabsenceofavalueofacertaintype.Optionalsofanytypecanbesettonil,notjustobjecttypes.

IfStatementsandForcedUnwrappingYoucanuseanifstatementtofindoutwhetheranoptionalcontainsavaluebycomparingtheoptionalagainstnil.Youperformthiscomparisonwiththe“equalto”operator(==)orthe“notequalto”operator(!=).

Ifanoptionalhasavalue,it’sconsideredtobe“notequalto”nil:

1 ifconvertedNumber!=nil{

2 print("convertedNumbercontainssomeintegervalue.")

3 }

4 //Prints"convertedNumbercontainssomeintegervalue."

Onceyou’resurethattheoptionaldoescontainavalue,youcanaccessitsunderlyingvaluebyaddinganexclamationmark(!)totheendoftheoptional’sname.Theexclamationmarkeffectivelysays,“Iknowthatthisoptionaldefinitelyhasavalue;pleaseuseit.”Thisisknownasforcedunwrappingoftheoptional’svalue:

1 ifconvertedNumber!=nil{

2 print("convertedNumberhasanintegervalueof\

(convertedNumber!).")

3 }

4 //Prints"convertedNumberhasanintegervalueof123."

Formoreabouttheifstatement,seeControlFlow.

NOTE

Tryingtouse!toaccessanonexistentoptionalvaluetriggersaruntimeerror.Alwaysmakesurethatanoptionalcontainsanon-nilvaluebeforeusing!toforce-unwrapitsvalue.

OptionalBindingYouuseoptionalbindingtofindoutwhetheranoptionalcontainsavalue,andif

so,tomakethatvalueavailableasatemporaryconstantorvariable.Optionalbindingcanbeusedwithifandwhilestatementstocheckforavalueinsideanoptional,andtoextractthatvalueintoaconstantorvariable,aspartofasingleaction.ifandwhilestatementsaredescribedinmoredetailinControlFlow.

Writeanoptionalbindingforanifstatementasfollows:

iflet constantName = someOptional {

statements

}

YoucanrewritethepossibleNumberexamplefromtheOptionalssectiontouseoptionalbindingratherthanforcedunwrapping:

1 ifletactualNumber=Int(possibleNumber){

2 print("Thestring\"\(possibleNumber)\"hasaninteger

valueof\(actualNumber)")

3 }else{

4 print("Thestring\"\(possibleNumber)\"couldnotbe

convertedtoaninteger")

5 }

6 //Prints"Thestring"123"hasanintegervalueof123"

Thiscodecanbereadas:

“IftheoptionalIntreturnedbyInt(possibleNumber)containsavalue,setanewconstantcalledactualNumbertothevaluecontainedintheoptional.”

Iftheconversionissuccessful,theactualNumberconstantbecomesavailableforusewithinthefirstbranchoftheifstatement.Ithasalreadybeeninitializedwiththevaluecontainedwithintheoptional,andsothere’snoneedtousethe!suffixtoaccessitsvalue.Inthisexample,actualNumberissimplyusedtoprinttheresultoftheconversion.

Youcanusebothconstantsandvariableswithoptionalbinding.Ifyouwantedto

manipulatethevalueofactualNumberwithinthefirstbranchoftheifstatement,youcouldwriteifvaractualNumberinstead,andthevaluecontainedwithintheoptionalwouldbemadeavailableasavariableratherthanaconstant.

YoucanincludeasmanyoptionalbindingsandBooleanconditionsinasingleifstatementasyouneedto,separatedbycommas.IfanyofthevaluesintheoptionalbindingsareniloranyBooleanconditionevaluatestofalse,thewholeifstatement’sconditionisconsideredtobefalse.Thefollowingifstatementsareequivalent:

1 ifletfirstNumber=Int("4"),letsecondNumber=Int("42"),

firstNumber<secondNumber&&secondNumber<100{

2 print("\(firstNumber)<\(secondNumber)<100")

3 }

4 //Prints"4<42<100"

5

6 ifletfirstNumber=Int("4"){

7 ifletsecondNumber=Int("42"){

8 iffirstNumber<secondNumber&&secondNumber<100{

9 print("\(firstNumber)<\(secondNumber)<100")

10 }

11 }

12 }

13 //Prints"4<42<100"

NOTE

Constantsandvariablescreatedwithoptionalbindinginanifstatementareavailableonlywithinthebodyoftheifstatement.Incontrast,theconstantsandvariablescreatedwithaguardstatementareavailableinthelinesofcodethatfollowtheguardstatement,asdescribedinEarlyExit.

ImplicitlyUnwrappedOptionals

Asdescribedabove,optionalsindicatethataconstantorvariableisallowedtohave“novalue”.Optionalscanbecheckedwithanifstatementtoseeifavalueexists,andcanbeconditionallyunwrappedwithoptionalbindingtoaccesstheoptional’svalueifitdoesexist.

Sometimesit’sclearfromaprogram’sstructurethatanoptionalwillalwayshaveavalue,afterthatvalueisfirstset.Inthesecases,it’susefultoremovetheneedtocheckandunwraptheoptional’svalueeverytimeit’saccessed,becauseitcanbesafelyassumedtohaveavalueallofthetime.

Thesekindsofoptionalsaredefinedasimplicitlyunwrappedoptionals.Youwriteanimplicitlyunwrappedoptionalbyplacinganexclamationmark(String!)ratherthanaquestionmark(String?)afterthetypethatyouwanttomakeoptional.

Implicitlyunwrappedoptionalsareusefulwhenanoptional’svalueisconfirmedtoexistimmediatelyaftertheoptionalisfirstdefinedandcandefinitelybeassumedtoexistateverypointthereafter.TheprimaryuseofimplicitlyunwrappedoptionalsinSwiftisduringclassinitialization,asdescribedinUnownedReferencesandImplicitlyUnwrappedOptionalProperties.

Animplicitlyunwrappedoptionalisanormaloptionalbehindthescenes,butcanalsobeusedlikeanon-optionalvalue,withouttheneedtounwraptheoptionalvalueeachtimeit’saccessed.ThefollowingexampleshowsthedifferenceinbehaviorbetweenanoptionalstringandanimplicitlyunwrappedoptionalstringwhenaccessingtheirwrappedvalueasanexplicitString:

1 letpossibleString:String?="Anoptionalstring."

2 letforcedString:String=possibleString!//requiresan

exclamationmark

3

4 letassumedString:String!="Animplicitlyunwrappedoptional

string."

5 letimplicitString:String=assumedString//noneedforan

exclamationmark

Youcanthinkofanimplicitlyunwrappedoptionalasgivingpermissionfortheoptionaltobeunwrappedautomaticallywheneverit’sused.Ratherthanplacinganexclamationmarkaftertheoptional’snameeachtimeyouuseit,youplaceanexclamationmarkaftertheoptional’stypewhenyoudeclareit.

NOTE

Ifanimplicitlyunwrappedoptionalisnilandyoutrytoaccessitswrappedvalue,you’lltriggeraruntimeerror.Theresultisexactlythesameasifyouplaceanexclamationmarkafteranormaloptionalthatdoesn’tcontainavalue.

Youcanstilltreatanimplicitlyunwrappedoptionallikeanormaloptional,tocheckifitcontainsavalue:

1 ifassumedString!=nil{

2 print(assumedString!)

3 }

4 //Prints"Animplicitlyunwrappedoptionalstring."

Youcanalsouseanimplicitlyunwrappedoptionalwithoptionalbinding,tocheckandunwrapitsvalueinasinglestatement:

1 ifletdefiniteString=assumedString{

2 print(definiteString)

3 }

4 //Prints"Animplicitlyunwrappedoptionalstring."

NOTE

Don’tuseanimplicitlyunwrappedoptionalwhenthere’sapossibilityofavariablebecomingnilatalaterpoint.Alwaysuseanormaloptionaltypeifyouneedtocheckforanilvalueduringthelifetimeofavariable.

ErrorHandling

Youuseerrorhandlingtorespondtoerrorconditionsyourprogrammayencounterduringexecution.

Incontrasttooptionals,whichcanusethepresenceorabsenceofavaluetocommunicatesuccessorfailureofafunction,errorhandlingallowsyoutodeterminetheunderlyingcauseoffailure,and,ifnecessary,propagatetheerrortoanotherpartofyourprogram.

Whenafunctionencountersanerrorcondition,itthrowsanerror.Thatfunction’scallercanthencatchtheerrorandrespondappropriately.

1 funccanThrowAnError()throws{

2 //thisfunctionmayormaynotthrowanerror

3 }

Afunctionindicatesthatitcanthrowanerrorbyincludingthethrowskeywordinitsdeclaration.Whenyoucallafunctionthatcanthrowanerror,youprependthetrykeywordtotheexpression.

Swiftautomaticallypropagateserrorsoutoftheircurrentscopeuntilthey’rehandledbyacatchclause.

1 do{

2 trycanThrowAnError()

3 //noerrorwasthrown

4 }catch{

5 //anerrorwasthrown

6 }

Adostatementcreatesanewcontainingscope,whichallowserrorstobepropagatedtooneormorecatchclauses.

Here’sanexampleofhowerrorhandlingcanbeusedtorespondtodifferenterrorconditions:

1 funcmakeASandwich()throws{

2 //...

3 }

4

5 do{

6 trymakeASandwich()

7 eatASandwich()

8 }catchSandwichError.outOfCleanDishes{

9 washDishes()

10 }catchSandwichError.missingIngredients(letingredients){

11 buyGroceries(ingredients)

12 }

Inthisexample,themakeASandwich()functionwillthrowanerrorifnocleandishesareavailableorifanyingredientsaremissing.BecausemakeASandwich()canthrowanerror,thefunctioncalliswrappedinatryexpression.Bywrappingthefunctioncallinadostatement,anyerrorsthatarethrownwillbepropagatedtotheprovidedcatchclauses.

Ifnoerroristhrown,theeatASandwich()functioniscalled.IfanerroristhrownanditmatchestheSandwichError.outOfCleanDishescase,thenthewashDishes()functionwillbecalled.IfanerroristhrownanditmatchestheSandwichError.missingIngredientscase,thenthebuyGroceries(_:)functioniscalledwiththeassociated[String]valuecapturedbythecatchpattern.

Throwing,catching,andpropagatingerrorsiscoveredingreaterdetailinErrorHandling.

AssertionsandPreconditions

Assertionsandpreconditionsarechecksthathappenatruntime.Youusethemtomakesureanessentialconditionissatisfiedbeforeexecutinganyfurthercode.If

theBooleanconditionintheassertionorpreconditionevaluatestotrue,codeexecutioncontinuesasusual.Iftheconditionevaluatestofalse,thecurrentstateoftheprogramisinvalid;codeexecutionends,andyourappisterminated.

Youuseassertionsandpreconditionstoexpresstheassumptionsyoumakeandtheexpectationsyouhavewhilecoding,soyoucanincludethemaspartofyourcode.Assertionshelpyoufindmistakesandincorrectassumptionsduringdevelopment,andpreconditionshelpyoudetectissuesinproduction.

Inadditiontoverifyingyourexpectationsatruntime,assertionsandpreconditionsalsobecomeausefulformofdocumentationwithinthecode.UnliketheerrorconditionsdiscussedinErrorHandlingabove,assertionsandpreconditionsaren’tusedforrecoverableorexpectederrors.Becauseafailedassertionorpreconditionindicatesaninvalidprogramstate,there’snowaytocatchafailedassertion.

Usingassertionsandpreconditionsisn’tasubstitutefordesigningyourcodeinsuchawaythatinvalidconditionsareunlikelytoarise.However,usingthemtoenforcevaliddataandstatecausesyourapptoterminatemorepredictablyifaninvalidstateoccurs,andhelpsmaketheproblemeasiertodebug.Stoppingexecutionassoonasaninvalidstateisdetectedalsohelpslimitthedamagecausedbythatinvalidstate.

Thedifferencebetweenassertionsandpreconditionsisinwhenthey’rechecked:Assertionsarecheckedonlyindebugbuilds,butpreconditionsarecheckedinbothdebugandproductionbuilds.Inproductionbuilds,theconditioninsideanassertionisn’tevaluated.Thismeansyoucanuseasmanyassertionsasyouwantduringyourdevelopmentprocess,withoutimpactingperformanceinproduction.

DebuggingwithAssertionsYouwriteanassertionbycallingtheassert(_:_:file:line:)functionfromtheSwiftstandardlibrary.Youpassthisfunctionanexpressionthatevaluatestotrueorfalseandamessagetodisplayiftheresultoftheconditionisfalse.Forexample:

1 letage=-3

2 assert(age>=0,"Aperson'sagecan'tbelessthanzero.")

3 //Thisassertionfailsbecause-3isnot>=0.

Inthisexample,codeexecutioncontinuesifage>=0evaluatestotrue,thatis,ifthevalueofageisnonnegative.Ifthevalueofageisnegative,asinthecodeabove,thenage>=0evaluatestofalse,andtheassertionfails,terminatingtheapplication.

Youcanomittheassertionmessage—forexample,whenitwouldjustrepeattheconditionasprose.

assert(age>=0)

Ifthecodealreadychecksthecondition,youusetheassertionFailure(_:file:line:)functiontoindicatethatanassertionhasfailed.Forexample:

1 ifage>10{

2 print("Youcanridetheroller-coasterortheferris

wheel.")

3 }elseifage>=0{

4 print("Youcanridetheferriswheel.")

5 }else{

6 assertionFailure("Aperson'sagecan'tbelessthanzero.")

7 }

EnforcingPreconditionsUseapreconditionwheneveraconditionhasthepotentialtobefalse,butmustdefinitelybetrueforyourcodetocontinueexecution.Forexample,useapreconditiontocheckthatasubscriptisnotoutofbounds,ortocheckthatafunctionhasbeenpassedavalidvalue.

Youwriteapreconditionbycallingtheprecondition(_:_:file:line:)function.

Youpassthisfunctionanexpressionthatevaluatestotrueorfalseandamessagetodisplayiftheresultoftheconditionisfalse.Forexample:

1 //Intheimplementationofasubscript...

2 precondition(index>0,"Indexmustbegreaterthanzero.")

YoucanalsocallthepreconditionFailure(_:file:line:)functiontoindicatethatafailurehasoccurred—forexample,ifthedefaultcaseofaswitchwastaken,butallvalidinputdatashouldhavebeenhandledbyoneoftheswitch’sothercases.

NOTE

Ifyoucompileinuncheckedmode(-Ounchecked),preconditionsaren’tchecked.Thecompilerassumesthatpreconditionsarealwaystrue,anditoptimizesyourcodeaccordingly.However,thefatalError(_:file:line:)functionalwayshaltsexecution,regardlessofoptimizationsettings.

YoucanusethefatalError(_:file:line:)functionduringprototypingandearlydevelopmenttocreatestubsforfunctionalitythathasn’tbeenimplementedyet,bywritingfatalError("Unimplemented")asthestubimplementation.Becausefatalerrorsareneveroptimizedout,unlikeassertionsorpreconditions,youcanbesurethatexecutionalwayshaltsifitencountersastubimplementation.

BasicOperators

Anoperatorisaspecialsymbolorphrasethatyouusetocheck,change,orcombinevalues.Forexample,theadditionoperator(+)addstwonumbers,asinleti=1+2,andthelogicalANDoperator(&&)combinestwoBooleanvalues,asinifenteredDoorCode&&passedRetinaScan.

SwiftsupportsmoststandardCoperatorsandimprovesseveralcapabilitiestoeliminatecommoncodingerrors.Theassignmentoperator(=)doesn’treturnavalue,topreventitfrombeingmistakenlyusedwhentheequaltooperator(==)isintended.Arithmeticoperators(+,-,*,/,%andsoforth)detectanddisallowvalueoverflow,toavoidunexpectedresultswhenworkingwithnumbersthatbecomelargerorsmallerthantheallowedvaluerangeofthetypethatstoresthem.YoucanoptintovalueoverflowbehaviorbyusingSwift’soverflowoperators,asdescribedinOverflowOperators.

Swiftalsoprovidesrangeoperatorsthataren’tfoundinC,suchasa..<banda...b,asashortcutforexpressingarangeofvalues.

ThischapterdescribesthecommonoperatorsinSwift.AdvancedOperatorscoversSwift’sadvancedoperators,anddescribeshowtodefineyourowncustomoperatorsandimplementthestandardoperatorsforyourowncustomtypes.

Terminology

Operatorsareunary,binary,orternary:

Unaryoperatorsoperateonasingletarget(suchas-a).Unaryprefixoperatorsappearimmediatelybeforetheirtarget(suchas!b),andunarypostfixoperatorsappearimmediatelyaftertheirtarget(suchasc!).

Binaryoperatorsoperateontwotargets(suchas2+3)andareinfixbecausetheyappearinbetweentheirtwotargets.

Ternaryoperatorsoperateonthreetargets.LikeC,Swifthasonlyoneternaryoperator,theternaryconditionaloperator(a?b:c).

Thevaluesthatoperatorsaffectareoperands.Intheexpression1+2,the+symbolisabinaryoperatoranditstwooperandsarethevalues1and2.

AssignmentOperator

Theassignmentoperator(a=b)initializesorupdatesthevalueofawiththevalueofb:

1 letb=10

2 vara=5

3 a=b

4 //aisnowequalto10

Iftherightsideoftheassignmentisatuplewithmultiplevalues,itselementscanbedecomposedintomultipleconstantsorvariablesatonce:

1 let(x,y)=(1,2)

2 //xisequalto1,andyisequalto2

UnliketheassignmentoperatorinCandObjective-C,theassignmentoperatorinSwiftdoesnotitselfreturnavalue.Thefollowingstatementisnotvalid:

1 ifx=y{

2 //Thisisnotvalid,becausex=ydoesnotreturna

value.

3 }

Thisfeaturepreventstheassignmentoperator(=)frombeingusedbyaccidentwhentheequaltooperator(==)isactuallyintended.Bymakingifx=yinvalid,Swifthelpsyoutoavoidthesekindsoferrorsinyourcode.

ArithmeticOperators

Swiftsupportsthefourstandardarithmeticoperatorsforallnumbertypes:

Addition(+)

Subtraction(-)

Multiplication(*)

Division(/)

1 1+2//equals3

2 5-3//equals2

3 2*3//equals6

4 10.0/2.5//equals4.0

UnlikethearithmeticoperatorsinCandObjective-C,theSwiftarithmeticoperatorsdon’tallowvaluestooverflowbydefault.YoucanoptintovalueoverflowbehaviorbyusingSwift’soverflowoperators(suchasa&+b).SeeOverflowOperators.

TheadditionoperatorisalsosupportedforStringconcatenation:

"hello,"+"world"//equals"hello,world"

RemainderOperatorTheremainderoperator(a%b)worksouthowmanymultiplesofbwillfitinsideaandreturnsthevaluethatisleftover(knownastheremainder).

NOTE

Theremainderoperator(%)isalsoknownasamodulooperatorinotherlanguages.However,itsbehaviorinSwiftfornegativenumbersmeansthat,strictlyspeaking,it’saremainderratherthanamodulooperation.

Here’showtheremainderoperatorworks.Tocalculate9%4,youfirstworkouthowmany4swillfitinside9:

Youcanfittwo4sinside9,andtheremainderis1(showninorange).

InSwift,thiswouldbewrittenas:

9%4//equals1

Todeterminetheanswerfora%b,the%operatorcalculatesthefollowingequationandreturnsremainderasitsoutput:

a=(bxsomemultiplier)+remainder

wheresomemultiplieristhelargestnumberofmultiplesofbthatwillfitinsidea.

Inserting9and4intothisequationyields:

9=(4x2)+1

Thesamemethodisappliedwhencalculatingtheremainderforanegativevalueofa:

-9%4//equals-1

Inserting-9and4intotheequationyields:

-9=(4x-2)+-1

givingaremaindervalueof-1.

Thesignofbisignoredfornegativevaluesofb.Thismeansthata%banda%-balwaysgivethesameanswer.

UnaryMinusOperatorThesignofanumericvaluecanbetoggledusingaprefixed-,knownastheunaryminusoperator:

1 letthree=3

2 letminusThree=-three//minusThreeequals-3

3 letplusThree=-minusThree//plusThreeequals3,or"minus

minusthree"

Theunaryminusoperator(-)isprependeddirectlybeforethevalueitoperateson,withoutanywhitespace.

UnaryPlusOperatorTheunaryplusoperator(+)simplyreturnsthevalueitoperateson,withoutanychange:

1 letminusSix=-6

2 letalsoMinusSix=+minusSix//alsoMinusSixequals-6

Althoughtheunaryplusoperatordoesn’tactuallydoanything,youcanuseittoprovidesymmetryinyourcodeforpositivenumberswhenalsousingtheunaryminusoperatorfornegativenumbers.

CompoundAssignmentOperators

LikeC,Swiftprovidescompoundassignmentoperatorsthatcombineassignment(=)withanotheroperation.Oneexampleistheadditionassignmentoperator(+=):

1 vara=1

2 a+=2

3 //aisnowequalto3

Theexpressiona+=2isshorthandfora=a+2.Effectively,theadditionandtheassignmentarecombinedintooneoperatorthatperformsbothtasksatthesametime.

NOTE

Thecompoundassignmentoperatorsdon’treturnavalue.Forexample,youcan’twriteletb=a+=2.

ForinformationabouttheoperatorsprovidedbytheSwiftstandardlibrary,seeOperatorDeclarations.

ComparisonOperators

SwiftsupportsallstandardCcomparisonoperators:

Equalto(a==b)

Notequalto(a!=b)

Greaterthan(a>b)

Lessthan(a<b)

Greaterthanorequalto(a>=b)

Lessthanorequalto(a<=b)

NOTE

Swiftalsoprovidestwoidentityoperators(===and!==),whichyouusetotestwhethertwoobjectreferencesbothrefertothesameobjectinstance.Formoreinformation,seeIdentityOperators.

EachofthecomparisonoperatorsreturnsaBoolvaluetoindicatewhetherornotthestatementistrue:

1 1==1//truebecause1isequalto1

2 2!=1//truebecause2isnotequalto1

3 2>1//truebecause2isgreaterthan1

4 1<2//truebecause1islessthan2

5 1>=1//truebecause1isgreaterthanorequalto1

6 2<=1//falsebecause2isnotlessthanorequalto1

Comparisonoperatorsareoftenusedinconditionalstatements,suchastheifstatement:

1 letname="world"

2 ifname=="world"{

3 print("hello,world")

4 }else{

5 print("I'msorry\(name),butIdon'trecognizeyou")

6 }

7 //Prints"hello,world",becausenameisindeedequalto

"world".

Formoreabouttheifstatement,seeControlFlow.

Youcancomparetwotuplesiftheyhavethesametypeandthesamenumberofvalues.Tuplesarecomparedfromlefttoright,onevalueatatime,untilthecomparisonfindstwovaluesthataren’tequal.Thosetwovaluesarecompared,andtheresultofthatcomparisondeterminestheoverallresultofthetuplecomparison.Ifalltheelementsareequal,thenthetuplesthemselvesareequal.Forexample:

1 (1,"zebra")<(2,"apple")//truebecause1islessthan2;

"zebra"and"apple"arenotcompared

2 (3,"apple")<(3,"bird")//truebecause3isequalto3,

and"apple"islessthan"bird"

3 (4,"dog")==(4,"dog")//truebecause4isequalto4,

and"dog"isequalto"dog"

Intheexampleabove,youcanseetheleft-to-rightcomparisonbehavioronthefirstline.Because1islessthan2,(1,"zebra")isconsideredlessthan(2,"apple"),regardlessofanyothervaluesinthetuples.Itdoesn’tmatterthat"zebra"isn’tlessthan"apple",becausethecomparisonisalreadydeterminedbythetuples’firstelements.However,whenthetuples’firstelementsarethesame,theirsecondelementsarecompared—thisiswhathappensonthesecondandthirdline.

Tuplescanbecomparedwithagivenoperatoronlyiftheoperatorcanbeappliedtoeachvalueintherespectivetuples.Forexample,asdemonstratedinthecodebelow,youcancomparetwotuplesoftype(String,Int)becausebothStringandIntvaluescanbecomparedusingthe<operator.Incontrast,twotuplesoftype(String,Bool)can’tbecomparedwiththe<operatorbecausethe<operatorcan’tbeappliedtoBoolvalues.

1 ("blue",-1)<("purple",1)//OK,evaluatestotrue

2 ("blue",false)<("purple",true)//Errorbecause<can't

compareBooleanvalues

NOTE

TheSwiftstandardlibraryincludestuplecomparisonoperatorsfortupleswithfewerthansevenelements.Tocomparetupleswithsevenormoreelements,youmustimplementthecomparisonoperatorsyourself.

TernaryConditionalOperator

Theternaryconditionaloperatorisaspecialoperatorwiththreeparts,whichtakestheformquestion?answer1:answer2.It’sashortcutforevaluatingoneoftwoexpressionsbasedonwhetherquestionistrueorfalse.Ifquestionistrue,itevaluatesanswer1andreturnsitsvalue;otherwise,itevaluatesanswer2

andreturnsitsvalue.

Theternaryconditionaloperatorisshorthandforthecodebelow:

1 ifquestion{

2 answer1

3 }else{

4 answer2

5 }

Here’sanexample,whichcalculatestheheightforatablerow.Therowheightshouldbe50pointstallerthanthecontentheightiftherowhasaheader,and20pointstalleriftherowdoesn’thaveaheader:

1 letcontentHeight=40

2 lethasHeader=true

3 letrowHeight=contentHeight+(hasHeader?50:20)

4 //rowHeightisequalto90

Theexampleaboveisshorthandforthecodebelow:

1 letcontentHeight=40

2 lethasHeader=true

3 letrowHeight:Int

4 ifhasHeader{

5 rowHeight=contentHeight+50

6 }else{

7 rowHeight=contentHeight+20

8 }

9 //rowHeightisequalto90

Thefirstexample’suseoftheternaryconditionaloperatormeansthatrowHeightcanbesettothecorrectvalueonasinglelineofcode,whichismoreconcise

thanthecodeusedinthesecondexample.

Theternaryconditionaloperatorprovidesanefficientshorthandfordecidingwhichoftwoexpressionstoconsider.Usetheternaryconditionaloperatorwithcare,however.Itsconcisenesscanleadtohard-to-readcodeifoverused.Avoidcombiningmultipleinstancesoftheternaryconditionaloperatorintoonecompoundstatement.

Nil-CoalescingOperator

Thenil-coalescingoperator(a??b)unwrapsanoptionalaifitcontainsavalue,orreturnsadefaultvaluebifaisnil.Theexpressionaisalwaysofanoptionaltype.Theexpressionbmustmatchthetypethatisstoredinsidea.

Thenil-coalescingoperatorisshorthandforthecodebelow:

a!=nil?a!:b

Thecodeaboveusestheternaryconditionaloperatorandforcedunwrapping(a!)toaccessthevaluewrappedinsideawhenaisnotnil,andtoreturnbotherwise.Thenil-coalescingoperatorprovidesamoreelegantwaytoencapsulatethisconditionalcheckingandunwrappinginaconciseandreadableform.

NOTE

Ifthevalueofaisnon-nil,thevalueofbisnotevaluated.Thisisknownasshort-circuitevaluation.

Theexamplebelowusesthenil-coalescingoperatortochoosebetweenadefaultcolornameandanoptionaluser-definedcolorname:

1 letdefaultColorName="red"

2 varuserDefinedColorName:String?//defaultstonil

3

4 varcolorNameToUse=userDefinedColorName??defaultColorName

5 //userDefinedColorNameisnil,socolorNameToUseissettothe

defaultof"red"

TheuserDefinedColorNamevariableisdefinedasanoptionalString,withadefaultvalueofnil.BecauseuserDefinedColorNameisofanoptionaltype,youcanusethenil-coalescingoperatortoconsideritsvalue.Intheexampleabove,theoperatorisusedtodetermineaninitialvalueforaStringvariablecalledcolorNameToUse.BecauseuserDefinedColorNameisnil,theexpressionuserDefinedColorName??defaultColorNamereturnsthevalueofdefaultColorName,or"red".

Ifyouassignanon-nilvaluetouserDefinedColorNameandperformthenil-coalescingoperatorcheckagain,thevaluewrappedinsideuserDefinedColorNameisusedinsteadofthedefault:

1 userDefinedColorName="green"

2 colorNameToUse=userDefinedColorName??defaultColorName

3 //userDefinedColorNameisnotnil,socolorNameToUseissetto

"green"

RangeOperators

Swiftincludesseveralrangeoperators,whichareshortcutsforexpressingarangeofvalues.

ClosedRangeOperatorTheclosedrangeoperator(a...b)definesarangethatrunsfromatob,andincludesthevaluesaandb.Thevalueofamustnotbegreaterthanb.

Theclosedrangeoperatorisusefulwheniteratingoverarangeinwhichyou

wantallofthevaluestobeused,suchaswithafor-inloop:

1 forindexin1...5{

2 print("\(index)times5is\(index*5)")

3 }

4 //1times5is5

5 //2times5is10

6 //3times5is15

7 //4times5is20

8 //5times5is25

Formoreaboutfor-inloops,seeControlFlow.

Half-OpenRangeOperatorThehalf-openrangeoperator(a..<b)definesarangethatrunsfromatob,butdoesn’tincludeb.It’ssaidtobehalf-openbecauseitcontainsitsfirstvalue,butnotitsfinalvalue.Aswiththeclosedrangeoperator,thevalueofamustnotbegreaterthanb.Ifthevalueofaisequaltob,thentheresultingrangewillbeempty.

Half-openrangesareparticularlyusefulwhenyouworkwithzero-basedlistssuchasarrays,whereit’susefultocountupto(butnotincluding)thelengthofthelist:

1 letnames=["Anna","Alex","Brian","Jack"]

2 letcount=names.count

3 foriin0..<count{

4 print("Person\(i+1)iscalled\(names[i])")

5 }

6 //Person1iscalledAnna

7 //Person2iscalledAlex

8 //Person3iscalledBrian

9 //Person4iscalledJack

Notethatthearraycontainsfouritems,but0..<countonlycountsasfaras3(theindexofthelastiteminthearray),becauseit’sahalf-openrange.Formoreaboutarrays,seeArrays.

One-SidedRangesTheclosedrangeoperatorhasanalternativeformforrangesthatcontinueasfaraspossibleinonedirection—forexample,arangethatincludesalltheelementsofanarrayfromindex2totheendofthearray.Inthesecases,youcanomitthevaluefromonesideoftherangeoperator.Thiskindofrangeiscalledaone-sidedrangebecausetheoperatorhasavalueononlyoneside.Forexample:

1 fornameinnames[2...]{

2 print(name)

3 }

4 //Brian

5 //Jack

6

7 fornameinnames[...2]{

8 print(name)

9 }

10 //Anna

11 //Alex

12 //Brian

Thehalf-openrangeoperatoralsohasaone-sidedformthat’swrittenwithonlyitsfinalvalue.Justlikewhenyouincludeavalueonbothsides,thefinalvalueisn’tpartoftherange.Forexample:

1 fornameinnames[..<2]{

2 print(name)

3 }

4 //Anna

5 //Alex

One-sidedrangescanbeusedinothercontexts,notjustinsubscripts.Youcan’titerateoveraone-sidedrangethatomitsafirstvalue,becauseitisn’tclearwhereiterationshouldbegin.Youcaniterateoveraone-sidedrangethatomitsitsfinalvalue;however,becausetherangecontinuesindefinitely,makesureyouaddanexplicitendconditionfortheloop.Youcanalsocheckwhetheraone-sidedrangecontainsaparticularvalue,asshowninthecodebelow.

1 letrange=...5

2 range.contains(7)//false

3 range.contains(4)//true

4 range.contains(-1)//true

LogicalOperators

LogicaloperatorsmodifyorcombinetheBooleanlogicvaluestrueandfalse.SwiftsupportsthethreestandardlogicaloperatorsfoundinC-basedlanguages:

LogicalNOT(!a)

LogicalAND(a&&b)

LogicalOR(a||b)

LogicalNOTOperatorThelogicalNOToperator(!a)invertsaBooleanvaluesothattruebecomes

false,andfalsebecomestrue.

ThelogicalNOToperatorisaprefixoperator,andappearsimmediatelybeforethevalueitoperateson,withoutanywhitespace.Itcanbereadas“nota”,asseeninthefollowingexample:

1 letallowedEntry=false

2 if!allowedEntry{

3 print("ACCESSDENIED")

4 }

5 //Prints"ACCESSDENIED"

Thephraseif!allowedEntrycanbereadas“ifnotallowedentry.”Thesubsequentlineisonlyexecutedif“notallowedentry”istrue;thatis,ifallowedEntryisfalse.

Asinthisexample,carefulchoiceofBooleanconstantandvariablenamescanhelptokeepcodereadableandconcise,whileavoidingdoublenegativesorconfusinglogicstatements.

LogicalANDOperatorThelogicalANDoperator(a&&b)createslogicalexpressionswherebothvaluesmustbetruefortheoverallexpressiontoalsobetrue.

Ifeithervalueisfalse,theoverallexpressionwillalsobefalse.Infact,ifthefirstvalueisfalse,thesecondvaluewon’tevenbeevaluated,becauseitcan’tpossiblymaketheoverallexpressionequatetotrue.Thisisknownasshort-circuitevaluation.

ThisexampleconsiderstwoBoolvaluesandonlyallowsaccessifbothvaluesaretrue:

1 letenteredDoorCode=true

2 letpassedRetinaScan=false

3 ifenteredDoorCode&&passedRetinaScan{

4 print("Welcome!")

5 }else{

6 print("ACCESSDENIED")

7 }

8 //Prints"ACCESSDENIED"

LogicalOROperatorThelogicalORoperator(a||b)isaninfixoperatormadefromtwoadjacentpipecharacters.Youuseittocreatelogicalexpressionsinwhichonlyoneofthetwovalueshastobetruefortheoverallexpressiontobetrue.

LiketheLogicalANDoperatorabove,theLogicalORoperatorusesshort-circuitevaluationtoconsideritsexpressions.IftheleftsideofaLogicalORexpressionistrue,therightsideisnotevaluated,becauseitcan’tchangetheoutcomeoftheoverallexpression.

Intheexamplebelow,thefirstBoolvalue(hasDoorKey)isfalse,butthesecondvalue(knowsOverridePassword)istrue.Becauseonevalueistrue,theoverallexpressionalsoevaluatestotrue,andaccessisallowed:

1 lethasDoorKey=false

2 letknowsOverridePassword=true

3 ifhasDoorKey||knowsOverridePassword{

4 print("Welcome!")

5 }else{

6 print("ACCESSDENIED")

7 }

8 //Prints"Welcome!"

CombiningLogicalOperatorsYoucancombinemultiplelogicaloperatorstocreatelongercompoundexpressions:

1 ifenteredDoorCode&&passedRetinaScan||hasDoorKey||

knowsOverridePassword{

2 print("Welcome!")

3 }else{

4 print("ACCESSDENIED")

5 }

6 //Prints"Welcome!"

Thisexampleusesmultiple&&and||operatorstocreatealongercompoundexpression.However,the&&and||operatorsstilloperateononlytwovalues,sothisisactuallythreesmallerexpressionschainedtogether.Theexamplecanbereadas:

Ifwe’veenteredthecorrectdoorcodeandpassedtheretinascan,orifwehaveavaliddoorkey,orifweknowtheemergencyoverridepassword,thenallowaccess.

BasedonthevaluesofenteredDoorCode,passedRetinaScan,andhasDoorKey,thefirsttwosubexpressionsarefalse.However,theemergencyoverridepasswordisknown,sotheoverallcompoundexpressionstillevaluatestotrue.

NOTE

TheSwiftlogicaloperators&&and||areleft-associative,meaningthatcompoundexpressionswithmultiplelogicaloperatorsevaluatetheleftmostsubexpressionfirst.

ExplicitParenthesesIt’ssometimesusefultoincludeparentheseswhenthey’renotstrictlyneeded,tomaketheintentionofacomplexexpressioneasiertoread.Inthedooraccessexampleabove,it’susefultoaddparenthesesaroundthefirstpartofthe

compoundexpressiontomakeitsintentexplicit:

1 if(enteredDoorCode&&passedRetinaScan)||hasDoorKey||

knowsOverridePassword{

2 print("Welcome!")

3 }else{

4 print("ACCESSDENIED")

5 }

6 //Prints"Welcome!"

Theparenthesesmakeitclearthatthefirsttwovaluesareconsideredaspartofaseparatepossiblestateintheoveralllogic.Theoutputofthecompoundexpressiondoesn’tchange,buttheoverallintentionisclearertothereader.Readabilityisalwayspreferredoverbrevity;useparentheseswheretheyhelptomakeyourintentionsclear.

StringsandCharacters

Astringisaseriesofcharacters,suchas"hello,world"or"albatross".SwiftstringsarerepresentedbytheStringtype.ThecontentsofaStringcanbeaccessedinvariousways,includingasacollectionofCharactervalues.

Swift’sStringandCharactertypesprovideafast,Unicode-compliantwaytoworkwithtextinyourcode.Thesyntaxforstringcreationandmanipulationislightweightandreadable,withastringliteralsyntaxthatissimilartoC.Stringconcatenationisassimpleascombiningtwostringswiththe+operator,andstringmutabilityismanagedbychoosingbetweenaconstantoravariable,justlikeanyothervalueinSwift.Youcanalsousestringstoinsertconstants,variables,literals,andexpressionsintolongerstrings,inaprocessknownasstringinterpolation.Thismakesiteasytocreatecustomstringvaluesfordisplay,storage,andprinting.

Despitethissimplicityofsyntax,Swift’sStringtypeisafast,modernstringimplementation.Everystringiscomposedofencoding-independentUnicodecharacters,andprovidessupportforaccessingthosecharactersinvariousUnicoderepresentations.

NOTE

Swift’sStringtypeisbridgedwithFoundation’sNSStringclass.FoundationalsoextendsStringtoexposemethodsdefinedbyNSString.Thismeans,ifyouimportFoundation,youcanaccessthoseNSStringmethodsonStringwithoutcasting.

FormoreinformationaboutusingStringwithFoundationandCocoa,seeBridgingBetweenStringandNSString.

StringLiterals

YoucanincludepredefinedStringvalueswithinyourcodeasstringliterals.Astringliteralisasequenceofcharacterssurroundedbydoublequotationmarks(").

Useastringliteralasaninitialvalueforaconstantorvariable:

letsomeString="Somestringliteralvalue"

NotethatSwiftinfersatypeofStringforthesomeStringconstantbecauseit’sinitializedwithastringliteralvalue.

MultilineStringLiteralsIfyouneedastringthatspansseverallines,useamultilinestringliteral—asequenceofcharacterssurroundedbythreedoublequotationmarks:

1 letquotation="""

2 TheWhiteRabbitputonhisspectacles."WhereshallIbegin,

3 pleaseyourMajesty?"heasked.

4

5 "Beginatthebeginning,"theKingsaidgravely,"andgoon

6 tillyoucometotheend;thenstop."

7 """

Amultilinestringliteralincludesallofthelinesbetweenitsopeningandclosingquotationmarks.Thestringbeginsonthefirstlineaftertheopeningquotationmarks(""")andendsonthelinebeforetheclosingquotationmarks,whichmeansthatneitherofthestringsbelowstartorendwithalinebreak:

1 letsingleLineString="Thesearethesame."

2 letmultilineString="""

3 Thesearethesame.

4 """

Whenyoursourcecodeincludesalinebreakinsideofamultilinestringliteral,thatlinebreakalsoappearsinthestring’svalue.Ifyouwanttouselinebreakstomakeyoursourcecodeeasiertoread,butyoudon’twantthelinebreakstobe

partofthestring’svalue,writeabackslash(\)attheendofthoselines:

1 letsoftWrappedQuotation="""

2 TheWhiteRabbitputonhisspectacles."WhereshallIbegin,

\

3 pleaseyourMajesty?"heasked.

4

5 "Beginatthebeginning,"theKingsaidgravely,"andgoon\

6 tillyoucometotheend;thenstop."

7 """

Tomakeamultilinestringliteralthatbeginsorendswithalinefeed,writeablanklineasthefirstorlastline.Forexample:

1 letlineBreaks="""

2

3 Thisstringstartswithalinebreak.

4 Italsoendswithalinebreak.

5

6 """

Amultilinestringcanbeindentedtomatchthesurroundingcode.Thewhitespacebeforetheclosingquotationmarks(""")tellsSwiftwhatwhitespacetoignorebeforealloftheotherlines.However,ifyouwritewhitespaceatthebeginningofalineinadditiontowhat’sbeforetheclosingquotationmarks,thatwhitespaceisincluded.

Intheexampleabove,eventhoughtheentiremultilinestringliteralisindented,

thefirstandlastlinesinthestringdon’tbeginwithanywhitespace.Themiddlelinehasmoreindentationthantheclosingquotationmarks,soitstartswiththatextrafour-spaceindentation.

SpecialCharactersinStringLiteralsStringliteralscanincludethefollowingspecialcharacters:

Theescapedspecialcharacters\0(nullcharacter),\\(backslash),\t(horizontaltab),\n(linefeed),\r(carriagereturn),\"(doublequotationmark)and\'(singlequotationmark)

AnarbitraryUnicodescalarvalue,writtenas\u{n},wherenisa1–8digithexadecimalnumber(UnicodeisdiscussedinUnicodebelow)

Thecodebelowshowsfourexamplesofthesespecialcharacters.ThewiseWordsconstantcontainstwoescapeddoublequotationmarks.ThedollarSign,blackHeart,andsparklingHeartconstantsdemonstratetheUnicodescalarformat:

1 letwiseWords="\"Imaginationismoreimportantthan

knowledge\"-Einstein"

2 //"Imaginationismoreimportantthanknowledge"-Einstein

3 letdollarSign="\u{24}"//$,UnicodescalarU+0024

4 letblackHeart="\u{2665}"//♥,UnicodescalarU+2665

5 letsparklingHeart="\u{1F496}"//,UnicodescalarU+1F496

Becausemultilinestringliteralsusethreedoublequotationmarksinsteadofjustone,youcanincludeadoublequotationmark(")insideofamultilinestringliteralwithoutescapingit.Toincludethetext"""inamultilinestring,escapeatleastoneofthequotationmarks.Forexample:

1 letthreeDoubleQuotationMarks="""

2 Escapingthefirstquotationmark\"""

3 Escapingallthreequotationmarks\"\"\"

4 """

ExtendedStringDelimitersYoucanplaceastringliteralwithinextendeddelimiterstoincludespecialcharactersinastringwithoutinvokingtheireffect.Youplaceyourstringwithinquotationmarks(")andsurroundthatwithnumbersigns(#).Forexample,printingthestringliteral#"Line1\nLine2"#printsthelinefeedescapesequence(\n)ratherthanprintingthestringacrosstwolines.

Ifyouneedthespecialeffectsofacharacterinastringliteral,matchthenumberofnumbersignswithinthestringfollowingtheescapecharacter(\).Forexample,ifyourstringis#"Line1\nLine2"#andyouwanttobreaktheline,youcanuse#"Line1\#nLine2"#instead.Similarly,###"Line1\###nLine2"###alsobreakstheline.

Stringliteralscreatedusingextendeddelimiterscanalsobemultilinestringliterals.Youcanuseextendeddelimiterstoincludethetext"""inamultilinestring,overridingthedefaultbehaviorthatendstheliteral.Forexample:

1 letthreeMoreDoubleQuotationMarks=#"""

2 Herearethreemoredoublequotes:"""

3 """#

InitializinganEmptyString

TocreateanemptyStringvalueasthestartingpointforbuildingalongerstring,eitherassignanemptystringliteraltoavariable,orinitializeanewStringinstancewithinitializersyntax:

1 varemptyString=""//emptystringliteral

2 varanotherEmptyString=String()//initializersyntax

3 //thesetwostringsarebothempty,andareequivalenttoeach

other

FindoutwhetheraStringvalueisemptybycheckingitsBooleanisEmptyproperty:

1 ifemptyString.isEmpty{

2 print("Nothingtoseehere")

3 }

4 //Prints"Nothingtoseehere"

StringMutability

YouindicatewhetheraparticularStringcanbemodified(ormutated)byassigningittoavariable(inwhichcaseitcanbemodified),ortoaconstant(inwhichcaseitcan’tbemodified):

1 varvariableString="Horse"

2 variableString+="andcarriage"

3 //variableStringisnow"Horseandcarriage"

4

5 letconstantString="Highlander"

6 constantString+="andanotherHighlander"

7 //thisreportsacompile-timeerror-aconstantstringcannot

bemodified

NOTE

ThisapproachisdifferentfromstringmutationinObjective-CandCocoa,whereyouchoosebetweentwoclasses(NSStringandNSMutableString)toindicatewhetherastringcanbemutated.

StringsAreValueTypes

Swift’sStringtypeisavaluetype.IfyoucreateanewStringvalue,thatStringvalueiscopiedwhenit’spassedtoafunctionormethod,orwhenit’sassignedtoaconstantorvariable.Ineachcase,anewcopyoftheexistingStringvalueiscreated,andthenewcopyispassedorassigned,nottheoriginalversion.ValuetypesaredescribedinStructuresandEnumerationsAreValueTypes.

Swift’scopy-by-defaultStringbehaviorensuresthatwhenafunctionormethodpassesyouaStringvalue,it’sclearthatyouownthatexactStringvalue,regardlessofwhereitcamefrom.Youcanbeconfidentthatthestringyouarepassedwon’tbemodifiedunlessyoumodifyityourself.

Behindthescenes,Swift’scompileroptimizesstringusagesothatactualcopyingtakesplaceonlywhenabsolutelynecessary.Thismeansyoualwaysgetgreatperformancewhenworkingwithstringsasvaluetypes.

WorkingwithCharacters

YoucanaccesstheindividualCharactervaluesforaStringbyiteratingoverthestringwithafor-inloop:

1 forcharacterin"Dog!" {

2 print(character)

3 }

4 //D

5 //o

6 //g

7 //!

8 //

Thefor-inloopisdescribedinFor-InLoops.

Alternatively,youcancreateastand-aloneCharacterconstantorvariablefromasingle-characterstringliteralbyprovidingaCharactertypeannotation:

letexclamationMark:Character="!"

StringvaluescanbeconstructedbypassinganarrayofCharactervaluesasanargumenttoitsinitializer:

1 letcatCharacters:[Character]=["C","a","t","!"," "]

2 letcatString=String(catCharacters)

3 print(catString)

4 //Prints"Cat! "

ConcatenatingStringsandCharacters

Stringvaluescanbeaddedtogether(orconcatenated)withtheadditionoperator(+)tocreateanewStringvalue:

1 letstring1="hello"

2 letstring2="there"

3 varwelcome=string1+string2

4 //welcomenowequals"hellothere"

YoucanalsoappendaStringvaluetoanexistingStringvariablewiththeadditionassignmentoperator(+=):

1 varinstruction="lookover"

2 instruction+=string2

3 //instructionnowequals"lookoverthere"

YoucanappendaCharactervaluetoaStringvariablewiththeStringtype’sappend()method:

1 letexclamationMark:Character="!"

2 welcome.append(exclamationMark)

3 //welcomenowequals"hellothere!"

NOTE

Youcan’tappendaStringorCharactertoanexistingCharactervariable,becauseaCharactervaluemustcontainasinglecharacteronly.

Ifyou’reusingmultilinestringliteralstobuildupthelinesofalongerstring,youwanteverylineinthestringtoendwithalinebreak,includingthelastline.Forexample:

1 letbadStart="""

2 one

3 two

4 """

5 letend="""

6 three

7 """

8 print(badStart+end)

9 //Printstwolines:

10 //one

11 //twothree

12

13 letgoodStart="""

14 one

15 two

16

17 """

18 print(goodStart+end)

19 //Printsthreelines:

20 //one

21 //two

22 //three

Inthecodeabove,concatenatingbadStartwithendproducesatwo-linestring,whichisn’tthedesiredresult.BecausethelastlineofbadStartdoesn’tendwithalinebreak,thatlinegetscombinedwiththefirstlineofend.Incontrast,bothlinesofgoodStartendwithalinebreak,sowhenit’scombinedwithendtheresulthasthreelines,asexpected.

StringInterpolation

StringinterpolationisawaytoconstructanewStringvaluefromamixofconstants,variables,literals,andexpressionsbyincludingtheirvaluesinsideastringliteral.Youcanusestringinterpolationinbothsingle-lineandmultilinestringliterals.Eachitemthatyouinsertintothestringliteraliswrappedinapairofparentheses,prefixedbyabackslash(\):

1 letmultiplier=3

2 letmessage="\(multiplier)times2.5is\(Double(multiplier)

*2.5)"

3 //messageis"3times2.5is7.5"

Intheexampleabove,thevalueofmultiplierisinsertedintoastringliteralas\(multiplier).Thisplaceholderisreplacedwiththeactualvalueofmultiplierwhenthestringinterpolationisevaluatedtocreateanactualstring.

Thevalueofmultiplierisalsopartofalargerexpressionlaterinthestring.ThisexpressioncalculatesthevalueofDouble(multiplier)*2.5andinsertstheresult(7.5)intothestring.Inthiscase,theexpressioniswrittenas\(Double(multiplier)*2.5)whenit’sincludedinsidethestringliteral.

Youcanuseextendedstringdelimiterstocreatestringscontainingcharacters

thatwouldotherwisebetreatedasastringinterpolation.Forexample:

1 print(#"WriteaninterpolatedstringinSwiftusing\

(multiplier)."#)

2 //Prints"WriteaninterpolatedstringinSwiftusing\

(multiplier)."

Tousestringinterpolationinsideastringthatusesextendeddelimiters,matchthenumberofnumbersignsbeforethebackslashtothenumberofnumbersignsatthebeginningandendofthestring.Forexample:

1 print(#"6times7is\#(6*7)."#)

2 //Prints"6times7is42."

NOTE

Theexpressionsyouwriteinsideparentheseswithinaninterpolatedstringcan’tcontainanunescapedbackslash(\),acarriagereturn,oralinefeed.However,theycancontainotherstringliterals.

Unicode

Unicodeisaninternationalstandardforencoding,representing,andprocessingtextindifferentwritingsystems.Itenablesyoutorepresentalmostanycharacterfromanylanguageinastandardizedform,andtoreadandwritethosecharacterstoandfromanexternalsourcesuchasatextfileorwebpage.Swift’sStringandCharactertypesarefullyUnicode-compliant,asdescribedinthissection.

UnicodeScalarValuesBehindthescenes,Swift’snativeStringtypeisbuiltfromUnicodescalarvalues.AUnicodescalarvalueisaunique21-bitnumberforacharacterormodifier,suchasU+0061forLATINSMALLLETTERA("a"),orU+1F425forFRONT-FACINGBABYCHICK("" ).

Notethatnotall21-bitUnicodescalarvaluesareassignedtoacharacter—somescalarsarereservedforfutureassignmentorforuseinUTF-16encoding.Scalarvaluesthathavebeenassignedtoacharactertypicallyalsohaveaname,suchasLATINSMALLLETTERAandFRONT-FACINGBABYCHICKintheexamplesabove.

ExtendedGraphemeClustersEveryinstanceofSwift’sCharactertyperepresentsasingleextendedgraphemecluster.AnextendedgraphemeclusterisasequenceofoneormoreUnicodescalarsthat(whencombined)produceasinglehuman-readablecharacter.

Here’sanexample.TheletterécanberepresentedasthesingleUnicodescalaré(LATINSMALLLETTEREWITHACUTE,orU+00E9).However,thesamelettercanalsoberepresentedasapairofscalars—astandardlettere(LATINSMALLLETTERE,orU+0065),followedbytheCOMBININGACUTEACCENTscalar(U+0301).TheCOMBININGACUTEACCENTscalarisgraphicallyappliedtothescalarthatprecedesit,turninganeintoanéwhenit’srenderedbyaUnicode-awaretext-renderingsystem.

Inbothcases,theletteréisrepresentedasasingleSwiftCharactervaluethatrepresentsanextendedgraphemecluster.Inthefirstcase,theclustercontainsasinglescalar;inthesecondcase,it’saclusteroftwoscalars:

1 leteAcute:Character="\u{E9}"//é

2 letcombinedEAcute:Character="\u{65}\u{301}"//e

followedby

3 //eAcuteisé,combinedEAcuteisé

ExtendedgraphemeclustersareaflexiblewaytorepresentmanycomplexscriptcharactersasasingleCharactervalue.Forexample,HangulsyllablesfromtheKoreanalphabetcanberepresentedaseitheraprecomposedordecomposedsequence.BothoftheserepresentationsqualifyasasingleCharactervalueinSwift:

1 letprecomposed:Character="\u{D55C}"//

2 letdecomposed:Character="\u{1112}\u{1161}\u{11AB}"//�,

�,�

3 //precomposedis�,decomposedis�

Extendedgraphemeclustersenablescalarsforenclosingmarks(suchasCOMBININGENCLOSINGCIRCLE,orU+20DD)toencloseotherUnicodescalarsaspartofasingleCharactervalue:

1 letenclosedEAcute:Character="\u{E9}\u{20DD}"

2 //enclosedEAcuteisé�

UnicodescalarsforregionalindicatorsymbolscanbecombinedinpairstomakeasingleCharactervalue,suchasthiscombinationofREGIONALINDICATORSYMBOLLETTERU(U+1F1FA)andREGIONALINDICATORSYMBOLLETTERS(U+1F1F8):

1 letregionalIndicatorForUS:Character="\u{1F1FA}\u{1F1F8}"

2 //regionalIndicatorForUSis��

CountingCharacters

ToretrieveacountoftheCharactervaluesinastring,usethecountpropertyofthestring:

1 letunusualMenagerie="Koala,Snail,Penguin,Dromedary

"

2 print("unusualMenageriehas\(unusualMenagerie.count)

characters")

3 //Prints"unusualMenageriehas40characters"

NotethatSwift’suseofextendedgraphemeclustersforCharactervaluesmeansthatstringconcatenationandmodificationmaynotalwaysaffectastring’scharactercount.

Forexample,ifyouinitializeanewstringwiththefour-characterwordcafe,andthenappendaCOMBININGACUTEACCENT(U+0301)totheendofthestring,theresultingstringwillstillhaveacharactercountof4,withafourthcharacterofé,note:

1 varword="cafe"

2 print("thenumberofcharactersin\(word)is\(word.count)")

3 //Prints"thenumberofcharactersincafeis4"

4

5 word+="\u{301}"//COMBININGACUTEACCENT,U+0301

6

7 print("thenumberofcharactersin\(word)is\(word.count)")

8 //Prints"thenumberofcharactersincaféis4"

NOTE

ExtendedgraphemeclusterscanbecomposedofmultipleUnicodescalars.Thismeansthatdifferentcharacters—anddifferentrepresentationsofthesamecharacter—canrequiredifferentamountsofmemorytostore.Becauseofthis,charactersinSwiftdon’teachtakeupthesameamountofmemorywithinastring’srepresentation.Asaresult,thenumberofcharactersinastringcan’tbecalculatedwithoutiteratingthroughthestringtodetermineitsextendedgraphemeclusterboundaries.Ifyouareworkingwithparticularlylongstringvalues,beawarethatthecountpropertymustiterateovertheUnicodescalarsintheentirestringinordertodeterminethecharactersforthatstring.

Thecountofthecharactersreturnedbythecountpropertyisn’talwaysthesameasthelengthpropertyofanNSStringthatcontainsthesamecharacters.ThelengthofanNSStringisbasedonthenumberof16-bitcodeunitswithinthestring’sUTF-16representationandnotthenumberofUnicodeextendedgraphemeclusterswithinthestring.

AccessingandModifyingaString

Youaccessandmodifyastringthroughitsmethodsandproperties,orbyusingsubscriptsyntax.

StringIndicesEachStringvaluehasanassociatedindextype,String.Index,whichcorrespondstothepositionofeachCharacterinthestring.

Asmentionedabove,differentcharacterscanrequiredifferentamountsofmemorytostore,soinordertodeterminewhichCharacterisataparticularposition,youmustiterateovereachUnicodescalarfromthestartorendofthatString.Forthisreason,Swiftstringscan’tbeindexedbyintegervalues.

UsethestartIndexpropertytoaccessthepositionofthefirstCharacterofaString.TheendIndexpropertyisthepositionafterthelastcharacterinaString.Asaresult,theendIndexpropertyisn’tavalidargumenttoastring’ssubscript.IfaStringisempty,startIndexandendIndexareequal.

Youaccesstheindicesbeforeandafteragivenindexusingtheindex(before:)andindex(after:)methodsofString.Toaccessanindexfartherawayfromthegivenindex,youcanusetheindex(_:offsetBy:)methodinsteadofcallingoneofthesemethodsmultipletimes.

YoucanusesubscriptsyntaxtoaccesstheCharacterataparticularStringindex.

1 letgreeting="GutenTag!"

2 greeting[greeting.startIndex]

3 //G

4 greeting[greeting.index(before:greeting.endIndex)]

5 //!

6 greeting[greeting.index(after:greeting.startIndex)]

7 //u

8 letindex=greeting.index(greeting.startIndex,offsetBy:7)

9 greeting[index]

10 //a

Attemptingtoaccessanindexoutsideofastring’srangeoraCharacteratanindexoutsideofastring’srangewilltriggeraruntimeerror.

1 greeting[greeting.endIndex]//Error

2 greeting.index(after:greeting.endIndex)//Error

Usetheindicespropertytoaccessalloftheindicesofindividualcharactersinastring.

1 forindexingreeting.indices{

2 print("\(greeting[index])",terminator:"")

3 }

4 //Prints"GutenTag!"

NOTE

YoucanusethestartIndexandendIndexpropertiesandtheindex(before:),index(after:),andindex(_:offsetBy:)methodsonanytypethatconformstotheCollectionprotocol.ThisincludesString,asshownhere,aswellascollectiontypessuchasArray,Dictionary,andSet.

InsertingandRemovingToinsertasinglecharacterintoastringataspecifiedindex,usetheinsert(_:at:)method,andtoinsertthecontentsofanotherstringataspecifiedindex,usetheinsert(contentsOf:at:)method.

1 varwelcome="hello"

2 welcome.insert("!",at:welcome.endIndex)

3 //welcomenowequals"hello!"

4

5 welcome.insert(contentsOf:"there",at:welcome.index(before:

welcome.endIndex))

6 //welcomenowequals"hellothere!"

Toremoveasinglecharacterfromastringataspecifiedindex,usetheremove(at:)method,andtoremoveasubstringataspecifiedrange,usethe

removeSubrange(_:)method:

1 welcome.remove(at:welcome.index(before:welcome.endIndex))

2 //welcomenowequals"hellothere"

3

4 letrange=welcome.index(welcome.endIndex,offsetBy:-6)..

<welcome.endIndex

5 welcome.removeSubrange(range)

6 //welcomenowequals"hello"

NOTE

Youcanusetheinsert(_:at:),insert(contentsOf:at:),remove(at:),andremoveSubrange(_:)methodsonanytypethatconformstotheRangeReplaceableCollectionprotocol.ThisincludesString,asshownhere,aswellascollectiontypessuchasArray,Dictionary,andSet.

Substrings

Whenyougetasubstringfromastring—forexample,usingasubscriptoramethodlikeprefix(_:)—theresultisaninstanceofSubstring,notanotherstring.SubstringsinSwifthavemostofthesamemethodsasstrings,whichmeansyoucanworkwithsubstringsthesamewayyouworkwithstrings.However,unlikestrings,youusesubstringsforonlyashortamountoftimewhileperformingactionsonastring.Whenyou’rereadytostoretheresultforalongertime,youconvertthesubstringtoaninstanceofString.Forexample:

1 letgreeting="Hello,world!"

2 letindex=greeting.firstIndex(of:",")??greeting.endIndex

3 letbeginning=greeting[..<index]

4 //beginningis"Hello"

5

6 //ConverttheresulttoaStringforlong-termstorage.

7 letnewString=String(beginning)

Likestrings,eachsubstringhasaregionofmemorywherethecharactersthatmakeupthesubstringarestored.Thedifferencebetweenstringsandsubstringsisthat,asaperformanceoptimization,asubstringcanreusepartofthememorythat’susedtostoretheoriginalstring,orpartofthememorythat’susedtostoreanothersubstring.(Stringshaveasimilaroptimization,butiftwostringssharememory,theyareequal.)Thisperformanceoptimizationmeansyoudon’thavetopaytheperformancecostofcopyingmemoryuntilyoumodifyeitherthestringorsubstring.Asmentionedabove,substringsaren’tsuitableforlong-termstorage—becausetheyreusethestorageoftheoriginalstring,theentireoriginalstringmustbekeptinmemoryaslongasanyofitssubstringsarebeingused.

Intheexampleabove,greetingisastring,whichmeansithasaregionofmemorywherethecharactersthatmakeupthestringarestored.Becausebeginningisasubstringofgreeting,itreusesthememorythatgreetinguses.Incontrast,newStringisastring—whenit’screatedfromthesubstring,ithasitsownstorage.Thefigurebelowshowstheserelationships:

NOTE

BothStringandSubstringconformtotheStringProtocolprotocol,whichmeansit’softenconvenientforstring-manipulationfunctionstoacceptaStringProtocolvalue.YoucancallsuchfunctionswitheitheraStringorSubstringvalue.

ComparingStrings

Swiftprovidesthreewaystocomparetextualvalues:stringandcharacterequality,prefixequality,andsuffixequality.

StringandCharacterEqualityStringandcharacterequalityischeckedwiththe“equalto”operator(==)andthe“notequalto”operator(!=),asdescribedinComparisonOperators:

1 letquotation="We'realotalike,youandI."

2 letsameQuotation="We'realotalike,youandI."

3 ifquotation==sameQuotation{

4 print("Thesetwostringsareconsideredequal")

5 }

6 //Prints"Thesetwostringsareconsideredequal"

TwoStringvalues(ortwoCharactervalues)areconsideredequaliftheirextendedgraphemeclustersarecanonicallyequivalent.Extendedgraphemeclustersarecanonicallyequivalentiftheyhavethesamelinguisticmeaningandappearance,evenifthey’recomposedfromdifferentUnicodescalarsbehindthescenes.

Forexample,LATINSMALLLETTEREWITHACUTE(U+00E9)iscanonicallyequivalenttoLATINSMALLLETTERE(U+0065)followedbyCOMBININGACUTEACCENT(U+0301).Bothoftheseextendedgraphemeclustersarevalidwaystorepresentthecharacteré,andsothey’reconsideredtobecanonicallyequivalent:

1 //"Voulez-vousuncafé?"usingLATINSMALLLETTEREWITHACUTE

2 leteAcuteQuestion="Voulez-vousuncaf\u{E9}?"

3

4 //"Voulez-vousuncafé?"usingLATINSMALLLETTEREand

COMBININGACUTEACCENT

5 letcombinedEAcuteQuestion="Voulez-vousuncaf\u{65}\u{301}?"

6

7 ifeAcuteQuestion==combinedEAcuteQuestion{

8 print("Thesetwostringsareconsideredequal")

9 }

10 //Prints"Thesetwostringsareconsideredequal"

Conversely,LATINCAPITALLETTERA(U+0041,or"A"),asusedinEnglish,isnotequivalenttoCYRILLICCAPITALLETTERA(U+0410,or"А"),asusedinRussian.Thecharactersarevisuallysimilar,butdon’thavethesamelinguisticmeaning:

1 letlatinCapitalLetterA:Character="\u{41}"

2

3 letcyrillicCapitalLetterA:Character="\u{0410}"

4

5 iflatinCapitalLetterA!=cyrillicCapitalLetterA{

6 print("Thesetwocharactersarenotequivalent.")

7 }

8 //Prints"Thesetwocharactersarenotequivalent."

NOTE

StringandcharactercomparisonsinSwiftarenotlocale-sensitive.

PrefixandSuffixEqualityTocheckwhetherastringhasaparticularstringprefixorsuffix,callthestring’shasPrefix(_:)andhasSuffix(_:)methods,bothofwhichtakeasingleargumentoftypeStringandreturnaBooleanvalue.

TheexamplesbelowconsideranarrayofstringsrepresentingthescenelocationsfromthefirsttwoactsofShakespeare’sRomeoandJuliet:

1 letromeoAndJuliet=[

2 "Act1Scene1:Verona,Apublicplace",

3 "Act1Scene2:Capulet'smansion",

4 "Act1Scene3:AroominCapulet'smansion",

5 "Act1Scene4:AstreetoutsideCapulet'smansion",

6 "Act1Scene5:TheGreatHallinCapulet'smansion",

7 "Act2Scene1:OutsideCapulet'smansion",

8 "Act2Scene2:Capulet'sorchard",

9 "Act2Scene3:OutsideFriarLawrence'scell",

10 "Act2Scene4:AstreetinVerona",

11 "Act2Scene5:Capulet'smansion",

12 "Act2Scene6:FriarLawrence'scell"

13 ]

YoucanusethehasPrefix(_:)methodwiththeromeoAndJulietarraytocountthenumberofscenesinAct1oftheplay:

1 varact1SceneCount=0

2 forsceneinromeoAndJuliet{

3 ifscene.hasPrefix("Act1"){

4 act1SceneCount+=1

5 }

6 }

7 print("Thereare\(act1SceneCount)scenesinAct1")

8 //Prints"Thereare5scenesinAct1"

Similarly,usethehasSuffix(_:)methodtocountthenumberofscenesthattakeplaceinoraroundCapulet’smansionandFriarLawrence’scell:

1 varmansionCount=0

2 varcellCount=0

3 forsceneinromeoAndJuliet{

4 ifscene.hasSuffix("Capulet'smansion"){

5 mansionCount+=1

6 }elseifscene.hasSuffix("FriarLawrence'scell"){

7 cellCount+=1

8 }

9 }

10 print("\(mansionCount)mansionscenes;\(cellCount)cell

scenes")

11 //Prints"6mansionscenes;2cellscenes"

NOTE

ThehasPrefix(_:)andhasSuffix(_:)methodsperformacharacter-by-charactercanonicalequivalencecomparisonbetweentheextendedgraphemeclustersineachstring,asdescribedinStringandCharacterEquality.

UnicodeRepresentationsofStrings

WhenaUnicodestringiswrittentoatextfileorsomeotherstorage,theUnicodescalarsinthatstringareencodedinoneofseveralUnicode-definedencodingforms.Eachformencodesthestringinsmallchunksknownascodeunits.TheseincludetheUTF-8encodingform(whichencodesastringas8-bitcodeunits),theUTF-16encodingform(whichencodesastringas16-bitcodeunits),andtheUTF-32encodingform(whichencodesastringas32-bitcodeunits).

SwiftprovidesseveraldifferentwaystoaccessUnicoderepresentationsofstrings.Youcaniterateoverthestringwithafor-instatement,toaccessitsindividualCharactervaluesasUnicodeextendedgraphemeclusters.ThisprocessisdescribedinWorkingwithCharacters.

Alternatively,accessaStringvalueinoneofthreeotherUnicode-compliant

representations:

AcollectionofUTF-8codeunits(accessedwiththestring’sutf8property)

AcollectionofUTF-16codeunits(accessedwiththestring’sutf16property)

Acollectionof21-bitUnicodescalarvalues,equivalenttothestring’sUTF-32encodingform(accessedwiththestring’sunicodeScalarsproperty)

Eachexamplebelowshowsadifferentrepresentationofthefollowingstring,whichismadeupofthecharactersD,o,g,‼(DOUBLEEXCLAMATIONMARK,orUnicodescalarU+203C),andthecharacter( DOGFACE,orUnicodescalarU+1F436):

letdogString="Dog‼"

UTF-8RepresentationYoucanaccessaUTF-8representationofaStringbyiteratingoveritsutf8property.ThispropertyisoftypeString.UTF8View,whichisacollectionofunsigned8-bit(UInt8)values,oneforeachbyteinthestring’sUTF-8representation:

1 forcodeUnitindogString.utf8{

2 print("\(codeUnit)",terminator:"")

3 }

4 print("")

5 //Prints"68111103226128188240159144182"

Intheexampleabove,thefirstthreedecimalcodeUnitvalues(68,111,103)representthecharactersD,o,andg,whoseUTF-8representationisthesameastheirASCIIrepresentation.ThenextthreedecimalcodeUnitvalues(226,128,188)areathree-byteUTF-8representationoftheDOUBLEEXCLAMATIONMARKcharacter.ThelastfourcodeUnitvalues(240,159,144,182)areafour-byteUTF-8representationoftheDOGFACEcharacter.

UTF-16RepresentationYoucanaccessaUTF-16representationofaStringbyiteratingoveritsutf16property.ThispropertyisoftypeString.UTF16View,whichisacollectionofunsigned16-bit(UInt16)values,oneforeach16-bitcodeunitinthestring’sUTF-16representation:

1 forcodeUnitindogString.utf16{

2 print("\(codeUnit)",terminator:"")

3 }

4 print("")

5 //Prints"6811110382525535756374"

Again,thefirstthreecodeUnitvalues(68,111,103)representthecharactersD,o,andg,whoseUTF-16codeunitshavethesamevaluesasinthestring’sUTF-8representation(becausetheseUnicodescalarsrepresentASCIIcharacters).

ThefourthcodeUnitvalue(8252)isadecimalequivalentofthehexadecimalvalue203C,whichrepresentstheUnicodescalarU+203CfortheDOUBLEEXCLAMATIONMARKcharacter.ThischaractercanberepresentedasasinglecodeunitinUTF-16.

ThefifthandsixthcodeUnitvalues(55357and56374)areaUTF-16surrogatepairrepresentationoftheDOGFACEcharacter.Thesevaluesareahigh-surrogatevalueofU+D83D(decimalvalue55357)andalow-surrogatevalueofU+DC36(decimalvalue56374).

UnicodeScalarRepresentationYoucanaccessaUnicodescalarrepresentationofaStringvaluebyiteratingoveritsunicodeScalarsproperty.ThispropertyisoftypeUnicodeScalarView,whichisacollectionofvaluesoftypeUnicodeScalar.

EachUnicodeScalarhasavaluepropertythatreturnsthescalar’s21-bitvalue,representedwithinaUInt32value:

1 forscalarindogString.unicodeScalars{

2 print("\(scalar.value)",terminator:"")

3 }

4 print("")

5 //Prints"681111038252128054"

ThevaluepropertiesforthefirstthreeUnicodeScalarvalues(68,111,103)onceagainrepresentthecharactersD,o,andg.

ThefourthcodeUnitvalue(8252)isagainadecimalequivalentofthehexadecimalvalue203C,whichrepresentstheUnicodescalarU+203CfortheDOUBLEEXCLAMATIONMARKcharacter.

ThevaluepropertyofthefifthandfinalUnicodeScalar,128054,isadecimalequivalentofthehexadecimalvalue1F436,whichrepresentstheUnicodescalarU+1F436fortheDOGFACEcharacter.

Asanalternativetoqueryingtheirvalueproperties,eachUnicodeScalarvaluecanalsobeusedtoconstructanewStringvalue,suchaswithstringinterpolation:

1 forscalarindogString.unicodeScalars{

2 print("\(scalar)")

3 }

4 //D

5 //o

6 //g

7 //‼

8 //

CollectionTypes

Swiftprovidesthreeprimarycollectiontypes,knownasarrays,sets,anddictionaries,forstoringcollectionsofvalues.Arraysareorderedcollectionsofvalues.Setsareunorderedcollectionsofuniquevalues.Dictionariesareunorderedcollectionsofkey-valueassociations.

Arrays,sets,anddictionariesinSwiftarealwaysclearaboutthetypesofvaluesandkeysthattheycanstore.Thismeansthatyoucannotinsertavalueofthewrongtypeintoacollectionbymistake.Italsomeansyoucanbeconfidentaboutthetypeofvaluesyouwillretrievefromacollection.

NOTE

Swift’sarray,set,anddictionarytypesareimplementedasgenericcollections.Formoreaboutgenerictypesandcollections,seeGenerics.

MutabilityofCollections

Ifyoucreateanarray,aset,oradictionary,andassignittoavariable,thecollectionthatiscreatedwillbemutable.Thismeansthatyoucanchange(ormutate)thecollectionafterit’screatedbyadding,removing,orchangingitemsinthecollection.Ifyouassignanarray,aset,oradictionarytoaconstant,that

collectionisimmutable,anditssizeandcontentscannotbechanged.

NOTE

Itisgoodpracticetocreateimmutablecollectionsinallcaseswherethecollectiondoesnotneedtochange.DoingsomakesiteasierforyoutoreasonaboutyourcodeandenablestheSwiftcompilertooptimizetheperformanceofthecollectionsyoucreate.

Arrays

Anarraystoresvaluesofthesametypeinanorderedlist.Thesamevaluecanappearinanarraymultipletimesatdifferentpositions.

NOTE

Swift’sArraytypeisbridgedtoFoundation’sNSArrayclass.

FormoreinformationaboutusingArraywithFoundationandCocoa,seeBridgingBetweenArrayandNSArray.

ArrayTypeShorthandSyntaxThetypeofaSwiftarrayiswritteninfullasArray<Element>,whereElementisthetypeofvaluesthearrayisallowedtostore.Youcanalsowritethetypeofanarrayinshorthandformas[Element].Althoughthetwoformsarefunctionallyidentical,theshorthandformispreferredandisusedthroughoutthisguidewhenreferringtothetypeofanarray.

CreatinganEmptyArrayYoucancreateanemptyarrayofacertaintypeusinginitializersyntax:

1 varsomeInts=[Int]()

2 print("someIntsisoftype[Int]with\(someInts.count)

items.")

3 //Prints"someIntsisoftype[Int]with0items."

NotethatthetypeofthesomeIntsvariableisinferredtobe[Int]fromthetypeoftheinitializer.

Alternatively,ifthecontextalreadyprovidestypeinformation,suchasafunctionargumentoranalreadytypedvariableorconstant,youcancreateanemptyarraywithanemptyarrayliteral,whichiswrittenas[](anemptypairofsquarebrackets):

1 someInts.append(3)

2 //someIntsnowcontains1valueoftypeInt

3 someInts=[]

4 //someIntsisnowanemptyarray,butisstilloftype[Int]

CreatinganArraywithaDefaultValueSwift’sArraytypealsoprovidesaninitializerforcreatinganarrayofacertainsizewithallofitsvaluessettothesamedefaultvalue.Youpassthisinitializeradefaultvalueoftheappropriatetype(calledrepeating):andthenumberoftimesthatvalueisrepeatedinthenewarray(calledcount):

1 varthreeDoubles=Array(repeating:0.0,count:3)

2 //threeDoublesisoftype[Double],andequals[0.0,0.0,0.0]

CreatinganArraybyAddingTwoArraysTogetherYoucancreateanewarraybyaddingtogethertwoexistingarrayswithcompatibletypeswiththeadditionoperator(+).Thenewarray’stypeisinferredfromthetypeofthetwoarraysyouaddtogether:

1 varanotherThreeDoubles=Array(repeating:2.5,count:3)

2 //anotherThreeDoublesisoftype[Double],andequals[2.5,

2.5,2.5]

3

4 varsixDoubles=threeDoubles+anotherThreeDoubles

5 //sixDoublesisinferredas[Double],andequals[0.0,0.0,

0.0,2.5,2.5,2.5]

CreatinganArraywithanArrayLiteralYoucanalsoinitializeanarraywithanarrayliteral,whichisashorthandwaytowriteoneormorevaluesasanarraycollection.Anarrayliteraliswrittenasalistofvalues,separatedbycommas,surroundedbyapairofsquarebrackets:

[ value1 , value2 , value3 ]

TheexamplebelowcreatesanarraycalledshoppingListtostoreStringvalues:

1 varshoppingList:[String]=["Eggs","Milk"]

2 //shoppingListhasbeeninitializedwithtwoinitialitems

TheshoppingListvariableisdeclaredas“anarrayofstringvalues”,writtenas[String].BecausethisparticulararrayhasspecifiedavaluetypeofString,itisallowedtostoreStringvaluesonly.Here,theshoppingListarrayisinitializedwithtwoStringvalues("Eggs"and"Milk"),writtenwithinanarrayliteral.

NOTE

TheshoppingListarrayisdeclaredasavariable(withthevarintroducer)andnotaconstant(withtheletintroducer)becausemoreitemsareaddedtotheshoppinglistintheexamplesbelow.

Inthiscase,thearrayliteralcontainstwoStringvaluesandnothingelse.ThismatchesthetypeoftheshoppingListvariable’sdeclaration(anarraythatcanonlycontainStringvalues),andsotheassignmentofthearrayliteralispermittedasawaytoinitializeshoppingListwithtwoinitialitems.

ThankstoSwift’stypeinference,youdon’thavetowritethetypeofthearrayifyou’reinitializingitwithanarrayliteralcontainingvaluesofthesametype.TheinitializationofshoppingListcouldhavebeenwritteninashorterforminstead:

varshoppingList=["Eggs","Milk"]

Becauseallvaluesinthearrayliteralareofthesametype,Swiftcaninferthat[String]isthecorrecttypetousefortheshoppingListvariable.

AccessingandModifyinganArrayYouaccessandmodifyanarraythroughitsmethodsandproperties,orbyusingsubscriptsyntax.

Tofindoutthenumberofitemsinanarray,checkitsread-onlycountproperty:

1 print("Theshoppinglistcontains\(shoppingList.count)

items.")

2 //Prints"Theshoppinglistcontains2items."

UsetheBooleanisEmptypropertyasashortcutforcheckingwhetherthecountpropertyisequalto0:

1 ifshoppingList.isEmpty{

2 print("Theshoppinglistisempty.")

3 }else{

4 print("Theshoppinglistisnotempty.")

5 }

6 //Prints"Theshoppinglistisnotempty."

Youcanaddanewitemtotheendofanarraybycallingthearray’sappend(_:)method:

1 shoppingList.append("Flour")

2 //shoppingListnowcontains3items,andsomeoneismaking

pancakes

Alternatively,appendanarrayofoneormorecompatibleitemswiththeadditionassignmentoperator(+=):

1 shoppingList+=["BakingPowder"]

2 //shoppingListnowcontains4items

3 shoppingList+=["ChocolateSpread","Cheese","Butter"]

4 //shoppingListnowcontains7items

Retrieveavaluefromthearraybyusingsubscriptsyntax,passingtheindexofthevalueyouwanttoretrievewithinsquarebracketsimmediatelyafterthenameofthearray:

1 varfirstItem=shoppingList[0]

2 //firstItemisequalto"Eggs"

NOTE

Thefirstiteminthearrayhasanindexof0,not1.ArraysinSwiftarealwayszero-indexed.

Youcanusesubscriptsyntaxtochangeanexistingvalueatagivenindex:

1 shoppingList[0]="Sixeggs"

2 //thefirstiteminthelistisnowequalto"Sixeggs"rather

than"Eggs"

Whenyouusesubscriptsyntax,theindexyouspecifyneedstobevalid.Forexample,writingshoppingList[shoppingList.count]="Salt"totrytoappendanitemtotheendofthearrayresultsinaruntimeerror.

Youcanalsousesubscriptsyntaxtochangearangeofvaluesatonce,evenifthereplacementsetofvalueshasadifferentlengththantherangeyouarereplacing.Thefollowingexamplereplaces"ChocolateSpread","Cheese",and"Butter"

with"Bananas"and"Apples":

1 shoppingList[4...6]=["Bananas","Apples"]

2 //shoppingListnowcontains6items

Toinsertanitemintothearrayataspecifiedindex,callthearray’sinsert(_:at:)method:

1 shoppingList.insert("MapleSyrup",at:0)

2 //shoppingListnowcontains7items

3 //"MapleSyrup"isnowthefirstiteminthelist

Thiscalltotheinsert(_:at:)methodinsertsanewitemwithavalueof"MapleSyrup"attheverybeginningoftheshoppinglist,indicatedbyanindexof0.

Similarly,youremoveanitemfromthearraywiththeremove(at:)method.Thismethodremovestheitematthespecifiedindexandreturnstheremoveditem(althoughyoucanignorethereturnedvalueifyoudonotneedit):

1 letmapleSyrup=shoppingList.remove(at:0)

2 //theitemthatwasatindex0hasjustbeenremoved

3 //shoppingListnowcontains6items,andnoMapleSyrup

4 //themapleSyrupconstantisnowequaltotheremoved"Maple

Syrup"string

NOTE

Ifyoutrytoaccessormodifyavalueforanindexthatisoutsideofanarray’sexistingbounds,youwilltriggeraruntimeerror.Youcancheckthatanindexisvalidbeforeusingitbycomparingittothearray’scountproperty.Thelargestvalidindexinanarrayiscount-1becausearraysareindexedfromzero—however,whencountis0(meaningthearrayisempty),therearenovalidindexes.

Anygapsinanarrayareclosedwhenanitemisremoved,andsothevalueatindex0isonceagainequalto"Sixeggs":

1 firstItem=shoppingList[0]

2 //firstItemisnowequalto"Sixeggs"

Ifyouwanttoremovethefinalitemfromanarray,usetheremoveLast()methodratherthantheremove(at:)methodtoavoidtheneedtoquerythearray’scountproperty.Liketheremove(at:)method,removeLast()returnstheremoveditem:

1 letapples=shoppingList.removeLast()

2 //thelastiteminthearrayhasjustbeenremoved

3 //shoppingListnowcontains5items,andnoapples

4 //theapplesconstantisnowequaltotheremoved"Apples"

string

IteratingOveranArrayYoucaniterateovertheentiresetofvaluesinanarraywiththefor-inloop:

1 foriteminshoppingList{

2 print(item)

3 }

4 //Sixeggs

5 //Milk

6 //Flour

7 //BakingPowder

8 //Bananas

Ifyouneedtheintegerindexofeachitemaswellasitsvalue,usetheenumerated()methodtoiterateoverthearrayinstead.Foreachiteminthearray,theenumerated()methodreturnsatuplecomposedofanintegerandtheitem.Theintegersstartatzeroandcountupbyoneforeachitem;ifyouenumerateoverawholearray,theseintegersmatchtheitems’indices.Youcandecomposethetupleintotemporaryconstantsorvariablesaspartoftheiteration:

1 for(index,value)inshoppingList.enumerated(){

2 print("Item\(index+1):\(value)")

3 }

4 //Item1:Sixeggs

5 //Item2:Milk

6 //Item3:Flour

7 //Item4:BakingPowder

8 //Item5:Bananas

Formoreaboutthefor-inloop,seeFor-InLoops.

Sets

Asetstoresdistinctvaluesofthesametypeinacollectionwithnodefinedordering.Youcanuseasetinsteadofanarraywhentheorderofitemsisnotimportant,orwhenyouneedtoensurethatanitemonlyappearsonce.

NOTE

Swift’sSettypeisbridgedtoFoundation’sNSSetclass.

FormoreinformationaboutusingSetwithFoundationandCocoa,seeBridgingBetweenSetandNSSet.

HashValuesforSetTypesAtypemustbehashableinordertobestoredinaset—thatis,thetypemustprovideawaytocomputeahashvalueforitself.AhashvalueisanIntvaluethatisthesameforallobjectsthatcompareequally,suchthatifa==b,itfollowsthata.hashValue==b.hashValue.

AllofSwift’sbasictypes(suchasString,Int,Double,andBool)arehashablebydefault,andcanbeusedassetvaluetypesordictionarykeytypes.Enumeration

casevalueswithoutassociatedvalues(asdescribedinEnumerations)arealsohashablebydefault.

NOTE

YoucanuseyourowncustomtypesassetvaluetypesordictionarykeytypesbymakingthemconformtotheHashableprotocolfromSwift’sstandardlibrary.TypesthatconformtotheHashableprotocolmustprovideagettableIntpropertycalledhashValue.Thevaluereturnedbyatype’shashValuepropertyisnotrequiredtobethesameacrossdifferentexecutionsofthesameprogram,orindifferentprograms.

BecausetheHashableprotocolconformstoEquatable,conformingtypesmustalsoprovideanimplementationoftheequalsoperator(==).TheEquatableprotocolrequiresanyconformingimplementationof==tobeanequivalencerelation.Thatis,animplementationof==mustsatisfythefollowingthreeconditions,forallvaluesa,b,andc:

a==a(Reflexivity)

a==bimpliesb==a(Symmetry)

a==b&&b==cimpliesa==c(Transitivity)

Formoreinformationaboutconformingtoprotocols,seeProtocols.

SetTypeSyntaxThetypeofaSwiftsetiswrittenasSet<Element>,whereElementisthetypethatthesetisallowedtostore.Unlikearrays,setsdonothaveanequivalentshorthandform.

CreatingandInitializinganEmptySetYoucancreateanemptysetofacertaintypeusinginitializersyntax:

1 varletters=Set<Character>()

2 print("lettersisoftypeSet<Character>with\(letters.count)

items.")

3 //Prints"lettersisoftypeSet<Character>with0items."

NOTE

ThetypeofthelettersvariableisinferredtobeSet<Character>,fromthetypeoftheinitializer.

Alternatively,ifthecontextalreadyprovidestypeinformation,suchasafunctionargumentoranalreadytypedvariableorconstant,youcancreateanemptysetwithanemptyarrayliteral:

1 letters.insert("a")

2 //lettersnowcontains1valueoftypeCharacter

3 letters=[]

4 //lettersisnowanemptyset,butisstilloftype

Set<Character>

CreatingaSetwithanArrayLiteralYoucanalsoinitializeasetwithanarrayliteral,asashorthandwaytowriteoneormorevaluesasasetcollection.

TheexamplebelowcreatesasetcalledfavoriteGenrestostoreStringvalues:

1 varfavoriteGenres:Set<String>=["Rock","Classical","Hip

hop"]

2 //favoriteGenreshasbeeninitializedwiththreeinitialitems

ThefavoriteGenresvariableisdeclaredas“asetofStringvalues”,writtenasSet<String>.BecausethisparticularsethasspecifiedavaluetypeofString,itisonlyallowedtostoreStringvalues.Here,thefavoriteGenressetisinitializedwiththreeStringvalues("Rock","Classical",and"Hiphop"),writtenwithinanarrayliteral.

NOTE

ThefavoriteGenressetisdeclaredasavariable(withthevarintroducer)andnotaconstant(withtheletintroducer)becauseitemsareaddedandremovedintheexamplesbelow.

Asettypecannotbeinferredfromanarrayliteralalone,sothetypeSetmustbeexplicitlydeclared.However,becauseofSwift’stypeinference,youdon’thavetowritethetypeoftheset’selementsifyou’reinitializingitwithanarrayliteralthatcontainsvaluesofjustonetype.TheinitializationoffavoriteGenrescouldhavebeenwritteninashorterforminstead:

varfavoriteGenres:Set=["Rock","Classical","Hiphop"]

Becauseallvaluesinthearrayliteralareofthesametype,SwiftcaninferthatSet<String>isthecorrecttypetouseforthefavoriteGenresvariable.

AccessingandModifyingaSetYouaccessandmodifyasetthroughitsmethodsandproperties.

Tofindoutthenumberofitemsinaset,checkitsread-onlycountproperty:

1 print("Ihave\(favoriteGenres.count)favoritemusicgenres.")

2 //Prints"Ihave3favoritemusicgenres."

UsetheBooleanisEmptypropertyasashortcutforcheckingwhetherthecountpropertyisequalto0:

1 iffavoriteGenres.isEmpty{

2 print("Asfarasmusicgoes,I'mnotpicky.")

3 }else{

4 print("Ihaveparticularmusicpreferences.")

5 }

6 //Prints"Ihaveparticularmusicpreferences."

Youcanaddanewitemintoasetbycallingtheset’sinsert(_:)method:

1 favoriteGenres.insert("Jazz")

2 //favoriteGenresnowcontains4items

Youcanremoveanitemfromasetbycallingtheset’sremove(_:)method,whichremovestheitemifit’samemberoftheset,andreturnstheremovedvalue,orreturnsnilifthesetdidnotcontainit.Alternatively,allitemsinasetcanberemovedwithitsremoveAll()method.

1 ifletremovedGenre=favoriteGenres.remove("Rock"){

2 print("\(removedGenre)?I'moverit.")

3 }else{

4 print("Inevermuchcaredforthat.")

5 }

6 //Prints"Rock?I'moverit."

Tocheckwhetherasetcontainsaparticularitem,usethecontains(_:)method.

1 iffavoriteGenres.contains("Funk"){

2 print("Igetuponthegoodfoot.")

3 }else{

4 print("It'stoofunkyinhere.")

5 }

6 //Prints"It'stoofunkyinhere."

IteratingOveraSetYoucaniterateoverthevaluesinasetwithafor-inloop.

1 forgenreinfavoriteGenres{

2 print("\(genre)")

3 }

4 //Classical

5 //Jazz

6 //Hiphop

Formoreaboutthefor-inloop,seeFor-InLoops.

Swift’sSettypedoesnothaveadefinedordering.Toiterateoverthevaluesofasetinaspecificorder,usethesorted()method,whichreturnstheset’selementsasanarraysortedusingthe<operator.

1 forgenreinfavoriteGenres.sorted(){

2 print("\(genre)")

3 }

4 //Classical

5 //Hiphop

6 //Jazz

PerformingSetOperations

Youcanefficientlyperformfundamentalsetoperations,suchascombiningtwosetstogether,determiningwhichvaluestwosetshaveincommon,ordeterminingwhethertwosetscontainall,some,ornoneofthesamevalues.

FundamentalSetOperationsTheillustrationbelowdepictstwosets—aandb—withtheresultsofvarioussetoperationsrepresentedbytheshadedregions.

Usetheintersection(_:)methodtocreateanewsetwithonlythevaluescommontobothsets.

UsethesymmetricDifference(_:)methodtocreateanewsetwithvaluesineitherset,butnotboth.

Usetheunion(_:)methodtocreateanewsetwithallofthevaluesinbothsets.

Usethesubtracting(_:)methodtocreateanewsetwithvaluesnotinthespecifiedset.

1 letoddDigits:Set=[1,3,5,7,9]

2 letevenDigits:Set=[0,2,4,6,8]

3 letsingleDigitPrimeNumbers:Set=[2,3,5,7]

4

5 oddDigits.union(evenDigits).sorted()

6 //[0,1,2,3,4,5,6,7,8,9]

7 oddDigits.intersection(evenDigits).sorted()

8 //[]

9 oddDigits.subtracting(singleDigitPrimeNumbers).sorted()

10 //[1,9]

11 oddDigits.symmetricDifference(singleDigitPrimeNumbers).sorted()

12 //[1,2,9]

SetMembershipandEqualityTheillustrationbelowdepictsthreesets—a,bandc—withoverlappingregionsrepresentingelementssharedamongsets.Setaisasupersetofsetb,becauseacontainsallelementsinb.Conversely,setbisasubsetofseta,becauseallelementsinbarealsocontainedbya.Setbandsetcaredisjointwithoneanother,becausetheysharenoelementsincommon.

Usethe“isequal”operator(==)todeterminewhethertwosetscontainallofthesamevalues.

UsetheisSubset(of:)methodtodeterminewhetherallofthevaluesofasetarecontainedinthespecifiedset.

UsetheisSuperset(of:)methodtodeterminewhetherasetcontainsallofthevaluesinaspecifiedset.

UsetheisStrictSubset(of:)orisStrictSuperset(of:)methodstodeterminewhetherasetisasubsetorsuperset,butnotequalto,aspecifiedset.

UsetheisDisjoint(with:)methodtodeterminewhethertwosetshavenovaluesincommon.

1 lethouseAnimals:Set=["" ," "]

2 letfarmAnimals:Set=[" ","" ,"" ,"" ," "]

3 letcityAnimals:Set=["" ," "]

4

5 houseAnimals.isSubset(of:farmAnimals)

6 //true

7 farmAnimals.isSuperset(of:houseAnimals)

8 //true

9 farmAnimals.isDisjoint(with:cityAnimals)

10 //true

Dictionaries

Adictionarystoresassociationsbetweenkeysofthesametypeandvaluesofthesametypeinacollectionwithnodefinedordering.Eachvalueisassociatedwithauniquekey,whichactsasanidentifierforthatvaluewithinthedictionary.Unlikeitemsinanarray,itemsinadictionarydonothaveaspecifiedorder.Youuseadictionarywhenyouneedtolookupvaluesbasedontheiridentifier,inmuchthesamewaythatareal-worlddictionaryisusedtolookupthedefinitionforaparticularword.

NOTE

Swift’sDictionarytypeisbridgedtoFoundation’sNSDictionaryclass.

FormoreinformationaboutusingDictionarywithFoundationandCocoa,seeBridgingBetweenDictionaryandNSDictionary.

DictionaryTypeShorthandSyntaxThetypeofaSwiftdictionaryiswritteninfullasDictionary<Key,Value>,whereKeyisthetypeofvaluethatcanbeusedasadictionarykey,andValueisthetypeofvaluethatthedictionarystoresforthosekeys.

NOTE

AdictionaryKeytypemustconformtotheHashableprotocol,likeaset’svaluetype.

Youcanalsowritethetypeofadictionaryinshorthandformas[Key:Value].Althoughthetwoformsarefunctionallyidentical,theshorthandformispreferredandisusedthroughoutthisguidewhenreferringtothetypeofadictionary.

CreatinganEmptyDictionaryAswitharrays,youcancreateanemptyDictionaryofacertaintypebyusinginitializersyntax:

1 varnamesOfIntegers=[Int:String]()

2 //namesOfIntegersisanempty[Int:String]dictionary

Thisexamplecreatesanemptydictionaryoftype[Int:String]tostorehuman-readablenamesofintegervalues.ItskeysareoftypeInt,anditsvaluesareoftypeString.

Ifthecontextalreadyprovidestypeinformation,youcancreateanemptydictionarywithanemptydictionaryliteral,whichiswrittenas[:](acoloninsideapairofsquarebrackets):

1 namesOfIntegers[16]="sixteen"

2 //namesOfIntegersnowcontains1key-valuepair

3 namesOfIntegers=[:]

4 //namesOfIntegersisonceagainanemptydictionaryoftype

[Int:String]

CreatingaDictionarywithaDictionaryLiteralYoucanalsoinitializeadictionarywithadictionaryliteral,whichhasasimilarsyntaxtothearrayliteralseenearlier.Adictionaryliteralisashorthandwaytowriteoneormorekey-valuepairsasaDictionarycollection.

Akey-valuepairisacombinationofakeyandavalue.Inadictionaryliteral,thekeyandvalueineachkey-valuepairareseparatedbyacolon.Thekey-valuepairsarewrittenasalist,separatedbycommas,surroundedbyapairofsquarebrackets:

[ key1 : value1 , key2 : value2 , key3 : value3 ]

Theexamplebelowcreatesadictionarytostorethenamesofinternationalairports.Inthisdictionary,thekeysarethree-letterInternationalAirTransportAssociationcodes,andthevaluesareairportnames:

varairports:[String:String]=["YYZ":"TorontoPearson",

"DUB":"Dublin"]

Theairportsdictionaryisdeclaredashavingatypeof[String:String],whichmeans“aDictionarywhosekeysareoftypeString,andwhosevaluesarealsooftypeString”.

NOTE

Theairportsdictionaryisdeclaredasavariable(withthevarintroducer),andnotaconstant(withtheletintroducer),becausemoreairportsareaddedtothedictionaryintheexamplesbelow.

Theairportsdictionaryisinitializedwithadictionaryliteralcontainingtwokey-valuepairs.Thefirstpairhasakeyof"YYZ"andavalueof"TorontoPearson".Thesecondpairhasakeyof"DUB"andavalueof"Dublin".

ThisdictionaryliteralcontainstwoString:Stringpairs.Thiskey-valuetypematchesthetypeoftheairportsvariabledeclaration(adictionarywithonlyStringkeys,andonlyStringvalues),andsotheassignmentofthedictionaryliteralispermittedasawaytoinitializetheairportsdictionarywithtwoinitialitems.

Aswitharrays,youdon’thavetowritethetypeofthedictionaryifyou’reinitializingitwithadictionaryliteralwhosekeysandvalueshaveconsistenttypes.Theinitializationofairportscouldhavebeenwritteninashorterforminstead:

varairports=["YYZ":"TorontoPearson","DUB":"Dublin"]

Becauseallkeysintheliteralareofthesametypeaseachother,andlikewiseallvaluesareofthesametypeaseachother,Swiftcaninferthat[String:String]isthecorrecttypetousefortheairportsdictionary.

AccessingandModifyingaDictionaryYouaccessandmodifyadictionarythroughitsmethodsandproperties,orbyusingsubscriptsyntax.

Aswithanarray,youfindoutthenumberofitemsinaDictionarybycheckingitsread-onlycountproperty:

1 print("Theairportsdictionarycontains\(airports.count)

items.")

2 //Prints"Theairportsdictionarycontains2items."

UsetheBooleanisEmptypropertyasashortcutforcheckingwhetherthecountpropertyisequalto0:

1 ifairports.isEmpty{

2 print("Theairportsdictionaryisempty.")

3 }else{

4 print("Theairportsdictionaryisnotempty.")

5 }

6 //Prints"Theairportsdictionaryisnotempty."

Youcanaddanewitemtoadictionarywithsubscriptsyntax.Useanewkeyoftheappropriatetypeasthesubscriptindex,andassignanewvalueoftheappropriatetype:

1 airports["LHR"]="London"

2 //theairportsdictionarynowcontains3items

Youcanalsousesubscriptsyntaxtochangethevalueassociatedwithaparticularkey:

1 airports["LHR"]="LondonHeathrow"

2 //thevaluefor"LHR"hasbeenchangedto"LondonHeathrow"

Asanalternativetosubscripting,useadictionary’supdateValue(_:forKey:)methodtosetorupdatethevalueforaparticularkey.Likethesubscriptexamplesabove,theupdateValue(_:forKey:)methodsetsavalueforakeyifnoneexists,orupdatesthevalueifthatkeyalreadyexists.Unlikeasubscript,however,theupdateValue(_:forKey:)methodreturnstheoldvalueafterperforminganupdate.Thisenablesyoutocheckwhetherornotanupdatetookplace.

TheupdateValue(_:forKey:)methodreturnsanoptionalvalueofthedictionary’svaluetype.ForadictionarythatstoresStringvalues,forexample,themethodreturnsavalueoftypeString?,or“optionalString”.Thisoptionalvaluecontainstheoldvalueforthatkeyifoneexistedbeforetheupdate,ornilifnovalueexisted:

1 ifletoldValue=airports.updateValue("DublinAirport",

forKey:"DUB"){

2 print("TheoldvalueforDUBwas\(oldValue).")

3 }

4 //Prints"TheoldvalueforDUBwasDublin."

Youcanalsousesubscriptsyntaxtoretrieveavaluefromthedictionaryforaparticularkey.Becauseitispossibletorequestakeyforwhichnovalueexists,adictionary’ssubscriptreturnsanoptionalvalueofthedictionary’svaluetype.Ifthedictionarycontainsavaluefortherequestedkey,thesubscriptreturnsanoptionalvaluecontainingtheexistingvalueforthatkey.Otherwise,thesubscriptreturnsnil:

1 ifletairportName=airports["DUB"]{

2 print("Thenameoftheairportis\(airportName).")

3 }else{

4 print("Thatairportisnotintheairportsdictionary.")

5 }

6 //Prints"ThenameoftheairportisDublinAirport."

Youcanusesubscriptsyntaxtoremoveakey-valuepairfromadictionarybyassigningavalueofnilforthatkey:

1 airports["APL"]="AppleInternational"

2 //"AppleInternational"isnottherealairportforAPL,so

deleteit

3 airports["APL"]=nil

4 //APLhasnowbeenremovedfromthedictionary

Alternatively,removeakey-valuepairfromadictionarywiththeremoveValue(forKey:)method.Thismethodremovesthekey-valuepairifitexistsandreturnstheremovedvalue,orreturnsnilifnovalueexisted:

1 ifletremovedValue=airports.removeValue(forKey:"DUB"){

2 print("Theremovedairport'snameis\(removedValue).")

3 }else{

4 print("Theairportsdictionarydoesnotcontainavaluefor

DUB.")

5 }

6 //Prints"Theremovedairport'snameisDublinAirport."

IteratingOveraDictionaryYoucaniterateoverthekey-valuepairsinadictionarywithafor-inloop.Eachiteminthedictionaryisreturnedasa(key,value)tuple,andyoucandecomposethetuple’smembersintotemporaryconstantsorvariablesaspartoftheiteration:

1 for(airportCode,airportName)inairports{

2 print("\(airportCode):\(airportName)")

3 }

4 //LHR:LondonHeathrow

5 //YYZ:TorontoPearson

Formoreaboutthefor-inloop,seeFor-InLoops.

Youcanalsoretrieveaniterablecollectionofadictionary’skeysorvaluesbyaccessingitskeysandvaluesproperties:

1 forairportCodeinairports.keys{

2 print("Airportcode:\(airportCode)")

3 }

4 //Airportcode:LHR

5 //Airportcode:YYZ

6

7 forairportNameinairports.values{

8 print("Airportname:\(airportName)")

9 }

10 //Airportname:LondonHeathrow

11 //Airportname:TorontoPearson

Ifyouneedtouseadictionary’skeysorvalueswithanAPIthattakesanArrayinstance,initializeanewarraywiththekeysorvaluesproperty:

1 letairportCodes=[String](airports.keys)

2 //airportCodesis["LHR","YYZ"]

3

4 letairportNames=[String](airports.values)

5 //airportNamesis["LondonHeathrow","TorontoPearson"]

Swift’sDictionarytypedoesnothaveadefinedordering.Toiterateoverthekeysorvaluesofadictionaryinaspecificorder,usethesorted()methodonitskeysorvaluesproperty.

ControlFlow

Swiftprovidesavarietyofcontrolflowstatements.Theseincludewhileloopstoperformataskmultipletimes;if,guard,andswitchstatementstoexecutedifferentbranchesofcodebasedoncertainconditions;andstatementssuchasbreakandcontinuetotransfertheflowofexecutiontoanotherpointinyourcode.

Swiftalsoprovidesafor-inloopthatmakesiteasytoiterateoverarrays,dictionaries,ranges,strings,andothersequences.

Swift’sswitchstatementisconsiderablymorepowerfulthanitscounterpartinmanyC-likelanguages.Casescanmatchmanydifferentpatterns,includingintervalmatches,tuples,andcaststoaspecifictype.Matchedvaluesinaswitchcasecanbeboundtotemporaryconstantsorvariablesforusewithinthecase’sbody,andcomplexmatchingconditionscanbeexpressedwithawhereclauseforeachcase.

For-InLoops

Youusethefor-inlooptoiterateoverasequence,suchasitemsinanarray,rangesofnumbers,orcharactersinastring.

Thisexampleusesafor-inlooptoiterateovertheitemsinanarray:

1 letnames=["Anna","Alex","Brian","Jack"]

2 fornameinnames{

3 print("Hello,\(name)!")

4 }

5 //Hello,Anna!

6 //Hello,Alex!

7 //Hello,Brian!

8 //Hello,Jack!

Youcanalsoiterateoveradictionarytoaccessitskey-valuepairs.Eachiteminthedictionaryisreturnedasa(key,value)tuplewhenthedictionaryisiterated,andyoucandecomposethe(key,value)tuple’smembersasexplicitlynamedconstantsforusewithinthebodyofthefor-inloop.Inthecodeexamplebelow,thedictionary’skeysaredecomposedintoaconstantcalledanimalName,andthedictionary’svaluesaredecomposedintoaconstantcalledlegCount.

1 letnumberOfLegs=["spider":8,"ant":6,"cat":4]

2 for(animalName,legCount)innumberOfLegs{

3 print("\(animalName)shave\(legCount)legs")

4 }

5 //catshave4legs

6 //antshave6legs

7 //spidershave8legs

ThecontentsofaDictionaryareinherentlyunordered,anditeratingoverthemdoesnotguaranteetheorderinwhichtheywillberetrieved.Inparticular,theorderyouinsertitemsintoaDictionarydoesn’tdefinetheordertheyareiterated.Formoreaboutarraysanddictionaries,seeCollectionTypes.

Youcanalsousefor-inloopswithnumericranges.Thisexampleprintsthefirstfewentriesinafive-timestable:

1 forindexin1...5{

2 print("\(index)times5is\(index*5)")

3 }

4 //1times5is5

5 //2times5is10

6 //3times5is15

7 //4times5is20

8 //5times5is25

Thesequencebeingiteratedoverisarangeofnumbersfrom1to5,inclusive,asindicatedbytheuseoftheclosedrangeoperator(...).Thevalueofindexissettothefirstnumberintherange(1),andthestatementsinsidetheloopareexecuted.Inthiscase,theloopcontainsonlyonestatement,whichprintsanentryfromthefive-timestableforthecurrentvalueofindex.Afterthestatementisexecuted,thevalueofindexisupdatedtocontainthesecondvalueintherange(2),andtheprint(_:separator:terminator:)functioniscalledagain.Thisprocesscontinuesuntiltheendoftherangeisreached.

Intheexampleabove,indexisaconstantwhosevalueisautomaticallysetatthestartofeachiterationoftheloop.Assuch,indexdoesnothavetobedeclaredbeforeitisused.Itisimplicitlydeclaredsimplybyitsinclusionintheloopdeclaration,withouttheneedforaletdeclarationkeyword.

Ifyoudon’tneedeachvaluefromasequence,youcanignorethevaluesbyusinganunderscoreinplaceofavariablename.

1 letbase=3

2 letpower=10

3 varanswer=1

4 for_in1...power{

5 answer*=base

6 }

7 print("\(base)tothepowerof\(power)is\(answer)")

8 //Prints"3tothepowerof10is59049"

Theexampleabovecalculatesthevalueofonenumbertothepowerofanother(inthiscase,3tothepowerof10).Itmultipliesastartingvalueof1(thatis,3tothepowerof0)by3,tentimes,usingaclosedrangethatstartswith1andendswith10.Forthiscalculation,theindividualcountervalueseachtimethroughtheloopareunnecessary—thecodesimplyexecutestheloopthecorrectnumberoftimes.Theunderscorecharacter(_)usedinplaceofaloopvariablecausestheindividualvaluestobeignoredanddoesnotprovideaccesstothecurrentvalueduringeachiterationoftheloop.

Insomesituations,youmightnotwanttouseclosedranges,whichincludeboth

endpoints.Considerdrawingthetickmarksforeveryminuteonawatchface.Youwanttodraw60tickmarks,startingwiththe0minute.Usethehalf-openrangeoperator(..<)toincludethelowerboundbutnottheupperbound.Formoreaboutranges,seeRangeOperators.

1 letminutes=60

2 fortickMarkin0..<minutes{

3 //renderthetickmarkeachminute(60times)

4 }

SomeusersmightwantfewertickmarksintheirUI.Theycouldpreferonemarkevery5minutesinstead.Usethestride(from:to:by:)functiontoskiptheunwantedmarks.

1 letminuteInterval=5

2 fortickMarkinstride(from:0,to:minutes,by:

minuteInterval){

3 //renderthetickmarkevery5minutes(0,5,10,15...

45,50,55)

4 }

Closedrangesarealsoavailable,byusingstride(from:through:by:)instead:

1 lethours=12

2 lethourInterval=3

3 fortickMarkinstride(from:3,through:hours,by:

hourInterval){

4 //renderthetickmarkevery3hours(3,6,9,12)

5 }

WhileLoops

Awhileloopperformsasetofstatementsuntilaconditionbecomesfalse.Thesekindsofloopsarebestusedwhenthenumberofiterationsisnotknownbeforethefirstiterationbegins.Swiftprovidestwokindsofwhileloops:

whileevaluatesitsconditionatthestartofeachpassthroughtheloop.

repeat-whileevaluatesitsconditionattheendofeachpassthroughtheloop.

WhileAwhileloopstartsbyevaluatingasinglecondition.Iftheconditionistrue,asetofstatementsisrepeateduntiltheconditionbecomesfalse.

Here’sthegeneralformofawhileloop:

while condition {

statements

}

ThisexampleplaysasimplegameofSnakesandLadders(alsoknownasChutesandLadders):

Therulesofthegameareasfollows:

Theboardhas25squares,andtheaimistolandonorbeyondsquare25.

Theplayer’sstartingsquareis“squarezero”,whichisjustoffthebottom-leftcorneroftheboard.

Eachturn,yourollasix-sideddiceandmovebythatnumberofsquares,followingthehorizontalpathindicatedbythedottedarrowabove.

Ifyourturnendsatthebottomofaladder,youmoveupthatladder.

Ifyourturnendsattheheadofasnake,youmovedownthatsnake.

ThegameboardisrepresentedbyanarrayofIntvalues.ItssizeisbasedonaconstantcalledfinalSquare,whichisusedtoinitializethearrayandalsotocheckforawinconditionlaterintheexample.Becausetheplayersstartofftheboard,on“squarezero”,theboardisinitializedwith26zeroIntvalues,not25.

1 letfinalSquare=25

2 varboard=[Int](repeating:0,count:finalSquare+1)

Somesquaresarethensettohavemorespecificvaluesforthesnakesandladders.Squareswithaladderbasehaveapositivenumbertomoveyouuptheboard,whereassquareswithasnakeheadhaveanegativenumbertomoveyoubackdowntheboard.

1 board[03]=+08;board[06]=+11;board[09]=+09;board[10]=

+02

2 board[14]=-10;board[19]=-11;board[22]=-02;board[24]=

-08

Square3containsthebottomofaladderthatmovesyouuptosquare11.Torepresentthis,board[03]isequalto+08,whichisequivalenttoanintegervalueof8(thedifferencebetween3and11).Toalignthevaluesandstatements,theunaryplusoperator(+i)isexplicitlyusedwiththeunaryminusoperator(-i)andnumberslowerthan10arepaddedwithzeros.(Neitherstylistictechniqueis

strictlynecessary,buttheyleadtoneatercode.)

1 varsquare=0

2 vardiceRoll=0

3 whilesquare<finalSquare{

4 //rollthedice

5 diceRoll+=1

6 ifdiceRoll==7{diceRoll=1}

7 //movebytherolledamount

8 square+=diceRoll

9 ifsquare<board.count{

10 //ifwe'restillontheboard,moveupordownfora

snakeoraladder

11 square+=board[square]

12 }

13 }

14 print("Gameover!")

Theexampleaboveusesaverysimpleapproachtodicerolling.Insteadofgeneratingarandomnumber,itstartswithadiceRollvalueof0.Eachtimethroughthewhileloop,diceRollisincrementedbyoneandisthencheckedtoseewhetherithasbecometoolarge.Wheneverthisreturnvalueequals7,thedicerollhasbecometoolargeandisresettoavalueof1.TheresultisasequenceofdiceRollvaluesthatisalways1,2,3,4,5,6,1,2andsoon.

Afterrollingthedice,theplayermovesforwardbydiceRollsquares.It’spossiblethatthedicerollmayhavemovedtheplayerbeyondsquare25,inwhichcasethegameisover.Tocopewiththisscenario,thecodechecksthatsquareislessthantheboardarray’scountproperty.Ifsquareisvalid,thevaluestoredinboard[square]isaddedtothecurrentsquarevaluetomovetheplayerupordownanyladdersorsnakes.

NOTE

Ifthischeckisnotperformed,board[square]mighttrytoaccessavalueoutsidetheboundsoftheboardarray,whichwouldtriggeraruntimeerror.

Thecurrentwhileloopexecutionthenends,andtheloop’sconditionischeckedtoseeiftheloopshouldbeexecutedagain.Iftheplayerhasmovedonorbeyondsquarenumber25,theloop’sconditionevaluatestofalseandthegameends.

Awhileloopisappropriateinthiscase,becausethelengthofthegameisnotclearatthestartofthewhileloop.Instead,theloopisexecuteduntilaparticularconditionissatisfied.

Repeat-WhileTheothervariationofthewhileloop,knownastherepeat-whileloop,performsasinglepassthroughtheloopblockfirst,beforeconsideringtheloop’scondition.Itthencontinuestorepeattheloopuntiltheconditionisfalse.

NOTE

Therepeat-whileloopinSwiftisanalogoustoado-whileloopinotherlanguages.

Here’sthegeneralformofarepeat-whileloop:

repeat{

statements

}while condition

Here’stheSnakesandLaddersexampleagain,writtenasarepeat-whileloopratherthanawhileloop.ThevaluesoffinalSquare,board,square,anddiceRollareinitializedinexactlythesamewayaswithawhileloop.

1 letfinalSquare=25

2 varboard=[Int](repeating:0,count:finalSquare+1)

3 board[03]=+08;board[06]=+11;board[09]=+09;board[10]=

+02

4 board[14]=-10;board[19]=-11;board[22]=-02;board[24]=

-08

5 varsquare=0

6 vardiceRoll=0

Inthisversionofthegame,thefirstactionintheloopistocheckforaladderorasnake.Noladderontheboardtakestheplayerstraighttosquare25,andsoitisn’tpossibletowinthegamebymovingupaladder.Therefore,it’ssafetocheckforasnakeoraladderasthefirstactionintheloop.

Atthestartofthegame,theplayerison“squarezero”.board[0]alwaysequals0andhasnoeffect.

1 repeat{

2 //moveupordownforasnakeorladder

3 square+=board[square]

4 //rollthedice

5 diceRoll+=1

6 ifdiceRoll==7{diceRoll=1}

7 //movebytherolledamount

8 square+=diceRoll

9 }whilesquare<finalSquare

10 print("Gameover!")

Afterthecodechecksforsnakesandladders,thediceisrolledandtheplayerismovedforwardbydiceRollsquares.Thecurrentloopexecutionthenends.

Theloop’scondition(whilesquare<finalSquare)isthesameasbefore,butthistimeit’snotevaluateduntiltheendofthefirstrunthroughtheloop.Thestructureoftherepeat-whileloopisbettersuitedtothisgamethanthewhileloopinthepreviousexample.Intherepeat-whileloopabove,square+=board[square]isalwaysexecutedimmediatelyaftertheloop’swhileconditionconfirmsthatsquareisstillontheboard.Thisbehaviorremovestheneedforthearrayboundscheckseeninthewhileloopversionofthegamedescribedearlier.

ConditionalStatements

Itisoftenusefultoexecutedifferentpiecesofcodebasedoncertainconditions.Youmightwanttorunanextrapieceofcodewhenanerroroccurs,ortodisplayamessagewhenavaluebecomestoohighortoolow.Todothis,youmakepartsofyourcodeconditional.

Swiftprovidestwowaystoaddconditionalbranchestoyourcode:theifstatementandtheswitchstatement.Typically,youusetheifstatementtoevaluatesimpleconditionswithonlyafewpossibleoutcomes.Theswitchstatementisbettersuitedtomorecomplexconditionswithmultiplepossiblepermutationsandisusefulinsituationswherepatternmatchingcanhelpselectanappropriatecodebranchtoexecute.

IfInitssimplestform,theifstatementhasasingleifcondition.Itexecutesasetofstatementsonlyifthatconditionistrue.

1 vartemperatureInFahrenheit=30

2 iftemperatureInFahrenheit<=32{

3 print("It'sverycold.Considerwearingascarf.")

4 }

5 //Prints"It'sverycold.Considerwearingascarf."

Theexampleabovecheckswhetherthetemperatureislessthanorequalto32degreesFahrenheit(thefreezingpointofwater).Ifitis,amessageisprinted.Otherwise,nomessageisprinted,andcodeexecutioncontinuesaftertheifstatement’sclosingbrace.

Theifstatementcanprovideanalternativesetofstatements,knownasanelseclause,forsituationswhentheifconditionisfalse.Thesestatementsareindicatedbytheelsekeyword.

1 temperatureInFahrenheit=40

2 iftemperatureInFahrenheit<=32{

3 print("It'sverycold.Considerwearingascarf.")

4 }else{

5 print("It'snotthatcold.Wearat-shirt.")

6 }

7 //Prints"It'snotthatcold.Wearat-shirt."

Oneofthesetwobranchesisalwaysexecuted.Becausethetemperaturehasincreasedto40degreesFahrenheit,itisnolongercoldenoughtoadvisewearingascarfandsotheelsebranchistriggeredinstead.

Youcanchainmultipleifstatementstogethertoconsideradditionalclauses.

1 temperatureInFahrenheit=90

2 iftemperatureInFahrenheit<=32{

3 print("It'sverycold.Considerwearingascarf.")

4 }elseiftemperatureInFahrenheit>=86{

5 print("It'sreallywarm.Don'tforgettowearsunscreen.")

6 }else{

7 print("It'snotthatcold.Wearat-shirt.")

8 }

9 //Prints"It'sreallywarm.Don'tforgettowearsunscreen."

Here,anadditionalifstatementwasaddedtorespondtoparticularlywarmtemperatures.Thefinalelseclauseremains,anditprintsaresponseforanytemperaturesthatareneithertoowarmnortoocold.

Thefinalelseclauseisoptional,however,andcanbeexcludedifthesetofconditionsdoesnotneedtobecomplete.

1 temperatureInFahrenheit=72

2 iftemperatureInFahrenheit<=32{

3 print("It'sverycold.Considerwearingascarf.")

4 }elseiftemperatureInFahrenheit>=86{

5 print("It'sreallywarm.Don'tforgettowearsunscreen.")

6 }

Becausethetemperatureisneithertoocoldnortoowarmtotriggertheiforelseifconditions,nomessageisprinted.

SwitchAswitchstatementconsidersavalueandcomparesitagainstseveralpossiblematchingpatterns.Itthenexecutesanappropriateblockofcode,basedonthefirstpatternthatmatchessuccessfully.Aswitchstatementprovidesanalternativetotheifstatementforrespondingtomultiplepotentialstates.

Initssimplestform,aswitchstatementcomparesavalueagainstoneormorevaluesofthesametype.

switch somevaluetoconsider {

case value1 :

respondtovalue1

case value2 ,

value3 :

respondtovalue2or3

default:

otherwise,dosomethingelse

}

Everyswitchstatementconsistsofmultiplepossiblecases,eachofwhichbeginswiththecasekeyword.Inadditiontocomparingagainstspecificvalues,Swiftprovidesseveralwaysforeachcasetospecifymorecomplexmatchingpatterns.Theseoptionsaredescribedlaterinthischapter.

Likethebodyofanifstatement,eachcaseisaseparatebranchofcodeexecution.Theswitchstatementdetermineswhichbranchshouldbeselected.

Thisprocedureisknownasswitchingonthevaluethatisbeingconsidered.

Everyswitchstatementmustbeexhaustive.Thatis,everypossiblevalueofthetypebeingconsideredmustbematchedbyoneoftheswitchcases.Ifit’snotappropriatetoprovideacaseforeverypossiblevalue,youcandefineadefaultcasetocoveranyvaluesthatarenotaddressedexplicitly.Thisdefaultcaseisindicatedbythedefaultkeyword,andmustalwaysappearlast.

ThisexampleusesaswitchstatementtoconsiderasinglelowercasecharactercalledsomeCharacter:

1 letsomeCharacter:Character="z"

2 switchsomeCharacter{

3 case"a":

4 print("Thefirstletterofthealphabet")

5 case"z":

6 print("Thelastletterofthealphabet")

7 default:

8 print("Someothercharacter")

9 }

10 //Prints"Thelastletterofthealphabet"

Theswitchstatement’sfirstcasematchesthefirstletteroftheEnglishalphabet,a,anditssecondcasematchesthelastletter,z.Becausetheswitchmusthaveacaseforeverypossiblecharacter,notjusteveryalphabeticcharacter,thisswitchstatementusesadefaultcasetomatchallcharactersotherthanaandz.Thisprovisionensuresthattheswitchstatementisexhaustive.

NoImplicitFallthrough

IncontrastwithswitchstatementsinCandObjective-C,switchstatementsinSwiftdonotfallthroughthebottomofeachcaseandintothenextonebydefault.Instead,theentireswitchstatementfinishesitsexecutionassoonasthefirstmatchingswitchcaseiscompleted,withoutrequiringanexplicitbreakstatement.Thismakestheswitchstatementsaferandeasiertousethantheone

inCandavoidsexecutingmorethanoneswitchcasebymistake.

NOTE

AlthoughbreakisnotrequiredinSwift,youcanuseabreakstatementtomatchandignoreaparticularcaseortobreakoutofamatchedcasebeforethatcasehascompleteditsexecution.Fordetails,seeBreakinaSwitchStatement.

Thebodyofeachcasemustcontainatleastoneexecutablestatement.Itisnotvalidtowritethefollowingcode,becausethefirstcaseisempty:

1 letanotherCharacter:Character="a"

2 switchanotherCharacter{

3 case"a"://Invalid,thecasehasanemptybody

4 case"A":

5 print("TheletterA")

6 default:

7 print("NottheletterA")

8 }

9 //Thiswillreportacompile-timeerror.

UnlikeaswitchstatementinC,thisswitchstatementdoesnotmatchboth"a"and"A".Rather,itreportsacompile-timeerrorthatcase"a":doesnotcontainanyexecutablestatements.Thisapproachavoidsaccidentalfallthroughfromonecasetoanotherandmakesforsafercodethatisclearerinitsintent.

Tomakeaswitchwithasinglecasethatmatchesboth"a"and"A",combinethetwovaluesintoacompoundcase,separatingthevalueswithcommas.

1 letanotherCharacter:Character="a"

2 switchanotherCharacter{

3 case"a","A":

4 print("TheletterA")

5 default:

6 print("NottheletterA")

7 }

8 //Prints"TheletterA"

Forreadability,acompoundcasecanalsobewrittenovermultiplelines.Formoreinformationaboutcompoundcases,seeCompoundCases.

NOTE

Toexplicitlyfallthroughattheendofaparticularswitchcase,usethefallthroughkeyword,asdescribedinFallthrough.

IntervalMatching

Valuesinswitchcasescanbecheckedfortheirinclusioninaninterval.Thisexampleusesnumberintervalstoprovideanatural-languagecountfornumbersofanysize:

1 letapproximateCount=62

2 letcountedThings="moonsorbitingSaturn"

3 letnaturalCount:String

4 switchapproximateCount{

5 case0:

6 naturalCount="no"

7 case1..<5:

8 naturalCount="afew"

9 case5..<12:

10 naturalCount="several"

11 case12..<100:

12 naturalCount="dozensof"

13 case100..<1000:

14 naturalCount="hundredsof"

15 default:

16 naturalCount="many"

17 }

18 print("Thereare\(naturalCount)\(countedThings).")

19 //Prints"TherearedozensofmoonsorbitingSaturn."

Intheaboveexample,approximateCountisevaluatedinaswitchstatement.Eachcasecomparesthatvaluetoanumberorinterval.BecausethevalueofapproximateCountfallsbetween12and100,naturalCountisassignedthevalue"dozensof",andexecutionistransferredoutoftheswitchstatement.

Tuples

Youcanusetuplestotestmultiplevaluesinthesameswitchstatement.Eachelementofthetuplecanbetestedagainstadifferentvalueorintervalofvalues.Alternatively,usetheunderscorecharacter(_),alsoknownasthewildcardpattern,tomatchanypossiblevalue.

Theexamplebelowtakesan(x,y)point,expressedasasimpletupleoftype(Int,Int),andcategorizesitonthegraphthatfollowstheexample.

1 letsomePoint=(1,1)

2 switchsomePoint{

3 case(0,0):

4 print("\(somePoint)isattheorigin")

5 case(_,0):

6 print("\(somePoint)isonthex-axis")

7 case(0,_):

8 print("\(somePoint)isonthey-axis")

9 case(-2...2,-2...2):

10 print("\(somePoint)isinsidethebox")

11 default:

12 print("\(somePoint)isoutsideofthebox")

13 }

14 //Prints"(1,1)isinsidethebox"

Theswitchstatementdetermineswhetherthepointisattheorigin(0,0),ontheredx-axis,ontheorangey-axis,insidetheblue4-by-4boxcenteredontheorigin,oroutsideofthebox.

UnlikeC,Swiftallowsmultipleswitchcasestoconsiderthesamevalueorvalues.Infact,thepoint(0,0)couldmatchallfourofthecasesinthisexample.However,ifmultiplematchesarepossible,thefirstmatchingcaseisalwaysused.Thepoint(0,0)wouldmatchcase(0,0)first,andsoallothermatchingcaseswouldbeignored.

ValueBindings

Aswitchcasecannamethevalueorvaluesitmatchestotemporaryconstantsorvariables,foruseinthebodyofthecase.Thisbehaviorisknownasvaluebinding,becausethevaluesareboundtotemporaryconstantsorvariableswithinthecase’sbody.

Theexamplebelowtakesan(x,y)point,expressedasatupleoftype(Int,Int),andcategorizesitonthegraphthatfollows:

1 letanotherPoint=(2,0)

2 switchanotherPoint{

3 case(letx,0):

4 print("onthex-axiswithanxvalueof\(x)")

5 case(0,lety):

6 print("onthey-axiswithayvalueof\(y)")

7 caselet(x,y):

8 print("somewhereelseat(\(x),\(y))")

9 }

10 //Prints"onthex-axiswithanxvalueof2"

Theswitchstatementdetermineswhetherthepointisontheredx-axis,ontheorangey-axis,orelsewhere(onneitheraxis).

Thethreeswitchcasesdeclareplaceholderconstantsxandy,whichtemporarilytakeononeorbothtuplevaluesfromanotherPoint.Thefirstcase,case(letx,0),matchesanypointwithayvalueof0andassignsthepoint’sxvaluetothetemporaryconstantx.Similarly,thesecondcase,case(0,lety),matchesanypointwithanxvalueof0andassignsthepoint’syvaluetothetemporaryconstanty.

Afterthetemporaryconstantsaredeclared,theycanbeusedwithinthecase’scodeblock.Here,theyareusedtoprintthecategorizationofthepoint.

Thisswitchstatementdoesnothaveadefaultcase.Thefinalcase,caselet

(x,y),declaresatupleoftwoplaceholderconstantsthatcanmatchanyvalue.BecauseanotherPointisalwaysatupleoftwovalues,thiscasematchesallpossibleremainingvalues,andadefaultcaseisnotneededtomaketheswitchstatementexhaustive.

Where

Aswitchcasecanuseawhereclausetocheckforadditionalconditions.

Theexamplebelowcategorizesan(x,y)pointonthefollowinggraph:

1 letyetAnotherPoint=(1,-1)

2 switchyetAnotherPoint{

3 caselet(x,y)wherex==y:

4 print("(\(x),\(y))isonthelinex==y")

5 caselet(x,y)wherex==-y:

6 print("(\(x),\(y))isonthelinex==-y")

7 caselet(x,y):

8 print("(\(x),\(y))isjustsomearbitrarypoint")

9 }

10 //Prints"(1,-1)isonthelinex==-y"

Theswitchstatementdetermineswhetherthepointisonthegreendiagonallinewherex==y,onthepurplediagonallinewherex==-y,orneither.

Thethreeswitchcasesdeclareplaceholderconstantsxandy,whichtemporarilytakeonthetwotuplevaluesfromyetAnotherPoint.Theseconstantsareusedaspartofawhereclause,tocreateadynamicfilter.Theswitchcasematchesthecurrentvalueofpointonlyifthewhereclause’sconditionevaluatestotrueforthatvalue.

Asinthepreviousexample,thefinalcasematchesallpossibleremainingvalues,andsoadefaultcaseisnotneededtomaketheswitchstatementexhaustive.

CompoundCases

Multipleswitchcasesthatsharethesamebodycanbecombinedbywritingseveralpatternsaftercase,withacommabetweeneachofthepatterns.Ifanyofthepatternsmatch,thenthecaseisconsideredtomatch.Thepatternscanbewrittenovermultiplelinesifthelistislong.Forexample:

1 letsomeCharacter:Character="e"

2 switchsomeCharacter{

3 case"a","e","i","o","u":

4 print("\(someCharacter)isavowel")

5 case"b","c","d","f","g","h","j","k","l","m",

6 "n","p","q","r","s","t","v","w","x","y","z":

7 print("\(someCharacter)isaconsonant")

8 default:

9 print("\(someCharacter)isnotavoweloraconsonant")

10 }

11 //Prints"eisavowel"

Theswitchstatement’sfirstcasematchesallfivelowercasevowelsintheEnglishlanguage.Similarly,itssecondcasematchesalllowercaseEnglishconsonants.Finally,thedefaultcasematchesanyothercharacter.

Compoundcasescanalsoincludevaluebindings.Allofthepatternsofacompoundcasehavetoincludethesamesetofvaluebindings,andeachbindinghastogetavalueofthesametypefromallofthepatternsinthecompoundcase.Thisensuresthat,nomatterwhichpartofthecompoundcasematched,thecodeinthebodyofthecasecanalwaysaccessavalueforthebindingsandthatthevaluealwayshasthesametype.

1 letstillAnotherPoint=(9,0)

2 switchstillAnotherPoint{

3 case(letdistance,0),(0,letdistance):

4 print("Onanaxis,\(distance)fromtheorigin")

5 default:

6 print("Notonanaxis")

7 }

8 //Prints"Onanaxis,9fromtheorigin"

Thecaseabovehastwopatterns:(letdistance,0)matchespointsonthex-axisand(0,letdistance)matchespointsonthey-axis.Bothpatternsincludeabindingfordistanceanddistanceisanintegerinbothpatterns—whichmeansthatthecodeinthebodyofthecasecanalwaysaccessavaluefordistance.

ControlTransferStatements

Controltransferstatementschangetheorderinwhichyourcodeisexecuted,bytransferringcontrolfromonepieceofcodetoanother.Swifthasfivecontroltransferstatements:

continue

break

fallthrough

return

throw

Thecontinue,break,andfallthroughstatementsaredescribedbelow.ThereturnstatementisdescribedinFunctions,andthethrowstatementisdescribedinPropagatingErrorsUsingThrowingFunctions.

ContinueThecontinuestatementtellsalooptostopwhatitisdoingandstartagainatthebeginningofthenextiterationthroughtheloop.Itsays“Iamdonewiththecurrentloopiteration”withoutleavingtheloopaltogether.

Thefollowingexampleremovesallvowelsandspacesfromalowercasestringtocreateacrypticpuzzlephrase:

1 letpuzzleInput="greatmindsthinkalike"

2 varpuzzleOutput=""

3 letcharactersToRemove:[Character]=["a","e","i","o","u",

""]

4 forcharacterinpuzzleInput{

5 ifcharactersToRemove.contains(character){

6 continue

7 }

8 puzzleOutput.append(character)

9 }

10 print(puzzleOutput)

11 //Prints"grtmndsthnklk"

Thecodeabovecallsthecontinuekeywordwheneveritmatchesavoweloraspace,causingthecurrentiterationofthelooptoendimmediatelyandtojumpstraighttothestartofthenextiteration.

BreakThebreakstatementendsexecutionofanentirecontrolflowstatementimmediately.Thebreakstatementcanbeusedinsideaswitchorloopstatementwhenyouwanttoterminatetheexecutionoftheswitchorloopstatementearlierthanwouldotherwisebethecase.

BreakinaLoopStatement

Whenusedinsidealoopstatement,breakendstheloop’sexecutionimmediatelyandtransferscontroltothecodeaftertheloop’sclosingbrace(}).Nofurthercodefromthecurrentiterationoftheloopisexecuted,andnofurtheriterationsofthelooparestarted.

BreakinaSwitchStatement

Whenusedinsideaswitchstatement,breakcausestheswitchstatementtoenditsexecutionimmediatelyandtotransfercontroltothecodeaftertheswitchstatement’sclosingbrace(}).

Thisbehaviorcanbeusedtomatchandignoreoneormorecasesinaswitchstatement.BecauseSwift’sswitchstatementisexhaustiveanddoesnotallowemptycases,itissometimesnecessarytodeliberatelymatchandignoreacaseinordertomakeyourintentionsexplicit.Youdothisbywritingthebreakstatementastheentirebodyofthecaseyouwanttoignore.Whenthatcaseismatchedbytheswitchstatement,thebreakstatementinsidethecaseendstheswitchstatement’sexecutionimmediately.

NOTE

Aswitchcasethatcontainsonlyacommentisreportedasacompile-timeerror.Commentsarenotstatementsanddonotcauseaswitchcasetobeignored.Alwaysuseabreakstatementtoignoreaswitchcase.

ThefollowingexampleswitchesonaCharactervalueanddetermineswhetheritrepresentsanumbersymbolinoneoffourlanguages.Forbrevity,multiplevaluesarecoveredinasingleswitchcase.

1 letnumberSymbol:Character="" //Chinesesymbolforthe

number3

2 varpossibleIntegerValue:Int?

3 switchnumberSymbol{

4 case"1","١","" ,"�":

5 possibleIntegerValue=1

6 case"2","٢","" ,"�":

7 possibleIntegerValue=2

8 case"3","٣","" ,"�":

9 possibleIntegerValue=3

10 case"4","٤","" ,"�":

11 possibleIntegerValue=4

12 default:

13 break

14 }

15 ifletintegerValue=possibleIntegerValue{

16 print("Theintegervalueof\(numberSymbol)is\

(integerValue).")

17 }else{

18 print("Anintegervaluecouldnotbefoundfor\

(numberSymbol).")

19 }

20 //Prints"Theintegervalueofis3."

ThisexamplechecksnumberSymboltodeterminewhetheritisaLatin,Arabic,Chinese,orThaisymbolforthenumbers1to4.Ifamatchisfound,oneoftheswitchstatement’scasessetsanoptionalInt?variablecalledpossibleIntegerValuetoanappropriateintegervalue.

Aftertheswitchstatementcompletesitsexecution,theexampleusesoptionalbindingtodeterminewhetheravaluewasfound.ThepossibleIntegerValue

variablehasanimplicitinitialvalueofnilbyvirtueofbeinganoptionaltype,andsotheoptionalbindingwillsucceedonlyifpossibleIntegerValuewassettoanactualvaluebyoneoftheswitchstatement’sfirstfourcases.

Becauseit’snotpracticaltolisteverypossibleCharactervalueintheexampleabove,adefaultcasehandlesanycharactersthatarenotmatched.Thisdefaultcasedoesnotneedtoperformanyaction,andsoitiswrittenwithasinglebreakstatementasitsbody.Assoonasthedefaultcaseismatched,thebreakstatementendstheswitchstatement’sexecution,andcodeexecutioncontinuesfromtheifletstatement.

FallthroughInSwift,switchstatementsdon’tfallthroughthebottomofeachcaseandintothenextone.Thatis,theentireswitchstatementcompletesitsexecutionassoonasthefirstmatchingcaseiscompleted.Bycontrast,Crequiresyoutoinsertanexplicitbreakstatementattheendofeveryswitchcasetopreventfallthrough.AvoidingdefaultfallthroughmeansthatSwiftswitchstatementsaremuchmoreconciseandpredictablethantheircounterpartsinC,andthustheyavoidexecutingmultipleswitchcasesbymistake.

IfyouneedC-stylefallthroughbehavior,youcanoptintothisbehavioronacase-by-casebasiswiththefallthroughkeyword.Theexamplebelowusesfallthroughtocreateatextualdescriptionofanumber.

1 letintegerToDescribe=5

2 vardescription="Thenumber\(integerToDescribe)is"

3 switchintegerToDescribe{

4 case2,3,5,7,11,13,17,19:

5 description+="aprimenumber,andalso"

6 fallthrough

7 default:

8 description+="aninteger."

9 }

10 print(description)

11 //Prints"Thenumber5isaprimenumber,andalsoan

integer."

ThisexampledeclaresanewStringvariablecalleddescriptionandassignsitaninitialvalue.ThefunctionthenconsidersthevalueofintegerToDescribeusingaswitchstatement.IfthevalueofintegerToDescribeisoneoftheprimenumbersinthelist,thefunctionappendstexttotheendofdescription,tonotethatthenumberisprime.Itthenusesthefallthroughkeywordto“fallinto”thedefaultcaseaswell.Thedefaultcaseaddssomeextratexttotheendofthedescription,andtheswitchstatementiscomplete.

UnlessthevalueofintegerToDescribeisinthelistofknownprimenumbers,itisnotmatchedbythefirstswitchcaseatall.Becausetherearenootherspecificcases,integerToDescribeismatchedbythedefaultcase.

Aftertheswitchstatementhasfinishedexecuting,thenumber’sdescriptionisprintedusingtheprint(_:separator:terminator:)function.Inthisexample,thenumber5iscorrectlyidentifiedasaprimenumber.

NOTE

Thefallthroughkeyworddoesnotcheckthecaseconditionsfortheswitchcasethatitcausesexecutiontofallinto.Thefallthroughkeywordsimplycausescodeexecutiontomovedirectlytothestatementsinsidethenextcase(ordefaultcase)block,asinC’sstandardswitchstatementbehavior.

LabeledStatementsInSwift,youcannestloopsandconditionalstatementsinsideotherloopsandconditionalstatementstocreatecomplexcontrolflowstructures.However,loopsandconditionalstatementscanbothusethebreakstatementtoendtheirexecutionprematurely.Therefore,itissometimesusefultobeexplicitaboutwhichlooporconditionalstatementyouwantabreakstatementtoterminate.Similarly,ifyouhavemultiplenestedloops,itcanbeusefultobeexplicitaboutwhichloopthecontinuestatementshouldaffect.

Toachievetheseaims,youcanmarkaloopstatementorconditionalstatementwithastatementlabel.Withaconditionalstatement,youcanuseastatementlabelwiththebreakstatementtoendtheexecutionofthelabeledstatement.Withaloopstatement,youcanuseastatementlabelwiththebreakorcontinuestatementtoendorcontinuetheexecutionofthelabeledstatement.

Alabeledstatementisindicatedbyplacingalabelonthesamelineasthestatement’sintroducerkeyword,followedbyacolon.Here’sanexampleofthissyntaxforawhileloop,althoughtheprincipleisthesameforallloopsandswitchstatements:

labelname :while condition {

statements

}

ThefollowingexampleusesthebreakandcontinuestatementswithalabeledwhileloopforanadaptedversionoftheSnakesandLaddersgamethatyousawearlierinthischapter.Thistimearound,thegamehasanextrarule:

Towin,youmustlandexactlyonsquare25.

Ifaparticulardicerollwouldtakeyoubeyondsquare25,youmustrollagainuntilyourolltheexactnumberneededtolandonsquare25.

Thegameboardisthesameasbefore.

ThevaluesoffinalSquare,board,square,anddiceRollareinitializedinthesamewayasbefore:

1 letfinalSquare=25

2 varboard=[Int](repeating:0,count:finalSquare+1)

3 board[03]=+08;board[06]=+11;board[09]=+09;board[10]=

+02

4 board[14]=-10;board[19]=-11;board[22]=-02;board[24]=

-08

5 varsquare=0

6 vardiceRoll=0

Thisversionofthegameusesawhileloopandaswitchstatementtoimplementthegame’slogic.ThewhileloophasastatementlabelcalledgameLooptoindicatethatitisthemaingameloopfortheSnakesandLaddersgame.

Thewhileloop’sconditioniswhilesquare!=finalSquare,toreflectthatyoumustlandexactlyonsquare25.

1 gameLoop:whilesquare!=finalSquare{

2 diceRoll+=1

3 ifdiceRoll==7{diceRoll=1}

4 switchsquare+diceRoll{

5 casefinalSquare:

6 //diceRollwillmoveustothefinalsquare,sothe

gameisover

7 breakgameLoop

8 caseletnewSquarewherenewSquare>finalSquare:

9 //diceRollwillmoveusbeyondthefinalsquare,so

rollagain

10 continuegameLoop

11 default:

12 //thisisavalidmove,sofindoutitseffect

13 square+=diceRoll

14 square+=board[square]

15 }

16 }

17 print("Gameover!")

Thediceisrolledatthestartofeachloop.Ratherthanmovingtheplayerimmediately,theloopusesaswitchstatementtoconsidertheresultofthemoveandtodeterminewhetherthemoveisallowed:

Ifthedicerollwillmovetheplayerontothefinalsquare,thegameisover.ThebreakgameLoopstatementtransferscontroltothefirstlineofcodeoutsideofthewhileloop,whichendsthegame.

Ifthedicerollwillmovetheplayerbeyondthefinalsquare,themoveisinvalidandtheplayerneedstorollagain.ThecontinuegameLoopstatementendsthecurrentwhileloopiterationandbeginsthenextiterationoftheloop.

Inallothercases,thedicerollisavalidmove.TheplayermovesforwardbydiceRollsquares,andthegamelogicchecksforanysnakesandladders.

Theloopthenends,andcontrolreturnstothewhileconditiontodecidewhetheranotherturnisrequired.

NOTE

IfthebreakstatementabovedidnotusethegameLooplabel,itwouldbreakoutoftheswitchstatement,notthewhilestatement.UsingthegameLooplabelmakesitclearwhichcontrolstatementshouldbeterminated.

ItisnotstrictlynecessarytousethegameLooplabelwhencallingcontinuegameLooptojumptothenextiterationoftheloop.Thereisonlyoneloopinthegame,andthereforenoambiguityastowhichloopthecontinuestatementwillaffect.However,thereisnoharminusingthegameLooplabelwiththecontinuestatement.Doingsoisconsistentwiththelabel’susealongsidethebreakstatementandhelpsmakethegame’slogicclearertoreadandunderstand.

EarlyExit

Aguardstatement,likeanifstatement,executesstatementsdependingontheBooleanvalueofanexpression.Youuseaguardstatementtorequirethataconditionmustbetrueinorderforthecodeaftertheguardstatementtobeexecuted.Unlikeanifstatement,aguardstatementalwayshasanelseclause—thecodeinsidetheelseclauseisexecutediftheconditionisnottrue.

1 funcgreet(person:[String:String]){

2 guardletname=person["name"]else{

3 return

4 }

5

6 print("Hello\(name)!")

7

8 guardletlocation=person["location"]else{

9 print("Ihopetheweatherisnicenearyou.")

10 return

11 }

12

13 print("Ihopetheweatherisnicein\(location).")

14 }

15

16 greet(person:["name":"John"])

17 //Prints"HelloJohn!"

18 //Prints"Ihopetheweatherisnicenearyou."

19 greet(person:["name":"Jane","location":"Cupertino"])

20 //Prints"HelloJane!"

21 //Prints"IhopetheweatherisniceinCupertino."

Iftheguardstatement’sconditionismet,codeexecutioncontinuesaftertheguardstatement’sclosingbrace.Anyvariablesorconstantsthatwereassignedvaluesusinganoptionalbindingaspartoftheconditionareavailablefortherestofthecodeblockthattheguardstatementappearsin.

Ifthatconditionisnotmet,thecodeinsidetheelsebranchisexecuted.Thatbranchmusttransfercontroltoexitthecodeblockinwhichtheguardstatementappears.Itcandothiswithacontroltransferstatementsuchasreturn,break,continue,orthrow,oritcancallafunctionormethodthatdoesn’treturn,suchasfatalError(_:file:line:).

Usingaguardstatementforrequirementsimprovesthereadabilityofyourcode,comparedtodoingthesamecheckwithanifstatement.Itletsyouwritethecodethat’stypicallyexecutedwithoutwrappingitinanelseblock,anditletsyoukeepthecodethathandlesaviolatedrequirementnexttotherequirement.

CheckingAPIAvailability

Swifthasbuilt-insupportforcheckingAPIavailability,whichensuresthatyoudon’taccidentallyuseAPIsthatareunavailableonagivendeploymenttarget.

ThecompilerusesavailabilityinformationintheSDKtoverifythatallofthe

APIsusedinyourcodeareavailableonthedeploymenttargetspecifiedbyyourproject.SwiftreportsanerroratcompiletimeifyoutrytouseanAPIthatisn’tavailable.

Youuseanavailabilityconditioninaniforguardstatementtoconditionallyexecuteablockofcode,dependingonwhethertheAPIsyouwanttouseareavailableatruntime.ThecompilerusestheinformationfromtheavailabilityconditionwhenitverifiesthattheAPIsinthatblockofcodeareavailable.

1 if#available(iOS10,macOS10.12,*){

2 //UseiOS10APIsoniOS,andusemacOS10.12APIson

macOS

3 }else{

4 //FallbacktoearlieriOSandmacOSAPIs

5 }

TheavailabilityconditionabovespecifiesthatiniOS,thebodyoftheifstatementexecutesonlyiniOS10andlater;inmacOS,onlyinmacOS10.12andlater.Thelastargument,*,isrequiredandspecifiesthatonanyotherplatform,thebodyoftheifexecutesontheminimumdeploymenttargetspecifiedbyyourtarget.

Initsgeneralform,theavailabilityconditiontakesalistofplatformnamesandversions.YouuseplatformnamessuchasiOS,macOS,watchOS,andtvOS—forthefulllist,seeDeclarationAttributes.InadditiontospecifyingmajorversionnumberslikeiOS8ormacOS10.10,youcanspecifyminorversionsnumberslikeiOS11.2.6andmacOS10.13.3.

if#available( platformname version , ... ,*){

statementstoexecuteiftheAPIsareavailable

}else{

fallbackstatementstoexecuteiftheAPIsareunavailable

}

Functions

Functionsareself-containedchunksofcodethatperformaspecifictask.Yougiveafunctionanamethatidentifieswhatitdoes,andthisnameisusedto“call”thefunctiontoperformitstaskwhenneeded.

Swift’sunifiedfunctionsyntaxisflexibleenoughtoexpressanythingfromasimpleC-stylefunctionwithnoparameternamestoacomplexObjective-C-stylemethodwithnamesandargumentlabelsforeachparameter.Parameterscanprovidedefaultvaluestosimplifyfunctioncallsandcanbepassedasin-outparameters,whichmodifyapassedvariableoncethefunctionhascompleteditsexecution.

EveryfunctioninSwifthasatype,consistingofthefunction’sparametertypesandreturntype.YoucanusethistypelikeanyothertypeinSwift,whichmakesiteasytopassfunctionsasparameterstootherfunctions,andtoreturnfunctionsfromfunctions.Functionscanalsobewrittenwithinotherfunctionstoencapsulateusefulfunctionalitywithinanestedfunctionscope.

DefiningandCallingFunctions

Whenyoudefineafunction,youcanoptionallydefineoneormorenamed,typedvaluesthatthefunctiontakesasinput,knownasparameters.Youcanalsooptionallydefineatypeofvaluethatthefunctionwillpassbackasoutputwhenitisdone,knownasitsreturntype.

Everyfunctionhasafunctionname,whichdescribesthetaskthatthefunctionperforms.Touseafunction,you“call”thatfunctionwithitsnameandpassitinputvalues(knownasarguments)thatmatchthetypesofthefunction’sparameters.Afunction’sargumentsmustalwaysbeprovidedinthesameorderasthefunction’sparameterlist.

Thefunctionintheexamplebelowiscalledgreet(person:),becausethat’swhatitdoes—ittakesaperson’snameasinputandreturnsagreetingforthatperson.

Toaccomplishthis,youdefineoneinputparameter—aStringvaluecalledperson—andareturntypeofString,whichwillcontainagreetingforthatperson:

1 funcgreet(person:String)->String{

2 letgreeting="Hello,"+person+"!"

3 returngreeting

4 }

Allofthisinformationisrolledupintothefunction’sdefinition,whichisprefixedwiththefunckeyword.Youindicatethefunction’sreturntypewiththereturnarrow->(ahyphenfollowedbyarightanglebracket),whichisfollowedbythenameofthetypetoreturn.

Thedefinitiondescribeswhatthefunctiondoes,whatitexpectstoreceive,andwhatitreturnswhenitisdone.Thedefinitionmakesiteasyforthefunctiontobecalledunambiguouslyfromelsewhereinyourcode:

1 print(greet(person:"Anna"))

2 //Prints"Hello,Anna!"

3 print(greet(person:"Brian"))

4 //Prints"Hello,Brian!"

Youcallthegreet(person:)functionbypassingitaStringvalueafterthepersonargumentlabel,suchasgreet(person:"Anna").BecausethefunctionreturnsaStringvalue,greet(person:)canbewrappedinacalltotheprint(_:separator:terminator:)functiontoprintthatstringandseeitsreturnvalue,asshownabove.

NOTE

Theprint(_:separator:terminator:)functiondoesn’thavealabelforitsfirstargument,anditsotherargumentsareoptionalbecausetheyhaveadefaultvalue.ThesevariationsonfunctionsyntaxarediscussedbelowinFunctionArgumentLabelsandParameterNamesandDefaultParameterValues.

Thebodyofthegreet(person:)functionstartsbydefininganewStringconstantcalledgreetingandsettingittoasimplegreetingmessage.Thisgreetingisthenpassedbackoutofthefunctionusingthereturnkeyword.Inthelineofcodethatsaysreturngreeting,thefunctionfinishesitsexecutionandreturnsthecurrentvalueofgreeting.

Youcancallthegreet(person:)functionmultipletimeswithdifferentinputvalues.Theexampleaboveshowswhathappensifitiscalledwithaninputvalueof"Anna",andaninputvalueof"Brian".Thefunctionreturnsatailoredgreetingineachcase.

Tomakethebodyofthisfunctionshorter,youcancombinethemessagecreationandthereturnstatementintooneline:

1 funcgreetAgain(person:String)->String{

2 return"Helloagain,"+person+"!"

3 }

4 print(greetAgain(person:"Anna"))

5 //Prints"Helloagain,Anna!"

FunctionParametersandReturnValues

FunctionparametersandreturnvaluesareextremelyflexibleinSwift.Youcandefineanythingfromasimpleutilityfunctionwithasingleunnamedparametertoacomplexfunctionwithexpressiveparameternamesanddifferentparameteroptions.

FunctionsWithoutParametersFunctionsarenotrequiredtodefineinputparameters.Here’safunctionwithnoinputparameters,whichalwaysreturnsthesameStringmessagewheneveritiscalled:

1 funcsayHelloWorld()->String{

2 return"hello,world"

3 }

4 print(sayHelloWorld())

5 //Prints"hello,world"

Thefunctiondefinitionstillneedsparenthesesafterthefunction’sname,eventhoughitdoesnottakeanyparameters.Thefunctionnameisalsofollowedbyanemptypairofparentheseswhenthefunctioniscalled.

FunctionsWithMultipleParametersFunctionscanhavemultipleinputparameters,whicharewrittenwithinthefunction’sparentheses,separatedbycommas.

Thisfunctiontakesaperson’snameandwhethertheyhavealreadybeengreetedasinput,andreturnsanappropriategreetingforthatperson:

1 funcgreet(person:String,alreadyGreeted:Bool)->String{

2 ifalreadyGreeted{

3 returngreetAgain(person:person)

4 }else{

5 returngreet(person:person)

6 }

7 }

8 print(greet(person:"Tim",alreadyGreeted:true))

9 //Prints"Helloagain,Tim!"

Youcallthegreet(person:alreadyGreeted:)functionbypassingitbothaStringargumentvaluelabeledpersonandaBoolargumentvaluelabeledalreadyGreetedinparentheses,separatedbycommas.Notethatthisfunctionisdistinctfromthegreet(person:)functionshowninanearliersection.Althoughbothfunctionshavenamesthatbeginwithgreet,the

greet(person:alreadyGreeted:)functiontakestwoargumentsbutthegreet(person:)functiontakesonlyone.

FunctionsWithoutReturnValuesFunctionsarenotrequiredtodefineareturntype.Here’saversionofthegreet(person:)function,whichprintsitsownStringvalueratherthanreturningit:

1 funcgreet(person:String){

2 print("Hello,\(person)!")

3 }

4 greet(person:"Dave")

5 //Prints"Hello,Dave!"

Becauseitdoesnotneedtoreturnavalue,thefunction’sdefinitiondoesnotincludethereturnarrow(->)orareturntype.

NOTE

Strictlyspeaking,thisversionofthegreet(person:)functiondoesstillreturnavalue,eventhoughnoreturnvalueisdefined.FunctionswithoutadefinedreturntypereturnaspecialvalueoftypeVoid.Thisissimplyanemptytuple,whichiswrittenas().

Thereturnvalueofafunctioncanbeignoredwhenitiscalled:

1 funcprintAndCount(string:String)->Int{

2 print(string)

3 returnstring.count

4 }

5 funcprintWithoutCounting(string:String){

6 let_=printAndCount(string:string)

7 }

8 printAndCount(string:"hello,world")

9 //prints"hello,world"andreturnsavalueof12

10 printWithoutCounting(string:"hello,world")

11 //prints"hello,world"butdoesnotreturnavalue

Thefirstfunction,printAndCount(string:),printsastring,andthenreturnsitscharactercountasanInt.Thesecondfunction,printWithoutCounting(string:),callsthefirstfunction,butignoresitsreturnvalue.Whenthesecondfunctioniscalled,themessageisstillprintedbythefirstfunction,butthereturnedvalueisnotused.

NOTE

Returnvaluescanbeignored,butafunctionthatsaysitwillreturnavaluemustalwaysdoso.Afunctionwithadefinedreturntypecannotallowcontroltofalloutofthebottomofthefunctionwithoutreturningavalue,andattemptingtodosowillresultinacompile-timeerror.

FunctionswithMultipleReturnValuesYoucanuseatupletypeasthereturntypeforafunctiontoreturnmultiplevaluesaspartofonecompoundreturnvalue.

TheexamplebelowdefinesafunctioncalledminMax(array:),whichfindsthesmallestandlargestnumbersinanarrayofIntvalues:

1 funcminMax(array:[Int])->(min:Int,max:Int){

2 varcurrentMin=array[0]

3 varcurrentMax=array[0]

4 forvalueinarray[1..<array.count]{

5 ifvalue<currentMin{

6 currentMin=value

7 }elseifvalue>currentMax{

8 currentMax=value

9 }

10 }

11 return(currentMin,currentMax)

12 }

TheminMax(array:)functionreturnsatuplecontainingtwoIntvalues.Thesevaluesarelabeledminandmaxsothattheycanbeaccessedbynamewhenqueryingthefunction’sreturnvalue.

ThebodyoftheminMax(array:)functionstartsbysettingtwoworkingvariablescalledcurrentMinandcurrentMaxtothevalueofthefirstintegerinthearray.ThefunctiontheniteratesovertheremainingvaluesinthearrayandcheckseachvaluetoseeifitissmallerorlargerthanthevaluesofcurrentMinandcurrentMaxrespectively.Finally,theoverallminimumandmaximumvaluesarereturnedasatupleoftwoIntvalues.

Becausethetuple’smembervaluesarenamedaspartofthefunction’sreturntype,theycanbeaccessedwithdotsyntaxtoretrievetheminimumandmaximumfoundvalues:

1 letbounds=minMax(array:[8,-6,2,109,3,71])

2 print("minis\(bounds.min)andmaxis\(bounds.max)")

3 //Prints"minis-6andmaxis109"

Notethatthetuple’smembersdonotneedtobenamedatthepointthatthetupleisreturnedfromthefunction,becausetheirnamesarealreadyspecifiedaspartofthefunction’sreturntype.

OptionalTupleReturnTypes

Ifthetupletypetobereturnedfromafunctionhasthepotentialtohave“novalue”fortheentiretuple,youcanuseanoptionaltuplereturntypetoreflectthefactthattheentiretuplecanbenil.Youwriteanoptionaltuplereturntypebyplacingaquestionmarkafterthetupletype’sclosingparenthesis,suchas(Int,Int)?or(String,Int,Bool)?.

NOTE

Anoptionaltupletypesuchas(Int,Int)?isdifferentfromatuplethatcontainsoptionaltypes

suchas(Int?,Int?).Withanoptionaltupletype,theentiretupleisoptional,notjusteachindividualvaluewithinthetuple.

TheminMax(array:)functionabovereturnsatuplecontainingtwoIntvalues.However,thefunctiondoesnotperformanysafetychecksonthearrayitispassed.Ifthearrayargumentcontainsanemptyarray,theminMax(array:)function,asdefinedabove,willtriggeraruntimeerrorwhenattemptingtoaccessarray[0].

Tohandleanemptyarraysafely,writetheminMax(array:)functionwithanoptionaltuplereturntypeandreturnavalueofnilwhenthearrayisempty:

1 funcminMax(array:[Int])->(min:Int,max:Int)?{

2 ifarray.isEmpty{returnnil}

3 varcurrentMin=array[0]

4 varcurrentMax=array[0]

5 forvalueinarray[1..<array.count]{

6 ifvalue<currentMin{

7 currentMin=value

8 }elseifvalue>currentMax{

9 currentMax=value

10 }

11 }

12 return(currentMin,currentMax)

13 }

YoucanuseoptionalbindingtocheckwhetherthisversionoftheminMax(array:)functionreturnsanactualtuplevalueornil:

1 ifletbounds=minMax(array:[8,-6,2,109,3,71]){

2 print("minis\(bounds.min)andmaxis\(bounds.max)")

3 }

4 //Prints"minis-6andmaxis109"

FunctionsWithanImplicitReturnIftheentirebodyofthefunctionisasingleexpression,thefunctionimplicitlyreturnsthatexpression.Forexample,bothfunctionsbelowhavethesamebehavior:

1 funcgreeting(forperson:String)->String{

2 "Hello,"+person+"!"

3 }

4 print(greeting(for:"Dave"))

5 //Prints"Hello,Dave!"

6

7 funcanotherGreeting(forperson:String)->String{

8 return"Hello,"+person+"!"

9 }

10 print(anotherGreeting(for:"Dave"))

11 //Prints"Hello,Dave!"

Theentiredefinitionofthegreeting(for:)functionisthegreetingmessagethatitreturns,whichmeansitcanusethisshorterform.TheanotherGreeting(for:)functionreturnsthesamegreetingmessage,usingthereturnkeywordlikealongerfunction.Anyfunctionthatyouwriteasjustonereturnlinecanomitthereturn.

Asyou’llseeinShorthandGetterDeclaration,propertygetterscanalsouseanimplicitreturn.

FunctionArgumentLabelsandParameterNames

Eachfunctionparameterhasbothanargumentlabelandaparametername.Theargumentlabelisusedwhencallingthefunction;eachargumentiswritteninthefunctioncallwithitsargumentlabelbeforeit.Theparameternameisusedintheimplementationofthefunction.Bydefault,parametersusetheirparametername

astheirargumentlabel.

1 funcsomeFunction(firstParameterName:Int,secondParameterName:

Int){

2 //Inthefunctionbody,firstParameterNameand

secondParameterName

3 //refertotheargumentvaluesforthefirstandsecond

parameters.

4 }

5 someFunction(firstParameterName:1,secondParameterName:2)

Allparametersmusthaveuniquenames.Althoughit’spossibleformultipleparameterstohavethesameargumentlabel,uniqueargumentlabelshelpmakeyourcodemorereadable.

SpecifyingArgumentLabelsYouwriteanargumentlabelbeforetheparametername,separatedbyaspace:

1 funcsomeFunction(argumentLabelparameterName:Int){

2 //Inthefunctionbody,parameterNamereferstothe

argumentvalue

3 //forthatparameter.

4 }

Here’savariationofthegreet(person:)functionthattakesaperson’snameandhometownandreturnsagreeting:

1 funcgreet(person:String,fromhometown:String)->String{

2 return"Hello\(person)!Gladyoucouldvisitfrom\

(hometown)."

3 }

4 print(greet(person:"Bill",from:"Cupertino"))

5 //Prints"HelloBill!GladyoucouldvisitfromCupertino."

Theuseofargumentlabelscanallowafunctiontobecalledinanexpressive,sentence-likemanner,whilestillprovidingafunctionbodythatisreadableandclearinintent.

OmittingArgumentLabelsIfyoudon’twantanargumentlabelforaparameter,writeanunderscore(_)insteadofanexplicitargumentlabelforthatparameter.

1 funcsomeFunction(_firstParameterName:Int,

secondParameterName:Int){

2 //Inthefunctionbody,firstParameterNameand

secondParameterName

3 //refertotheargumentvaluesforthefirstandsecond

parameters.

4 }

5 someFunction(1,secondParameterName:2)

Ifaparameterhasanargumentlabel,theargumentmustbelabeledwhenyoucallthefunction.

DefaultParameterValuesYoucandefineadefaultvalueforanyparameterinafunctionbyassigningavaluetotheparameterafterthatparameter’stype.Ifadefaultvalueisdefined,youcanomitthatparameterwhencallingthefunction.

1 funcsomeFunction(parameterWithoutDefault:Int,

parameterWithDefault:Int=12){

2 //Ifyouomitthesecondargumentwhencallingthis

function,then

3 //thevalueofparameterWithDefaultis12insidethe

functionbody.

4 }

5 someFunction(parameterWithoutDefault:3,parameterWithDefault:

6)//parameterWithDefaultis6

6 someFunction(parameterWithoutDefault:4)//

parameterWithDefaultis12

Placeparametersthatdon’thavedefaultvaluesatthebeginningofafunction’sparameterlist,beforetheparametersthathavedefaultvalues.Parametersthatdon’thavedefaultvaluesareusuallymoreimportanttothefunction’smeaning—writingthemfirstmakesiteasiertorecognizethatthesamefunctionisbeingcalled,regardlessofwhetheranydefaultparametersareomitted.

VariadicParametersAvariadicparameteracceptszeroormorevaluesofaspecifiedtype.Youuseavariadicparametertospecifythattheparametercanbepassedavaryingnumberofinputvalueswhenthefunctioniscalled.Writevariadicparametersbyinsertingthreeperiodcharacters(...)aftertheparameter’stypename.

Thevaluespassedtoavariadicparameteraremadeavailablewithinthefunction’sbodyasanarrayoftheappropriatetype.Forexample,avariadicparameterwithanameofnumbersandatypeofDouble...ismadeavailablewithinthefunction’sbodyasaconstantarraycallednumbersoftype[Double].

Theexamplebelowcalculatesthearithmeticmean(alsoknownastheaverage)foralistofnumbersofanylength:

1 funcarithmeticMean(_numbers:Double...)->Double{

2 vartotal:Double=0

3 fornumberinnumbers{

4 total+=number

5 }

6 returntotal/Double(numbers.count)

7 }

8 arithmeticMean(1,2,3,4,5)

9 //returns3.0,whichisthearithmeticmeanofthesefive

numbers

10 arithmeticMean(3,8.25,18.75)

11 //returns10.0,whichisthearithmeticmeanofthesethree

numbers

NOTE

Afunctionmayhaveatmostonevariadicparameter.

In-OutParametersFunctionparametersareconstantsbydefault.Tryingtochangethevalueofafunctionparameterfromwithinthebodyofthatfunctionresultsinacompile-timeerror.Thismeansthatyoucan’tchangethevalueofaparameterbymistake.Ifyouwantafunctiontomodifyaparameter’svalue,andyouwantthosechangestopersistafterthefunctioncallhasended,definethatparameterasanin-outparameterinstead.

Youwriteanin-outparameterbyplacingtheinoutkeywordrightbeforeaparameter’stype.Anin-outparameterhasavaluethatispassedintothefunction,ismodifiedbythefunction,andispassedbackoutofthefunctiontoreplacetheoriginalvalue.Foradetaileddiscussionofthebehaviorofin-outparametersandassociatedcompileroptimizations,seeIn-OutParameters.

Youcanonlypassavariableastheargumentforanin-outparameter.Youcannotpassaconstantoraliteralvalueastheargument,becauseconstantsandliteralscannotbemodified.Youplaceanampersand(&)directlybeforeavariable’s

namewhenyoupassitasanargumenttoanin-outparameter,toindicatethatitcanbemodifiedbythefunction.

NOTE

In-outparameterscannothavedefaultvalues,andvariadicparameterscannotbemarkedasinout.

Here’sanexampleofafunctioncalledswapTwoInts(_:_:),whichhastwoin-outintegerparameterscalledaandb:

1 funcswapTwoInts(_a:inoutInt,_b:inoutInt){

2 lettemporaryA=a

3 a=b

4 b=temporaryA

5 }

TheswapTwoInts(_:_:)functionsimplyswapsthevalueofbintoa,andthevalueofaintob.ThefunctionperformsthisswapbystoringthevalueofainatemporaryconstantcalledtemporaryA,assigningthevalueofbtoa,andthenassigningtemporaryAtob.

YoucancalltheswapTwoInts(_:_:)functionwithtwovariablesoftypeInttoswaptheirvalues.NotethatthenamesofsomeIntandanotherIntareprefixedwithanampersandwhentheyarepassedtotheswapTwoInts(_:_:)function:

1 varsomeInt=3

2 varanotherInt=107

3 swapTwoInts(&someInt,&anotherInt)

4 print("someIntisnow\(someInt),andanotherIntisnow\

(anotherInt)")

5 //Prints"someIntisnow107,andanotherIntisnow3"

TheexampleaboveshowsthattheoriginalvaluesofsomeIntandanotherIntaremodifiedbytheswapTwoInts(_:_:)function,eventhoughtheywereoriginallydefinedoutsideofthefunction.

NOTE

In-outparametersarenotthesameasreturningavaluefromafunction.TheswapTwoIntsexampleabovedoesnotdefineareturntypeorreturnavalue,butitstillmodifiesthevaluesofsomeIntandanotherInt.In-outparametersareanalternativewayforafunctiontohaveaneffectoutsideofthescopeofitsfunctionbody.

FunctionTypes

Everyfunctionhasaspecificfunctiontype,madeupoftheparametertypesandthereturntypeofthefunction.

Forexample:

1 funcaddTwoInts(_a:Int,_b:Int)->Int{

2 returna+b

3 }

4 funcmultiplyTwoInts(_a:Int,_b:Int)->Int{

5 returna*b

6 }

ThisexampledefinestwosimplemathematicalfunctionscalledaddTwoIntsandmultiplyTwoInts.ThesefunctionseachtaketwoIntvalues,andreturnanIntvalue,whichistheresultofperforminganappropriatemathematicaloperation.

Thetypeofbothofthesefunctionsis(Int,Int)->Int.Thiscanbereadas:

“Afunctionthathastwoparameters,bothoftypeInt,andthatreturnsavalueoftypeInt.”

Here’sanotherexample,forafunctionwithnoparametersorreturnvalue:

1 funcprintHelloWorld(){

2 print("hello,world")

3 }

Thetypeofthisfunctionis()->Void,or“afunctionthathasnoparameters,andreturnsVoid.”

UsingFunctionTypesYouusefunctiontypesjustlikeanyothertypesinSwift.Forexample,youcandefineaconstantorvariabletobeofafunctiontypeandassignanappropriatefunctiontothatvariable:

varmathFunction:(Int,Int)->Int=addTwoInts

Thiscanbereadas:

“DefineavariablecalledmathFunction,whichhasatypeof‘afunctionthattakestwoIntvalues,andreturnsanIntvalue.’SetthisnewvariabletorefertothefunctioncalledaddTwoInts.”

TheaddTwoInts(_:_:)functionhasthesametypeasthemathFunctionvariable,andsothisassignmentisallowedbySwift’stype-checker.

YoucannowcalltheassignedfunctionwiththenamemathFunction:

1 print("Result:\(mathFunction(2,3))")

2 //Prints"Result:5"

Adifferentfunctionwiththesamematchingtypecanbeassignedtothesamevariable,inthesamewayasfornonfunctiontypes:

1 mathFunction=multiplyTwoInts

2 print("Result:\(mathFunction(2,3))")

3 //Prints"Result:6"

Aswithanyothertype,youcanleaveittoSwifttoinferthefunctiontypewhen

youassignafunctiontoaconstantorvariable:

1 letanotherMathFunction=addTwoInts

2 //anotherMathFunctionisinferredtobeoftype(Int,Int)->

Int

FunctionTypesasParameterTypesYoucanuseafunctiontypesuchas(Int,Int)->Intasaparametertypeforanotherfunction.Thisenablesyoutoleavesomeaspectsofafunction’simplementationforthefunction’scallertoprovidewhenthefunctioniscalled.

Here’sanexampletoprinttheresultsofthemathfunctionsfromabove:

1 funcprintMathResult(_mathFunction:(Int,Int)->Int,_a:

Int,_b:Int){

2 print("Result:\(mathFunction(a,b))")

3 }

4 printMathResult(addTwoInts,3,5)

5 //Prints"Result:8"

ThisexampledefinesafunctioncalledprintMathResult(_:_:_:),whichhasthreeparameters.ThefirstparameteriscalledmathFunction,andisoftype(Int,Int)->Int.Youcanpassanyfunctionofthattypeastheargumentforthisfirstparameter.Thesecondandthirdparametersarecalledaandb,andarebothoftypeInt.Theseareusedasthetwoinputvaluesfortheprovidedmathfunction.

WhenprintMathResult(_:_:_:)iscalled,itispassedtheaddTwoInts(_:_:)function,andtheintegervalues3and5.Itcallstheprovidedfunctionwiththevalues3and5,andprintstheresultof8.

TheroleofprintMathResult(_:_:_:)istoprinttheresultofacalltoamathfunctionofanappropriatetype.Itdoesn’tmatterwhatthatfunction’simplementationactuallydoes—itmattersonlythatthefunctionisofthecorrect

type.ThisenablesprintMathResult(_:_:_:)tohandoffsomeofitsfunctionalitytothecallerofthefunctioninatype-safeway.

FunctionTypesasReturnTypesYoucanuseafunctiontypeasthereturntypeofanotherfunction.Youdothisbywritingacompletefunctiontypeimmediatelyafterthereturnarrow(->)ofthereturningfunction.

ThenextexampledefinestwosimplefunctionscalledstepForward(_:)andstepBackward(_:).ThestepForward(_:)functionreturnsavalueonemorethanitsinputvalue,andthestepBackward(_:)functionreturnsavalueonelessthanitsinputvalue.Bothfunctionshaveatypeof(Int)->Int:

1 funcstepForward(_input:Int)->Int{

2 returninput+1

3 }

4 funcstepBackward(_input:Int)->Int{

5 returninput-1

6 }

Here’safunctioncalledchooseStepFunction(backward:),whosereturntypeis(Int)->Int.ThechooseStepFunction(backward:)functionreturnsthestepForward(_:)functionorthestepBackward(_:)functionbasedonaBooleanparametercalledbackward:

1 funcchooseStepFunction(backward:Bool)->(Int)->Int{

2 returnbackward?stepBackward:stepForward

3 }

YoucannowusechooseStepFunction(backward:)toobtainafunctionthatwillstepinonedirectionortheother:

1 varcurrentValue=3

2 letmoveNearerToZero=chooseStepFunction(backward:

currentValue>0)

3 //moveNearerToZeronowreferstothestepBackward()function

TheexampleabovedetermineswhetherapositiveornegativestepisneededtomoveavariablecalledcurrentValueprogressivelyclosertozero.currentValuehasaninitialvalueof3,whichmeansthatcurrentValue>0returnstrue,causingchooseStepFunction(backward:)toreturnthestepBackward(_:)function.AreferencetothereturnedfunctionisstoredinaconstantcalledmoveNearerToZero.

NowthatmoveNearerToZeroreferstothecorrectfunction,itcanbeusedtocounttozero:

1 print("Countingtozero:")

2 //Countingtozero:

3 whilecurrentValue!=0{

4 print("\(currentValue)...")

5 currentValue=moveNearerToZero(currentValue)

6 }

7 print("zero!")

8 //3...

9 //2...

10 //1...

11 //zero!

NestedFunctions

Allofthefunctionsyouhaveencounteredsofarinthischapterhavebeenexamplesofglobalfunctions,whicharedefinedataglobalscope.Youcanalsodefinefunctionsinsidethebodiesofotherfunctions,knownasnestedfunctions.

Nestedfunctionsarehiddenfromtheoutsideworldbydefault,butcanstillbecalledandusedbytheirenclosingfunction.Anenclosingfunctioncanalsoreturnoneofitsnestedfunctionstoallowthenestedfunctiontobeusedinanotherscope.

YoucanrewritethechooseStepFunction(backward:)exampleabovetouseandreturnnestedfunctions:

1 funcchooseStepFunction(backward:Bool)->(Int)->Int{

2 funcstepForward(input:Int)->Int{returninput+1}

3 funcstepBackward(input:Int)->Int{returninput-1}

4 returnbackward?stepBackward:stepForward

5 }

6 varcurrentValue=-4

7 letmoveNearerToZero=chooseStepFunction(backward:

currentValue>0)

8 //moveNearerToZeronowreferstothenestedstepForward()

function

9 whilecurrentValue!=0{

10 print("\(currentValue)...")

11 currentValue=moveNearerToZero(currentValue)

12 }

13 print("zero!")

14 //-4...

15 //-3...

16 //-2...

17 //-1...

18 //zero!

Closures

Closuresareself-containedblocksoffunctionalitythatcanbepassedaroundandusedinyourcode.ClosuresinSwiftaresimilartoblocksinCandObjective-Candtolambdasinotherprogramminglanguages.

Closurescancaptureandstorereferencestoanyconstantsandvariablesfromthecontextinwhichtheyaredefined.Thisisknownasclosingoverthoseconstantsandvariables.Swifthandlesallofthememorymanagementofcapturingforyou.

NOTE

Don’tworryifyouarenotfamiliarwiththeconceptofcapturing.ItisexplainedindetailbelowinCapturingValues.

Globalandnestedfunctions,asintroducedinFunctions,areactuallyspecialcasesofclosures.Closurestakeoneofthreeforms:

Globalfunctionsareclosuresthathaveanameanddonotcaptureanyvalues.

Nestedfunctionsareclosuresthathaveanameandcancapturevaluesfromtheirenclosingfunction.

Closureexpressionsareunnamedclosureswritteninalightweightsyntaxthatcancapturevaluesfromtheirsurroundingcontext.

Swift’sclosureexpressionshaveaclean,clearstyle,withoptimizationsthatencouragebrief,clutter-freesyntaxincommonscenarios.Theseoptimizationsinclude:

Inferringparameterandreturnvaluetypesfromcontext

Implicitreturnsfromsingle-expressionclosures

Shorthandargumentnames

Trailingclosuresyntax

ClosureExpressions

Nestedfunctions,asintroducedinNestedFunctions,areaconvenientmeansofnaminganddefiningself-containedblocksofcodeaspartofalargerfunction.However,itissometimesusefultowriteshorterversionsoffunction-likeconstructswithoutafulldeclarationandname.Thisisparticularlytruewhenyouworkwithfunctionsormethodsthattakefunctionsasoneormoreoftheirarguments.

Closureexpressionsareawaytowriteinlineclosuresinabrief,focusedsyntax.Closureexpressionsprovideseveralsyntaxoptimizationsforwritingclosuresinashortenedformwithoutlossofclarityorintent.Theclosureexpressionexamplesbelowillustratetheseoptimizationsbyrefiningasingleexampleofthesorted(by:)methodoverseveraliterations,eachofwhichexpressesthesamefunctionalityinamoresuccinctway.

TheSortedMethodSwift’sstandardlibraryprovidesamethodcalledsorted(by:),whichsortsanarrayofvaluesofaknowntype,basedontheoutputofasortingclosurethatyouprovide.Onceitcompletesthesortingprocess,thesorted(by:)methodreturnsanewarrayofthesametypeandsizeastheoldone,withitselementsinthecorrectsortedorder.Theoriginalarrayisnotmodifiedbythesorted(by:)method.

Theclosureexpressionexamplesbelowusethesorted(by:)methodtosortanarrayofStringvaluesinreversealphabeticalorder.Here’stheinitialarraytobesorted:

letnames=["Chris","Alex","Ewa","Barry","Daniella"]

Thesorted(by:)methodacceptsaclosurethattakestwoargumentsofthesame

typeasthearray’scontents,andreturnsaBoolvaluetosaywhetherthefirstvalueshouldappearbeforeorafterthesecondvalueoncethevaluesaresorted.Thesortingclosureneedstoreturntrueifthefirstvalueshouldappearbeforethesecondvalue,andfalseotherwise.

ThisexampleissortinganarrayofStringvalues,andsothesortingclosureneedstobeafunctionoftype(String,String)->Bool.

Onewaytoprovidethesortingclosureistowriteanormalfunctionofthecorrecttype,andtopassitinasanargumenttothesorted(by:)method:

1 funcbackward(_s1:String,_s2:String)->Bool{

2 returns1>s2

3 }

4 varreversedNames=names.sorted(by:backward)

5 //reversedNamesisequalto["Ewa","Daniella","Chris",

"Barry","Alex"]

Ifthefirststring(s1)isgreaterthanthesecondstring(s2),thebackward(_:_:)functionwillreturntrue,indicatingthats1shouldappearbefores2inthesortedarray.Forcharactersinstrings,“greaterthan”means“appearslaterinthealphabetthan”.Thismeansthattheletter"B"is“greaterthan”theletter"A",andthestring"Tom"isgreaterthanthestring"Tim".Thisgivesareversealphabeticalsort,with"Barry"beingplacedbefore"Alex",andsoon.

However,thisisaratherlong-windedwaytowritewhatisessentiallyasingle-expressionfunction(a>b).Inthisexample,itwouldbepreferabletowritethesortingclosureinline,usingclosureexpressionsyntax.

ClosureExpressionSyntaxClosureexpressionsyntaxhasthefollowinggeneralform:

{( parameters )-> returntype in

statements

}

Theparametersinclosureexpressionsyntaxcanbein-outparameters,buttheycan’thaveadefaultvalue.Variadicparameterscanbeusedifyounamethevariadicparameter.Tuplescanalsobeusedasparametertypesandreturntypes.

Theexamplebelowshowsaclosureexpressionversionofthebackward(_:_:)functionfromabove:

1 reversedNames=names.sorted(by:{(s1:String,s2:String)->

Boolin

2 returns1>s2

3 })

Notethatthedeclarationofparametersandreturntypeforthisinlineclosureisidenticaltothedeclarationfromthebackward(_:_:)function.Inbothcases,itiswrittenas(s1:String,s2:String)->Bool.However,fortheinlineclosureexpression,theparametersandreturntypearewritteninsidethecurlybraces,notoutsideofthem.

Thestartoftheclosure’sbodyisintroducedbytheinkeyword.Thiskeywordindicatesthatthedefinitionoftheclosure’sparametersandreturntypehasfinished,andthebodyoftheclosureisabouttobegin.

Becausethebodyoftheclosureissoshort,itcanevenbewrittenonasingleline:

reversedNames=names.sorted(by:{(s1:String,s2:String)->

Boolinreturns1>s2})

Thisillustratesthattheoverallcalltothesorted(by:)methodhasremainedthesame.Apairofparenthesesstillwraptheentireargumentforthemethod.However,thatargumentisnowaninlineclosure.

InferringTypeFromContextBecausethesortingclosureispassedasanargumenttoamethod,Swiftcaninferthetypesofitsparametersandthetypeofthevalueitreturns.Thesorted(by:)methodisbeingcalledonanarrayofstrings,soitsargumentmustbeafunctionoftype(String,String)->Bool.Thismeansthatthe(String,String)andBooltypesdonotneedtobewrittenaspartoftheclosureexpression’sdefinition.Becauseallofthetypescanbeinferred,thereturnarrow(->)andtheparenthesesaroundthenamesoftheparameterscanalsobeomitted:

reversedNames=names.sorted(by:{s1,s2inreturns1>s2})

Itisalwayspossibletoinfertheparametertypesandreturntypewhenpassingaclosuretoafunctionormethodasaninlineclosureexpression.Asaresult,youneverneedtowriteaninlineclosureinitsfullestformwhentheclosureisusedasafunctionormethodargument.

Nonetheless,youcanstillmakethetypesexplicitifyouwish,anddoingsoisencouragedifitavoidsambiguityforreadersofyourcode.Inthecaseofthesorted(by:)method,thepurposeoftheclosureisclearfromthefactthatsortingistakingplace,anditissafeforareadertoassumethattheclosureislikelytobeworkingwithStringvalues,becauseitisassistingwiththesortingofanarrayofstrings.

ImplicitReturnsfromSingle-ExpressionClosuresSingle-expressionclosurescanimplicitlyreturntheresultoftheirsingleexpressionbyomittingthereturnkeywordfromtheirdeclaration,asinthisversionofthepreviousexample:

reversedNames=names.sorted(by:{s1,s2ins1>s2})

Here,thefunctiontypeofthesorted(by:)method’sargumentmakesitclearthataBoolvaluemustbereturnedbytheclosure.Becausetheclosure’sbodycontainsasingleexpression(s1>s2)thatreturnsaBoolvalue,thereisnoambiguity,andthereturnkeywordcanbeomitted.

ShorthandArgumentNamesSwiftautomaticallyprovidesshorthandargumentnamestoinlineclosures,whichcanbeusedtorefertothevaluesoftheclosure’sargumentsbythenames$0,$1,$2,andsoon.

Ifyouusetheseshorthandargumentnameswithinyourclosureexpression,youcanomittheclosure’sargumentlistfromitsdefinition,andthenumberandtypeoftheshorthandargumentnameswillbeinferredfromtheexpectedfunctiontype.Theinkeywordcanalsobeomitted,becausetheclosureexpressionismadeupentirelyofitsbody:

reversedNames=names.sorted(by:{$0>$1})

Here,$0and$1refertotheclosure’sfirstandsecondStringarguments.

OperatorMethodsThere’sactuallyanevenshorterwaytowritetheclosureexpressionabove.Swift’sStringtypedefinesitsstring-specificimplementationofthegreater-thanoperator(>)asamethodthathastwoparametersoftypeString,andreturnsavalueoftypeBool.Thisexactlymatchesthemethodtypeneededbythesorted(by:)method.Therefore,youcansimplypassinthegreater-thanoperator,andSwiftwillinferthatyouwanttouseitsstring-specificimplementation:

reversedNames=names.sorted(by:>)

Formoreaboutoperatormethod,seeOperatorMethods.

TrailingClosures

Ifyouneedtopassaclosureexpressiontoafunctionasthefunction’sfinalargumentandtheclosureexpressionislong,itcanbeusefultowriteitasa

trailingclosureinstead.Atrailingclosureiswrittenafterthefunctioncall’sparentheses,eventhoughitisstillanargumenttothefunction.Whenyouusethetrailingclosuresyntax,youdon’twritetheargumentlabelfortheclosureaspartofthefunctioncall.

1 funcsomeFunctionThatTakesAClosure(closure:()->Void){

2 //functionbodygoeshere

3 }

4

5 //Here'showyoucallthisfunctionwithoutusingatrailing

closure:

6

7 someFunctionThatTakesAClosure(closure:{

8 //closure'sbodygoeshere

9 })

10

11 //Here'showyoucallthisfunctionwithatrailingclosure

instead:

12

13 someFunctionThatTakesAClosure(){

14 //trailingclosure'sbodygoeshere

15 }

Thestring-sortingclosurefromtheClosureExpressionSyntaxsectionabovecanbewrittenoutsideofthesorted(by:)method’sparenthesesasatrailingclosure:

reversedNames=names.sorted(){$0>$1}

Ifaclosureexpressionisprovidedasthefunctionormethod’sonlyargumentandyouprovidethatexpressionasatrailingclosure,youdonotneedtowriteapairofparentheses()afterthefunctionormethod’snamewhenyoucallthefunction:

reversedNames=names.sorted{$0>$1}

Trailingclosuresaremostusefulwhentheclosureissufficientlylongthatitisnotpossibletowriteitinlineonasingleline.Asanexample,Swift’sArraytypehasamap(_:)methodwhichtakesaclosureexpressionasitssingleargument.Theclosureiscalledonceforeachiteminthearray,andreturnsanalternativemappedvalue(possiblyofsomeothertype)forthatitem.Thenatureofthemappingandthetypeofthereturnedvalueisleftuptotheclosuretospecify.

Afterapplyingtheprovidedclosuretoeacharrayelement,themap(_:)methodreturnsanewarraycontainingallofthenewmappedvalues,inthesameorderastheircorrespondingvaluesintheoriginalarray.

Here’showyoucanusethemap(_:)methodwithatrailingclosuretoconvertanarrayofIntvaluesintoanarrayofStringvalues.Thearray[16,58,510]isusedtocreatethenewarray["OneSix","FiveEight","FiveOneZero"]:

1 letdigitNames=[

2 0:"Zero",1:"One",2:"Two",3:"Three",4:"Four",

3 5:"Five",6:"Six",7:"Seven",8:"Eight",9:"Nine"

4 ]

5 letnumbers=[16,58,510]

ThecodeabovecreatesadictionaryofmappingsbetweentheintegerdigitsandEnglish-languageversionsoftheirnames.Italsodefinesanarrayofintegers,readytobeconvertedintostrings.

YoucannowusethenumbersarraytocreateanarrayofStringvalues,bypassingaclosureexpressiontothearray’smap(_:)methodasatrailingclosure:

1 letstrings=numbers.map{(number)->Stringin

2 varnumber=number

3 varoutput=""

4 repeat{

5 output=digitNames[number%10]!+output

6 number/=10

7 }whilenumber>0

8 returnoutput

9 }

10 //stringsisinferredtobeoftype[String]

11 //itsvalueis["OneSix","FiveEight","FiveOneZero"]

Themap(_:)methodcallstheclosureexpressiononceforeachiteminthearray.Youdonotneedtospecifythetypeoftheclosure’sinputparameter,number,becausethetypecanbeinferredfromthevaluesinthearraytobemapped.

Inthisexample,thevariablenumberisinitializedwiththevalueoftheclosure’snumberparameter,sothatthevaluecanbemodifiedwithintheclosurebody.(Theparameterstofunctionsandclosuresarealwaysconstants.)TheclosureexpressionalsospecifiesareturntypeofString,toindicatethetypethatwillbestoredinthemappedoutputarray.

Theclosureexpressionbuildsastringcalledoutputeachtimeitiscalled.Itcalculatesthelastdigitofnumberbyusingtheremainderoperator(number%10),andusesthisdigittolookupanappropriatestringinthedigitNamesdictionary.Theclosurecanbeusedtocreateastringrepresentationofanyintegergreaterthanzero.

NOTE

ThecalltothedigitNamesdictionary’ssubscriptisfollowedbyanexclamationmark(!),becausedictionarysubscriptsreturnanoptionalvaluetoindicatethatthedictionarylookupcanfailifthekeydoesnotexist.Intheexampleabove,itisguaranteedthatnumber%10willalwaysbeavalidsubscriptkeyforthedigitNamesdictionary,andsoanexclamationmarkisusedtoforce-unwraptheStringvaluestoredinthesubscript’soptionalreturnvalue.

ThestringretrievedfromthedigitNamesdictionaryisaddedtothefrontofoutput,effectivelybuildingastringversionofthenumberinreverse.(Theexpressionnumber%10givesavalueof6for16,8for58,and0for510.)

Thenumbervariableisthendividedby10.Becauseitisaninteger,itisroundeddownduringthedivision,so16becomes1,58becomes5,and510becomes51.

Theprocessisrepeateduntilnumberisequalto0,atwhichpointtheoutputstringisreturnedbytheclosure,andisaddedtotheoutputarraybythemap(_:)method.

Theuseoftrailingclosuresyntaxintheexampleaboveneatlyencapsulatestheclosure’sfunctionalityimmediatelyafterthefunctionthatclosuresupports,withoutneedingtowraptheentireclosurewithinthemap(_:)method’souterparentheses.

CapturingValues

Aclosurecancaptureconstantsandvariablesfromthesurroundingcontextinwhichitisdefined.Theclosurecanthenrefertoandmodifythevaluesofthoseconstantsandvariablesfromwithinitsbody,eveniftheoriginalscopethatdefinedtheconstantsandvariablesnolongerexists.

InSwift,thesimplestformofaclosurethatcancapturevaluesisanestedfunction,writtenwithinthebodyofanotherfunction.Anestedfunctioncancaptureanyofitsouterfunction’sargumentsandcanalsocaptureanyconstantsandvariablesdefinedwithintheouterfunction.

Here’sanexampleofafunctioncalledmakeIncrementer,whichcontainsanestedfunctioncalledincrementer.Thenestedincrementer()functioncapturestwovalues,runningTotalandamount,fromitssurroundingcontext.Aftercapturingthesevalues,incrementerisreturnedbymakeIncrementerasaclosurethatincrementsrunningTotalbyamounteachtimeitiscalled.

1 funcmakeIncrementer(forIncrementamount:Int)->()->Int{

2 varrunningTotal=0

3 funcincrementer()->Int{

4 runningTotal+=amount

5 returnrunningTotal

6 }

7 returnincrementer

8 }

ThereturntypeofmakeIncrementeris()->Int.Thismeansthatitreturnsafunction,ratherthanasimplevalue.Thefunctionitreturnshasnoparameters,andreturnsanIntvalueeachtimeitiscalled.Tolearnhowfunctionscanreturnotherfunctions,seeFunctionTypesasReturnTypes.

ThemakeIncrementer(forIncrement:)functiondefinesanintegervariablecalledrunningTotal,tostorethecurrentrunningtotaloftheincrementerthatwillbereturned.Thisvariableisinitializedwithavalueof0.

ThemakeIncrementer(forIncrement:)functionhasasingleIntparameterwithanargumentlabelofforIncrement,andaparameternameofamount.TheargumentvaluepassedtothisparameterspecifieshowmuchrunningTotalshouldbeincrementedbyeachtimethereturnedincrementerfunctioniscalled.ThemakeIncrementerfunctiondefinesanestedfunctioncalledincrementer,whichperformstheactualincrementing.ThisfunctionsimplyaddsamounttorunningTotal,andreturnstheresult.

Whenconsideredinisolation,thenestedincrementer()functionmightseemunusual:

1 funcincrementer()->Int{

2 runningTotal+=amount

3 returnrunningTotal

4 }

Theincrementer()functiondoesn’thaveanyparameters,andyetitreferstorunningTotalandamountfromwithinitsfunctionbody.ItdoesthisbycapturingareferencetorunningTotalandamountfromthesurroundingfunctionandusingthemwithinitsownfunctionbody.CapturingbyreferenceensuresthatrunningTotalandamountdonotdisappearwhenthecalltomakeIncrementerends,andalsoensuresthatrunningTotalisavailablethenexttimetheincrementerfunctioniscalled.

NOTE

Asanoptimization,Swiftmayinsteadcaptureandstoreacopyofavalueifthatvalueisnotmutatedbyaclosure,andifthevalueisnotmutatedaftertheclosureiscreated.

Swiftalsohandlesallmemorymanagementinvolvedindisposingofvariableswhentheyarenolongerneeded.

Here’sanexampleofmakeIncrementerinaction:

letincrementByTen=makeIncrementer(forIncrement:10)

ThisexamplesetsaconstantcalledincrementByTentorefertoanincrementerfunctionthatadds10toitsrunningTotalvariableeachtimeitiscalled.Callingthefunctionmultipletimesshowsthisbehaviorinaction:

1 incrementByTen()

2 //returnsavalueof10

3 incrementByTen()

4 //returnsavalueof20

5 incrementByTen()

6 //returnsavalueof30

Ifyoucreateasecondincrementer,itwillhaveitsownstoredreferencetoanew,separaterunningTotalvariable:

1 letincrementBySeven=makeIncrementer(forIncrement:7)

2 incrementBySeven()

3 //returnsavalueof7

Callingtheoriginalincrementer(incrementByTen)againcontinuestoincrementitsownrunningTotalvariable,anddoesnotaffectthevariablecapturedbyincrementBySeven:

1 incrementByTen()

2 //returnsavalueof40

NOTE

Ifyouassignaclosuretoapropertyofaclassinstance,andtheclosurecapturesthatinstancebyreferringtotheinstanceoritsmembers,youwillcreateastrongreferencecyclebetweentheclosureandtheinstance.Swiftusescaptureliststobreakthesestrongreferencecycles.Formoreinformation,seeStrongReferenceCyclesforClosures.

ClosuresAreReferenceTypes

Intheexampleabove,incrementBySevenandincrementByTenareconstants,buttheclosurestheseconstantsrefertoarestillabletoincrementtherunningTotalvariablesthattheyhavecaptured.Thisisbecausefunctionsandclosuresarereferencetypes.

Wheneveryouassignafunctionoraclosuretoaconstantoravariable,youareactuallysettingthatconstantorvariabletobeareferencetothefunctionorclosure.Intheexampleabove,itisthechoiceofclosurethatincrementByTenreferstothatisconstant,andnotthecontentsoftheclosureitself.

Thisalsomeansthatifyouassignaclosuretotwodifferentconstantsorvariables,bothofthoseconstantsorvariablesrefertothesameclosure.

1 letalsoIncrementByTen=incrementByTen

2 alsoIncrementByTen()

3 //returnsavalueof50

4

5 incrementByTen()

6 //returnsavalueof60

TheexampleaboveshowsthatcallingalsoIncrementByTenisthesameascallingincrementByTen.Becausebothofthemrefertothesameclosure,theybothincrementandreturnthesamerunningtotal.

EscapingClosures

Aclosureissaidtoescapeafunctionwhentheclosureispassedasanargumenttothefunction,butiscalledafterthefunctionreturns.Whenyoudeclareafunctionthattakesaclosureasoneofitsparameters,youcanwrite@escapingbeforetheparameter’stypetoindicatethattheclosureisallowedtoescape.

Onewaythataclosurecanescapeisbybeingstoredinavariablethatisdefinedoutsidethefunction.Asanexample,manyfunctionsthatstartanasynchronousoperationtakeaclosureargumentasacompletionhandler.Thefunctionreturnsafteritstartstheoperation,buttheclosureisn’tcalleduntiltheoperationiscompleted—theclosureneedstoescape,tobecalledlater.Forexample:

1 varcompletionHandlers:[()->Void]=[]

2 funcsomeFunctionWithEscapingClosure(completionHandler:

@escaping()->Void){

3 completionHandlers.append(completionHandler)

4 }

ThesomeFunctionWithEscapingClosure(_:)functiontakesaclosureasitsargumentandaddsittoanarraythat’sdeclaredoutsidethefunction.Ifyoudidn’tmarktheparameterofthisfunctionwith@escaping,youwouldgetacompile-timeerror.

Markingaclosurewith@escapingmeansyouhavetorefertoselfexplicitlywithintheclosure.Forexample,inthecodebelow,theclosurepassedtosomeFunctionWithEscapingClosure(_:)isanescapingclosure,whichmeansitneedstorefertoselfexplicitly.Incontrast,theclosurepassedtosomeFunctionWithNonescapingClosure(_:)isanonescapingclosure,whichmeansitcanrefertoselfimplicitly.

1 funcsomeFunctionWithNonescapingClosure(closure:()->Void){

2 closure()

3 }

4

5 classSomeClass{

6 varx=10

7 funcdoSomething(){

8 someFunctionWithEscapingClosure{self.x=100}

9 someFunctionWithNonescapingClosure{x=200}

10 }

11 }

12

13 letinstance=SomeClass()

14 instance.doSomething()

15 print(instance.x)

16 //Prints"200"

17

18 completionHandlers.first?()

19 print(instance.x)

20 //Prints"100"

Autoclosures

Anautoclosureisaclosurethatisautomaticallycreatedtowrapanexpressionthat’sbeingpassedasanargumenttoafunction.Itdoesn’ttakeanyarguments,andwhenit’scalled,itreturnsthevalueoftheexpressionthat’swrappedinsideofit.Thissyntacticconvenienceletsyouomitbracesaroundafunction’sparameterbywritinganormalexpressioninsteadofanexplicitclosure.

It’scommontocallfunctionsthattakeautoclosures,butit’snotcommontoimplementthatkindoffunction.Forexample,theassert(condition:message:file:line:)functiontakesanautoclosureforitsconditionandmessageparameters;itsconditionparameterisevaluatedonlyindebugbuildsanditsmessageparameterisevaluatedonlyifconditionisfalse.

Anautoclosureletsyoudelayevaluation,becausethecodeinsideisn’trununtilyoucalltheclosure.Delayingevaluationisusefulforcodethathassideeffectsoriscomputationallyexpensive,becauseitletsyoucontrolwhenthatcodeisevaluated.Thecodebelowshowshowaclosuredelaysevaluation.

1 varcustomersInLine=["Chris","Alex","Ewa","Barry",

"Daniella"]

2 print(customersInLine.count)

3 //Prints"5"

4

5 letcustomerProvider={customersInLine.remove(at:0)}

6 print(customersInLine.count)

7 //Prints"5"

8

9 print("Nowserving\(customerProvider())!")

10 //Prints"NowservingChris!"

11 print(customersInLine.count)

12 //Prints"4"

EventhoughthefirstelementofthecustomersInLinearrayisremovedbythecodeinsidetheclosure,thearrayelementisn’tremoveduntiltheclosureisactuallycalled.Iftheclosureisnevercalled,theexpressioninsidetheclosureisneverevaluated,whichmeansthearrayelementisneverremoved.NotethatthetypeofcustomerProviderisnotStringbut()->String—afunctionwithnoparametersthatreturnsastring.

Yougetthesamebehaviorofdelayedevaluationwhenyoupassaclosureasanargumenttoafunction.

1 //customersInLineis["Alex","Ewa","Barry","Daniella"]

2 funcserve(customercustomerProvider:()->String){

3 print("Nowserving\(customerProvider())!")

4 }

5 serve(customer:{customersInLine.remove(at:0)})

6 //Prints"NowservingAlex!"

Theserve(customer:)functioninthelistingabovetakesanexplicitclosurethatreturnsacustomer’sname.Theversionofserve(customer:)belowperformsthesameoperationbut,insteadoftakinganexplicitclosure,ittakesanautoclosurebymarkingitsparameter’stypewiththe@autoclosureattribute.NowyoucancallthefunctionasifittookaStringargumentinsteadofaclosure.Theargumentisautomaticallyconvertedtoaclosure,becausethecustomerProviderparameter’stypeismarkedwiththe@autoclosureattribute.

1 //customersInLineis["Ewa","Barry","Daniella"]

2 funcserve(customercustomerProvider:@autoclosure()->

String){

3 print("Nowserving\(customerProvider())!")

4 }

5 serve(customer:customersInLine.remove(at:0))

6 //Prints"NowservingEwa!"

NOTE

Overusingautoclosurescanmakeyourcodehardtounderstand.Thecontextandfunctionnameshouldmakeitclearthatevaluationisbeingdeferred.

Ifyouwantanautoclosurethatisallowedtoescape,useboththe@autoclosureand@escapingattributes.The@escapingattributeisdescribedaboveinEscapingClosures.

1 //customersInLineis["Barry","Daniella"]

2 varcustomerProviders:[()->String]=[]

3 funccollectCustomerProviders(_customerProvider:@autoclosure

@escaping()->String){

4 customerProviders.append(customerProvider)

5 }

6 collectCustomerProviders(customersInLine.remove(at:0))

7 collectCustomerProviders(customersInLine.remove(at:0))

8

9 print("Collected\(customerProviders.count)closures.")

10 //Prints"Collected2closures."

11 forcustomerProviderincustomerProviders{

12 print("Nowserving\(customerProvider())!")

13 }

14 //Prints"NowservingBarry!"

15 //Prints"NowservingDaniella!"

Inthecodeabove,insteadofcallingtheclosurepassedtoitasitscustomerProviderargument,thecollectCustomerProviders(_:)functionappendstheclosuretothecustomerProvidersarray.Thearrayisdeclaredoutsidethescopeofthefunction,whichmeanstheclosuresinthearraycanbeexecutedafterthefunctionreturns.Asaresult,thevalueofthecustomerProviderargumentmustbeallowedtoescapethefunction’sscope.

Enumerations

Anenumerationdefinesacommontypeforagroupofrelatedvaluesandenablesyoutoworkwiththosevaluesinatype-safewaywithinyourcode.

IfyouarefamiliarwithC,youwillknowthatCenumerationsassignrelatednamestoasetofintegervalues.EnumerationsinSwiftaremuchmoreflexible,anddon’thavetoprovideavalueforeachcaseoftheenumeration.Ifavalue(knownasarawvalue)isprovidedforeachenumerationcase,thevaluecanbeastring,acharacter,oravalueofanyintegerorfloating-pointtype.

Alternatively,enumerationcasescanspecifyassociatedvaluesofanytypetobestoredalongwitheachdifferentcasevalue,muchasunionsorvariantsdoinotherlanguages.Youcandefineacommonsetofrelatedcasesaspartofoneenumeration,eachofwhichhasadifferentsetofvaluesofappropriatetypesassociatedwithit.

EnumerationsinSwiftarefirst-classtypesintheirownright.Theyadoptmanyfeaturestraditionallysupportedonlybyclasses,suchascomputedpropertiestoprovideadditionalinformationabouttheenumeration’scurrentvalue,andinstancemethodstoprovidefunctionalityrelatedtothevaluestheenumerationrepresents.Enumerationscanalsodefineinitializerstoprovideaninitialcasevalue;canbeextendedtoexpandtheirfunctionalitybeyondtheiroriginalimplementation;andcanconformtoprotocolstoprovidestandardfunctionality.

Formoreaboutthesecapabilities,seeProperties,Methods,Initialization,Extensions,andProtocols.

EnumerationSyntax

Youintroduceenumerationswiththeenumkeywordandplacetheirentiredefinitionwithinapairofbraces:

1 enumSomeEnumeration{

2 //enumerationdefinitiongoeshere

3 }

Here’sanexampleforthefourmainpointsofacompass:

1 enumCompassPoint{

2 casenorth

3 casesouth

4 caseeast

5 casewest

6 }

Thevaluesdefinedinanenumeration(suchasnorth,south,east,andwest)areitsenumerationcases.Youusethecasekeywordtointroducenewenumerationcases.

NOTE

Swiftenumerationcasesdon’thaveanintegervaluesetbydefault,unlikelanguageslikeCandObjective-C.IntheCompassPointexampleabove,north,south,eastandwestdon’timplicitlyequal0,1,2and3.Instead,thedifferentenumerationcasesarevaluesintheirownright,withanexplicitlydefinedtypeofCompassPoint.

Multiplecasescanappearonasingleline,separatedbycommas:

1 enumPlanet{

2 casemercury,venus,earth,mars,jupiter,saturn,uranus,

neptune

3 }

Eachenumerationdefinitiondefinesanewtype.LikeothertypesinSwift,theirnames(suchasCompassPointandPlanet)startwithacapitalletter.Giveenumerationtypessingularratherthanpluralnames,sothattheyreadasself-evident:

vardirectionToHead=CompassPoint.west

ThetypeofdirectionToHeadisinferredwhenit’sinitializedwithoneofthepossiblevaluesofCompassPoint.OncedirectionToHeadisdeclaredasaCompassPoint,youcansetittoadifferentCompassPointvalueusingashorterdotsyntax:

directionToHead=.east

ThetypeofdirectionToHeadisalreadyknown,andsoyoucandropthetypewhensettingitsvalue.Thismakesforhighlyreadablecodewhenworkingwithexplicitlytypedenumerationvalues.

MatchingEnumerationValueswithaSwitchStatement

Youcanmatchindividualenumerationvalueswithaswitchstatement:

1 directionToHead=.south

2 switchdirectionToHead{

3 case.north:

4 print("Lotsofplanetshaveanorth")

5 case.south:

6 print("Watchoutforpenguins")

7 case.east:

8 print("Wherethesunrises")

9 case.west:

10 print("Wheretheskiesareblue")

11 }

12 //Prints"Watchoutforpenguins"

Youcanreadthiscodeas:

“ConsiderthevalueofdirectionToHead.Inthecasewhereitequals.north,print"Lotsofplanetshaveanorth".Inthecasewhereitequals.south,print"Watchoutforpenguins".”

…andsoon.

AsdescribedinControlFlow,aswitchstatementmustbeexhaustivewhenconsideringanenumeration’scases.Ifthecasefor.westisomitted,thiscodedoesn’tcompile,becauseitdoesn’tconsiderthecompletelistofCompassPointcases.Requiringexhaustivenessensuresthatenumerationcasesaren’taccidentallyomitted.

Whenitisn’tappropriatetoprovideacaseforeveryenumerationcase,youcanprovideadefaultcasetocoveranycasesthataren’taddressedexplicitly:

1 letsomePlanet=Planet.earth

2 switchsomePlanet{

3 case.earth:

4 print("Mostlyharmless")

5 default:

6 print("Notasafeplaceforhumans")

7 }

8 //Prints"Mostlyharmless"

IteratingoverEnumerationCases

Forsomeenumerations,it’susefultohaveacollectionofallofthatenumeration’scases.Youenablethisbywriting:CaseIterableaftertheenumeration’sname.SwiftexposesacollectionofallthecasesasanallCasespropertyoftheenumerationtype.Here’sanexample:

1 enumBeverage:CaseIterable{

2 casecoffee,tea,juice

3 }

4 letnumberOfChoices=Beverage.allCases.count

5 print("\(numberOfChoices)beveragesavailable")

6 //Prints"3beveragesavailable"

Intheexampleabove,youwriteBeverage.allCasestoaccessacollectionthatcontainsallofthecasesoftheBeverageenumeration.YoucanuseallCaseslikeanyothercollection—thecollection’selementsareinstancesoftheenumerationtype,sointhiscasethey’reBeveragevalues.Theexampleabovecountshowmanycasesthereare,andtheexamplebelowusesaforlooptoiterateoverallthecases.

1 forbeverageinBeverage.allCases{

2 print(beverage)

3 }

4 //coffee

5 //tea

6 //juice

ThesyntaxusedintheexamplesabovemarkstheenumerationasconformingtotheCaseIterableprotocol.Forinformationaboutprotocols,seeProtocols.

AssociatedValues

Theexamplesintheprevioussectionshowhowthecasesofanenumerationareadefined(andtyped)valueintheirownright.YoucansetaconstantorvariabletoPlanet.earth,andcheckforthisvaluelater.However,it’ssometimesusefultobeabletostorevaluesofothertypesalongsidethesecasevalues.Thisadditionalinformationiscalledanassociatedvalue,anditvarieseachtimeyouusethatcaseasavalueinyourcode.

YoucandefineSwiftenumerationstostoreassociatedvaluesofanygiventype,

andthevaluetypescanbedifferentforeachcaseoftheenumerationifneeded.Enumerationssimilartotheseareknownasdiscriminatedunions,taggedunions,orvariantsinotherprogramminglanguages.

Forexample,supposeaninventorytrackingsystemneedstotrackproductsbytwodifferenttypesofbarcode.Someproductsarelabeledwith1DbarcodesinUPCformat,whichusesthenumbers0to9.Eachbarcodehasanumbersystemdigit,followedbyfivemanufacturercodedigitsandfiveproductcodedigits.Thesearefollowedbyacheckdigittoverifythatthecodehasbeenscannedcorrectly:

Otherproductsarelabeledwith2DbarcodesinQRcodeformat,whichcanuseanyISO8859-1characterandcanencodeastringupto2,953characterslong:

It’sconvenientforaninventorytrackingsystemtostoreUPCbarcodesasatupleoffourintegers,andQRcodebarcodesasastringofanylength.

InSwift,anenumerationtodefineproductbarcodesofeithertypemightlooklikethis:

1 enumBarcode{

2 caseupc(Int,Int,Int,Int)

3 caseqrCode(String)

4 }

Thiscanbereadas:

“DefineanenumerationtypecalledBarcode,whichcantakeeitheravalueofupcwithanassociatedvalueoftype(Int,Int,Int,Int),oravalueofqrCodewithanassociatedvalueoftypeString.”

Thisdefinitiondoesn’tprovideanyactualIntorStringvalues—itjustdefinesthetypeofassociatedvaluesthatBarcodeconstantsandvariablescanstorewhentheyareequaltoBarcode.upcorBarcode.qrCode.

Youcanthencreatenewbarcodesusingeithertype:

varproductBarcode=Barcode.upc(8,85909,51226,3)

ThisexamplecreatesanewvariablecalledproductBarcodeandassignsitavalueofBarcode.upcwithanassociatedtuplevalueof(8,85909,51226,3).

Youcanassignthesameproductadifferenttypeofbarcode:

productBarcode=.qrCode("ABCDEFGHIJKLMNOP")

Atthispoint,theoriginalBarcode.upcanditsintegervaluesarereplacedbythenewBarcode.qrCodeanditsstringvalue.ConstantsandvariablesoftypeBarcodecanstoreeithera.upcora.qrCode(togetherwiththeirassociatedvalues),buttheycanstoreonlyoneofthematanygiventime.

Youcancheckthedifferentbarcodetypesusingaswitchstatement,similartotheexampleinMatchingEnumerationValueswithaSwitchStatement.Thistime,however,theassociatedvaluesareextractedaspartoftheswitchstatement.Youextracteachassociatedvalueasaconstant(withtheletprefix)oravariable(withthevarprefix)forusewithintheswitchcase’sbody:

1 switchproductBarcode{

2 case.upc(letnumberSystem,letmanufacturer,letproduct,let

check):

3 print("UPC:\(numberSystem),\(manufacturer),\(product),\

(check).")

4 case.qrCode(letproductCode):

5 print("QRcode:\(productCode).")

6 }

7 //Prints"QRcode:ABCDEFGHIJKLMNOP."

Ifalloftheassociatedvaluesforanenumerationcaseareextractedasconstants,orifallareextractedasvariables,youcanplaceasinglevarorletannotationbeforethecasename,forbrevity:

1 switchproductBarcode{

2 caselet.upc(numberSystem,manufacturer,product,check):

3 print("UPC:\(numberSystem),\(manufacturer),\(product),

\(check).")

4 caselet.qrCode(productCode):

5 print("QRcode:\(productCode).")

6 }

7 //Prints"QRcode:ABCDEFGHIJKLMNOP."

RawValues

ThebarcodeexampleinAssociatedValuesshowshowcasesofanenumerationcandeclarethattheystoreassociatedvaluesofdifferenttypes.Asanalternativetoassociatedvalues,enumerationcasescancomeprepopulatedwithdefaultvalues(calledrawvalues),whichareallofthesametype.

Here’sanexamplethatstoresrawASCIIvaluesalongsidenamedenumerationcases:

1 enumASCIIControlCharacter:Character{

2 casetab="\t"

3 caselineFeed="\n"

4 casecarriageReturn="\r"

5 }

Here,therawvaluesforanenumerationcalledASCIIControlCharacteraredefinedtobeoftypeCharacter,andaresettosomeofthemorecommonASCIIcontrolcharacters.CharactervaluesaredescribedinStringsandCharacters.

Rawvaluescanbestrings,characters,oranyoftheintegerorfloating-pointnumbertypes.Eachrawvaluemustbeuniquewithinitsenumerationdeclaration.

NOTE

Rawvaluesarenotthesameasassociatedvalues.Rawvaluesaresettoprepopulatedvalueswhenyoufirstdefinetheenumerationinyourcode,likethethreeASCIIcodesabove.Therawvalueforaparticularenumerationcaseisalwaysthesame.Associatedvaluesaresetwhenyoucreateanewconstantorvariablebasedononeoftheenumeration’scases,andcanbedifferenteachtimeyoudoso.

ImplicitlyAssignedRawValuesWhenyou’reworkingwithenumerationsthatstoreintegerorstringrawvalues,youdon’thavetoexplicitlyassignarawvalueforeachcase.Whenyoudon’t,Swiftautomaticallyassignsthevaluesforyou.

Forexample,whenintegersareusedforrawvalues,theimplicitvalueforeachcaseisonemorethanthepreviouscase.Ifthefirstcasedoesn’thaveavalueset,itsvalueis0.

TheenumerationbelowisarefinementoftheearlierPlanetenumeration,withintegerrawvaluestorepresenteachplanet’sorderfromthesun:

1 enumPlanet:Int{

2 casemercury=1,venus,earth,mars,jupiter,saturn,

uranus,neptune

3 }

Intheexampleabove,Planet.mercuryhasanexplicitrawvalueof1,Planet.venushasanimplicitrawvalueof2,andsoon.

Whenstringsareusedforrawvalues,theimplicitvalueforeachcaseisthetextofthatcase’sname.

TheenumerationbelowisarefinementoftheearlierCompassPointenumeration,withstringrawvaluestorepresenteachdirection’sname:

1 enumCompassPoint:String{

2 casenorth,south,east,west

3 }

Intheexampleabove,CompassPoint.southhasanimplicitrawvalueof"south",andsoon.

YouaccesstherawvalueofanenumerationcasewithitsrawValueproperty:

1 letearthsOrder=Planet.earth.rawValue

2 //earthsOrderis3

3

4 letsunsetDirection=CompassPoint.west.rawValue

5 //sunsetDirectionis"west"

InitializingfromaRawValueIfyoudefineanenumerationwitharaw-valuetype,theenumerationautomaticallyreceivesaninitializerthattakesavalueoftherawvalue’stype(asaparametercalledrawValue)andreturnseitheranenumerationcaseornil.Youcanusethisinitializertotrytocreateanewinstanceoftheenumeration.

ThisexampleidentifiesUranusfromitsrawvalueof7:

1 letpossiblePlanet=Planet(rawValue:7)

2 //possiblePlanetisoftypePlanet?andequalsPlanet.uranus

NotallpossibleIntvalueswillfindamatchingplanet,however.Becauseofthis,therawvalueinitializeralwaysreturnsanoptionalenumerationcase.Intheexampleabove,possiblePlanetisoftypePlanet?,or“optionalPlanet.”

NOTE

Therawvalueinitializerisafailableinitializer,becausenoteveryrawvaluewillreturnanenumerationcase.Formoreinformation,seeFailableInitializers.

Ifyoutrytofindaplanetwithapositionof11,theoptionalPlanetvaluereturnedbytherawvalueinitializerwillbenil:

1 letpositionToFind=11

2 ifletsomePlanet=Planet(rawValue:positionToFind){

3 switchsomePlanet{

4 case.earth:

5 print("Mostlyharmless")

6 default:

7 print("Notasafeplaceforhumans")

8 }

9 }else{

10 print("Thereisn'taplanetatposition\

(positionToFind)")

11 }

12 //Prints"Thereisn'taplanetatposition11"

Thisexampleusesoptionalbindingtotrytoaccessaplanetwitharawvalueof11.ThestatementifletsomePlanet=Planet(rawValue:11)createsanoptionalPlanet,andsetssomePlanettothevalueofthatoptionalPlanetifitcan

beretrieved.Inthiscase,itisn’tpossibletoretrieveaplanetwithapositionof11,andsotheelsebranchisexecutedinstead.

RecursiveEnumerations

Arecursiveenumerationisanenumerationthathasanotherinstanceoftheenumerationastheassociatedvalueforoneormoreoftheenumerationcases.Youindicatethatanenumerationcaseisrecursivebywritingindirectbeforeit,whichtellsthecompilertoinsertthenecessarylayerofindirection.

Forexample,hereisanenumerationthatstoressimplearithmeticexpressions:

1 enumArithmeticExpression{

2 casenumber(Int)

3 indirectcaseaddition(ArithmeticExpression,

ArithmeticExpression)

4 indirectcasemultiplication(ArithmeticExpression,

ArithmeticExpression)

5 }

Youcanalsowriteindirectbeforethebeginningoftheenumerationtoenableindirectionforalloftheenumeration’scasesthathaveanassociatedvalue:

1 indirectenumArithmeticExpression{

2 casenumber(Int)

3 caseaddition(ArithmeticExpression,ArithmeticExpression)

4 casemultiplication(ArithmeticExpression,

ArithmeticExpression)

5 }

Thisenumerationcanstorethreekindsofarithmeticexpressions:aplainnumber,theadditionoftwoexpressions,andthemultiplicationoftwo

expressions.Theadditionandmultiplicationcaseshaveassociatedvaluesthatarealsoarithmeticexpressions—theseassociatedvaluesmakeitpossibletonestexpressions.Forexample,theexpression(5+4)*2hasanumberontheright-handsideofthemultiplicationandanotherexpressionontheleft-handsideofthemultiplication.Becausethedataisnested,theenumerationusedtostorethedataalsoneedstosupportnesting—thismeanstheenumerationneedstoberecursive.ThecodebelowshowstheArithmeticExpressionrecursiveenumerationbeingcreatedfor(5+4)*2:

1 letfive=ArithmeticExpression.number(5)

2 letfour=ArithmeticExpression.number(4)

3 letsum=ArithmeticExpression.addition(five,four)

4 letproduct=ArithmeticExpression.multiplication(sum,

ArithmeticExpression.number(2))

Arecursivefunctionisastraightforwardwaytoworkwithdatathathasarecursivestructure.Forexample,here’safunctionthatevaluatesanarithmeticexpression:

1 funcevaluate(_expression:ArithmeticExpression)->Int{

2 switchexpression{

3 caselet.number(value):

4 returnvalue

5 caselet.addition(left,right):

6 returnevaluate(left)+evaluate(right)

7 caselet.multiplication(left,right):

8 returnevaluate(left)*evaluate(right)

9 }

10 }

11

12 print(evaluate(product))

13 //Prints"18"

Thisfunctionevaluatesaplainnumberbysimplyreturningtheassociatedvalue.Itevaluatesanadditionormultiplicationbyevaluatingtheexpressionontheleft-handside,evaluatingtheexpressionontheright-handside,andthenaddingthemormultiplyingthem.

StructuresandClasses

Structuresandclassesaregeneral-purpose,flexibleconstructsthatbecomethebuildingblocksofyourprogram’scode.Youdefinepropertiesandmethodstoaddfunctionalitytoyourstructuresandclassesusingthesamesyntaxyouusetodefineconstants,variables,andfunctions.

Unlikeotherprogramminglanguages,Swiftdoesn’trequireyoutocreateseparateinterfaceandimplementationfilesforcustomstructuresandclasses.InSwift,youdefineastructureorclassinasinglefile,andtheexternalinterfacetothatclassorstructureisautomaticallymadeavailableforothercodetouse.

NOTE

Aninstanceofaclassistraditionallyknownasanobject.However,Swiftstructuresandclassesaremuchcloserinfunctionalitythaninotherlanguages,andmuchofthischapterdescribesfunctionalitythatappliestoinstancesofeitheraclassorastructuretype.Becauseofthis,themoregeneralterminstanceisused.

ComparingStructuresandClasses

StructuresandclassesinSwifthavemanythingsincommon.Bothcan:

Definepropertiestostorevalues

Definemethodstoprovidefunctionality

Definesubscriptstoprovideaccesstotheirvaluesusingsubscriptsyntax

Defineinitializerstosetuptheirinitialstate

Beextendedtoexpandtheirfunctionalitybeyondadefaultimplementation

Conformtoprotocolstoprovidestandardfunctionalityofacertainkind

Formoreinformation,seeProperties,Methods,Subscripts,Initialization,Extensions,andProtocols.

Classeshaveadditionalcapabilitiesthatstructuresdon’thave:

Inheritanceenablesoneclasstoinheritthecharacteristicsofanother.

Typecastingenablesyoutocheckandinterpretthetypeofaclassinstanceatruntime.

Deinitializersenableaninstanceofaclasstofreeupanyresourcesithasassigned.

Referencecountingallowsmorethanonereferencetoaclassinstance.

Formoreinformation,seeInheritance,TypeCasting,Deinitialization,andAutomaticReferenceCounting.

Theadditionalcapabilitiesthatclassessupportcomeatthecostofincreasedcomplexity.Asageneralguideline,preferstructuresbecausethey’reeasiertoreasonabout,anduseclasseswhenthey’reappropriateornecessary.Inpractice,thismeansmostofthecustomdatatypesyoudefinewillbestructuresandenumerations.Foramoredetailedcomparison,seeChoosingBetweenStructuresandClasses.

DefinitionSyntaxStructuresandclasseshaveasimilardefinitionsyntax.Youintroducestructureswiththestructkeywordandclasseswiththeclasskeyword.Bothplacetheirentiredefinitionwithinapairofbraces:

1 structSomeStructure{

2 //structuredefinitiongoeshere

3 }

4 classSomeClass{

5 //classdefinitiongoeshere

6 }

NOTE

Wheneveryoudefineanewstructureorclass,youdefineanewSwifttype.GivetypesUpperCamelCasenames(suchasSomeStructureandSomeClasshere)tomatchthecapitalizationofstandardSwifttypes(suchasString,Int,andBool).GivepropertiesandmethodslowerCamelCasenames(suchasframeRateandincrementCount)todifferentiatethemfromtypenames.

Here’sanexampleofastructuredefinitionandaclassdefinition:

1 structResolution{

2 varwidth=0

3 varheight=0

4 }

5 classVideoMode{

6 varresolution=Resolution()

7 varinterlaced=false

8 varframeRate=0.0

9 varname:String?

10 }

TheexampleabovedefinesanewstructurecalledResolution,todescribeapixel-baseddisplayresolution.Thisstructurehastwostoredpropertiescalledwidthandheight.Storedpropertiesareconstantsorvariablesthatarebundledupandstoredaspartofthestructureorclass.ThesetwopropertiesareinferredtobeoftypeIntbysettingthemtoaninitialintegervalueof0.

TheexampleabovealsodefinesanewclasscalledVideoMode,todescribeaspecificvideomodeforvideodisplay.Thisclasshasfourvariablestoredproperties.Thefirst,resolution,isinitializedwithanewResolutionstructureinstance,whichinfersapropertytypeofResolution.Fortheotherthreeproperties,newVideoModeinstanceswillbeinitializedwithaninterlacedsettingoffalse(meaning“noninterlacedvideo”),aplaybackframerateof0.0,andanoptionalStringvaluecalledname.Thenamepropertyisautomatically

givenadefaultvalueofnil,or“nonamevalue”,becauseit’sofanoptionaltype.

StructureandClassInstancesTheResolutionstructuredefinitionandtheVideoModeclassdefinitiononlydescribewhataResolutionorVideoModewilllooklike.Theythemselvesdon’tdescribeaspecificresolutionorvideomode.Todothat,youneedtocreateaninstanceofthestructureorclass.

Thesyntaxforcreatinginstancesisverysimilarforbothstructuresandclasses:

1 letsomeResolution=Resolution()

2 letsomeVideoMode=VideoMode()

Structuresandclassesbothuseinitializersyntaxfornewinstances.Thesimplestformofinitializersyntaxusesthetypenameoftheclassorstructurefollowedbyemptyparentheses,suchasResolution()orVideoMode().Thiscreatesanewinstanceoftheclassorstructure,withanypropertiesinitializedtotheirdefaultvalues.ClassandstructureinitializationisdescribedinmoredetailinInitialization.

AccessingPropertiesYoucanaccessthepropertiesofaninstanceusingdotsyntax.Indotsyntax,youwritethepropertynameimmediatelyaftertheinstancename,separatedbyaperiod(.),withoutanyspaces:

1 print("ThewidthofsomeResolutionis\(someResolution.width)")

2 //Prints"ThewidthofsomeResolutionis0"

Inthisexample,someResolution.widthreferstothewidthpropertyofsomeResolution,andreturnsitsdefaultinitialvalueof0.

Youcandrilldownintosubproperties,suchasthewidthpropertyintheresolutionpropertyofaVideoMode:

1 print("ThewidthofsomeVideoModeis\

(someVideoMode.resolution.width)")

2 //Prints"ThewidthofsomeVideoModeis0"

Youcanalsousedotsyntaxtoassignanewvaluetoavariableproperty:

1 someVideoMode.resolution.width=1280

2 print("ThewidthofsomeVideoModeisnow\

(someVideoMode.resolution.width)")

3 //Prints"ThewidthofsomeVideoModeisnow1280"

MemberwiseInitializersforStructureTypesAllstructureshaveanautomaticallygeneratedmemberwiseinitializer,whichyoucanusetoinitializethememberpropertiesofnewstructureinstances.Initialvaluesforthepropertiesofthenewinstancecanbepassedtothememberwiseinitializerbyname:

letvga=Resolution(width:640,height:480)

Unlikestructures,classinstancesdon’treceiveadefaultmemberwiseinitializer.InitializersaredescribedinmoredetailinInitialization.

StructuresandEnumerationsAreValueTypes

Avaluetypeisatypewhosevalueiscopiedwhenit’sassignedtoavariableorconstant,orwhenit’spassedtoafunction.

You’veactuallybeenusingvaluetypesextensivelythroughoutthepreviouschapters.Infact,allofthebasictypesinSwift—integers,floating-pointnumbers,Booleans,strings,arraysanddictionaries—arevaluetypes,andareimplementedasstructuresbehindthescenes.

AllstructuresandenumerationsarevaluetypesinSwift.Thismeansthatanystructureandenumerationinstancesyoucreate—andanyvaluetypestheyhaveasproperties—arealwayscopiedwhentheyarepassedaroundinyourcode.

NOTE

Collectionsdefinedbythestandardlibrarylikearrays,dictionaries,andstringsuseanoptimizationtoreducetheperformancecostofcopying.Insteadofmakingacopyimmediately,thesecollectionssharethememorywheretheelementsarestoredbetweentheoriginalinstanceandanycopies.Ifoneofthecopiesofthecollectionismodified,theelementsarecopiedjustbeforethemodification.Thebehavioryouseeinyourcodeisalwaysasifacopytookplaceimmediately.

Considerthisexample,whichusestheResolutionstructurefromthepreviousexample:

1 lethd=Resolution(width:1920,height:1080)

2 varcinema=hd

ThisexampledeclaresaconstantcalledhdandsetsittoaResolutioninstanceinitializedwiththewidthandheightoffullHDvideo(1920pixelswideby1080pixelshigh).

Itthendeclaresavariablecalledcinemaandsetsittothecurrentvalueofhd.BecauseResolutionisastructure,acopyoftheexistinginstanceismade,andthisnewcopyisassignedtocinema.Eventhoughhdandcinemanowhavethesamewidthandheight,theyaretwocompletelydifferentinstancesbehindthescenes.

Next,thewidthpropertyofcinemaisamendedtobethewidthoftheslightlywider2Kstandardusedfordigitalcinemaprojection(2048pixelswideand1080pixelshigh):

cinema.width=2048

Checkingthewidthpropertyofcinemashowsthatithasindeedchangedtobe2048:

1 print("cinemaisnow\(cinema.width)pixelswide")

2 //Prints"cinemaisnow2048pixelswide"

However,thewidthpropertyoftheoriginalhdinstancestillhastheoldvalueof1920:

1 print("hdisstill\(hd.width)pixelswide")

2 //Prints"hdisstill1920pixelswide"

Whencinemawasgiventhecurrentvalueofhd,thevaluesstoredinhdwerecopiedintothenewcinemainstance.Theendresultwastwocompletelyseparateinstancesthatcontainedthesamenumericvalues.However,becausetheyareseparateinstances,settingthewidthofcinemato2048doesn’taffectthewidthstoredinhd,asshowninthefigurebelow:

Thesamebehaviorappliestoenumerations:

1 enumCompassPoint{

2 casenorth,south,east,west

3 mutatingfuncturnNorth(){

4 self=.north

5 }

6 }

7 varcurrentDirection=CompassPoint.west

8 letrememberedDirection=currentDirection

9 currentDirection.turnNorth()

10

11 print("Thecurrentdirectionis\(currentDirection)")

12 print("Theremembereddirectionis\(rememberedDirection)")

13 //Prints"Thecurrentdirectionisnorth"

14 //Prints"Theremembereddirectioniswest"

WhenrememberedDirectionisassignedthevalueofcurrentDirection,it’sactuallysettoacopyofthatvalue.ChangingthevalueofcurrentDirectionthereafterdoesn’taffectthecopyoftheoriginalvaluethatwasstoredinrememberedDirection.

ClassesAreReferenceTypes

Unlikevaluetypes,referencetypesarenotcopiedwhentheyareassignedtoavariableorconstant,orwhentheyarepassedtoafunction.Ratherthanacopy,areferencetothesameexistinginstanceisused.

Here’sanexample,usingtheVideoModeclassdefinedabove:

1 lettenEighty=VideoMode()

2 tenEighty.resolution=hd

3 tenEighty.interlaced=true

4 tenEighty.name="1080i"

5 tenEighty.frameRate=25.0

ThisexampledeclaresanewconstantcalledtenEightyandsetsittorefertoanewinstanceoftheVideoModeclass.ThevideomodeisassignedacopyoftheHDresolutionof1920by1080frombefore.It’ssettobeinterlaced,itsnameissetto"1080i",anditsframerateissetto25.0framespersecond.

Next,tenEightyisassignedtoanewconstant,calledalsoTenEighty,andtheframerateofalsoTenEightyismodified:

1 letalsoTenEighty=tenEighty

2 alsoTenEighty.frameRate=30.0

Becauseclassesarereferencetypes,tenEightyandalsoTenEightyactuallybothrefertothesameVideoModeinstance.Effectively,theyarejusttwodifferentnamesforthesamesingleinstance,asshowninthefigurebelow:

CheckingtheframeRatepropertyoftenEightyshowsthatitcorrectlyreportsthenewframerateof30.0fromtheunderlyingVideoModeinstance:

1 print("TheframeRatepropertyoftenEightyisnow\

(tenEighty.frameRate)")

2 //Prints"TheframeRatepropertyoftenEightyisnow30.0"

Thisexamplealsoshowshowreferencetypescanbehardertoreasonabout.IftenEightyandalsoTenEightywerefarapartinyourprogram’scode,itcouldbedifficulttofindallthewaysthatthevideomodeischanged.WhereveryouusetenEighty,youalsohavetothinkaboutthecodethatusesalsoTenEighty,andviceversa.Incontrast,valuetypesareeasiertoreasonaboutbecauseallofthecodethatinteractswiththesamevalueisclosetogetherinyoursourcefiles.

NotethattenEightyandalsoTenEightyaredeclaredasconstants,ratherthanvariables.However,youcanstillchangetenEighty.frameRateandalsoTenEighty.frameRatebecausethevaluesofthetenEightyandalsoTenEightyconstantsthemselvesdon’tactuallychange.tenEightyandalsoTenEightythemselvesdon’t“store”theVideoModeinstance—instead,theybothrefertoaVideoModeinstancebehindthescenes.It’stheframeRatepropertyoftheunderlyingVideoModethatischanged,notthevaluesoftheconstantreferencestothatVideoMode.

IdentityOperatorsBecauseclassesarereferencetypes,it’spossibleformultipleconstantsandvariablestorefertothesamesingleinstanceofaclassbehindthescenes.(Thesameisn’ttrueforstructuresandenumerations,becausetheyarealwayscopiedwhentheyareassignedtoaconstantorvariable,orpassedtoafunction.)

Itcansometimesbeusefultofindoutwhethertwoconstantsorvariablesrefertoexactlythesameinstanceofaclass.Toenablethis,Swiftprovidestwoidentityoperators:

Identicalto(===)

Notidenticalto(!==)

Usetheseoperatorstocheckwhethertwoconstantsorvariablesrefertothesamesingleinstance:

1 iftenEighty===alsoTenEighty{

2 print("tenEightyandalsoTenEightyrefertothesame

VideoModeinstance.")

3 }

4 //Prints"tenEightyandalsoTenEightyrefertothesame

VideoModeinstance."

Notethatidenticalto(representedbythreeequalssigns,or===)doesn’tmeanthesamethingasequalto(representedbytwoequalssigns,or==).Identicaltomeansthattwoconstantsorvariablesofclasstyperefertoexactlythesameclassinstance.Equaltomeansthattwoinstancesareconsideredequalorequivalentinvalue,forsomeappropriatemeaningofequal,asdefinedbythetype’sdesigner.

Whenyoudefineyourowncustomstructuresandclasses,it’syourresponsibilitytodecidewhatqualifiesastwoinstancesbeingequal.Theprocessofdefiningyourownimplementationsofthe==and!=operatorsisdescribedinEquivalenceOperators.

PointersIfyouhaveexperiencewithC,C++,orObjective-C,youmayknowthattheselanguagesusepointerstorefertoaddressesinmemory.ASwiftconstantorvariablethatreferstoaninstanceofsomereferencetypeissimilartoapointerinC,butisn’tadirectpointertoanaddressinmemory,anddoesn’trequireyoutowriteanasterisk(*)toindicatethatyouarecreatingareference.Instead,thesereferencesaredefinedlikeanyotherconstantorvariableinSwift.Thestandardlibraryprovidespointerandbuffertypesthatyoucanuseifyouneedtointeractwithpointersdirectly—seeManualMemoryManagement.

Properties

Propertiesassociatevalueswithaparticularclass,structure,orenumeration.Storedpropertiesstoreconstantandvariablevaluesaspartofaninstance,whereascomputedpropertiescalculate(ratherthanstore)avalue.Computedpropertiesareprovidedbyclasses,structures,andenumerations.Storedpropertiesareprovidedonlybyclassesandstructures.

Storedandcomputedpropertiesareusuallyassociatedwithinstancesofaparticulartype.However,propertiescanalsobeassociatedwiththetypeitself.Suchpropertiesareknownastypeproperties.

Inaddition,youcandefinepropertyobserverstomonitorchangesinaproperty’svalue,whichyoucanrespondtowithcustomactions.Propertyobserverscanbeaddedtostoredpropertiesyoudefineyourself,andalsotopropertiesthatasubclassinheritsfromitssuperclass.

Youcanalsouseapropertywrappertoreusecodeinthegetterandsetterofmultipleproperties.

StoredProperties

Initssimplestform,astoredpropertyisaconstantorvariablethatisstoredaspartofaninstanceofaparticularclassorstructure.Storedpropertiescanbeeithervariablestoredproperties(introducedbythevarkeyword)orconstantstoredproperties(introducedbytheletkeyword).

Youcanprovideadefaultvalueforastoredpropertyaspartofitsdefinition,asdescribedinDefaultPropertyValues.Youcanalsosetandmodifytheinitialvalueforastoredpropertyduringinitialization.Thisistrueevenforconstantstoredproperties,asdescribedinAssigningConstantPropertiesDuringInitialization.

TheexamplebelowdefinesastructurecalledFixedLengthRange,whichdescribes

arangeofintegerswhoserangelengthcannotbechangedafteritiscreated:

1 structFixedLengthRange{

2 varfirstValue:Int

3 letlength:Int

4 }

5 varrangeOfThreeItems=FixedLengthRange(firstValue:0,length:

3)

6 //therangerepresentsintegervalues0,1,and2

7 rangeOfThreeItems.firstValue=6

8 //therangenowrepresentsintegervalues6,7,and8

InstancesofFixedLengthRangehaveavariablestoredpropertycalledfirstValueandaconstantstoredpropertycalledlength.Intheexampleabove,lengthisinitializedwhenthenewrangeiscreatedandcannotbechangedthereafter,becauseitisaconstantproperty.

StoredPropertiesofConstantStructureInstancesIfyoucreateaninstanceofastructureandassignthatinstancetoaconstant,youcannotmodifytheinstance’sproperties,eveniftheyweredeclaredasvariableproperties:

1 letrangeOfFourItems=FixedLengthRange(firstValue:0,length:

4)

2 //thisrangerepresentsintegervalues0,1,2,and3

3 rangeOfFourItems.firstValue=6

4 //thiswillreportanerror,eventhoughfirstValueisa

variableproperty

BecauserangeOfFourItemsisdeclaredasaconstant(withtheletkeyword),itisnotpossibletochangeitsfirstValueproperty,eventhoughfirstValueisa

variableproperty.

Thisbehaviorisduetostructuresbeingvaluetypes.Whenaninstanceofavaluetypeismarkedasaconstant,soareallofitsproperties.

Thesameisnottrueforclasses,whicharereferencetypes.Ifyouassignaninstanceofareferencetypetoaconstant,youcanstillchangethatinstance’svariableproperties.

LazyStoredPropertiesAlazystoredpropertyisapropertywhoseinitialvalueisnotcalculateduntilthefirsttimeitisused.Youindicatealazystoredpropertybywritingthelazymodifierbeforeitsdeclaration.

NOTE

Youmustalwaysdeclarealazypropertyasavariable(withthevarkeyword),becauseitsinitialvaluemightnotberetrieveduntilafterinstanceinitializationcompletes.Constantpropertiesmustalwayshaveavaluebeforeinitializationcompletes,andthereforecannotbedeclaredaslazy.

Lazypropertiesareusefulwhentheinitialvalueforapropertyisdependentonoutsidefactorswhosevaluesarenotknownuntilafteraninstance’sinitializationiscomplete.Lazypropertiesarealsousefulwhentheinitialvalueforapropertyrequirescomplexorcomputationallyexpensivesetupthatshouldnotbeperformedunlessoruntilitisneeded.

Theexamplebelowusesalazystoredpropertytoavoidunnecessaryinitializationofacomplexclass.ThisexampledefinestwoclassescalledDataImporterandDataManager,neitherofwhichisshowninfull:

1 classDataImporter{

2 /*

3 DataImporterisaclasstoimportdatafromanexternal

file.

4 Theclassisassumedtotakeanontrivialamountoftimeto

initialize.

5 */

6 varfilename="data.txt"

7 //theDataImporterclasswouldprovidedataimporting

functionalityhere

8 }

9

10 classDataManager{

11 lazyvarimporter=DataImporter()

12 vardata=[String]()

13 //theDataManagerclasswouldprovidedatamanagement

functionalityhere

14 }

15

16 letmanager=DataManager()

17 manager.data.append("Somedata")

18 manager.data.append("Somemoredata")

19 //theDataImporterinstancefortheimporterpropertyhasnot

yetbeencreated

TheDataManagerclasshasastoredpropertycalleddata,whichisinitializedwithanew,emptyarrayofStringvalues.Althoughtherestofitsfunctionalityisnotshown,thepurposeofthisDataManagerclassistomanageandprovideaccesstothisarrayofStringdata.

PartofthefunctionalityoftheDataManagerclassistheabilitytoimportdatafromafile.ThisfunctionalityisprovidedbytheDataImporterclass,whichisassumedtotakeanontrivialamountoftimetoinitialize.ThismightbebecauseaDataImporterinstanceneedstoopenafileandreaditscontentsintomemorywhentheDataImporterinstanceisinitialized.

ItispossibleforaDataManagerinstancetomanageitsdatawithouteverimportingdatafromafile,sothereisnoneedtocreateanewDataImporter

instancewhentheDataManageritselfiscreated.Instead,itmakesmoresensetocreatetheDataImporterinstanceifandwhenitisfirstused.

Becauseitismarkedwiththelazymodifier,theDataImporterinstancefortheimporterpropertyisonlycreatedwhentheimporterpropertyisfirstaccessed,suchaswhenitsfilenamepropertyisqueried:

1 print(manager.importer.filename)

2 //theDataImporterinstancefortheimporterpropertyhasnow

beencreated

3 //Prints"data.txt"

NOTE

Ifapropertymarkedwiththelazymodifierisaccessedbymultiplethreadssimultaneouslyandthepropertyhasnotyetbeeninitialized,thereisnoguaranteethatthepropertywillbeinitializedonlyonce.

StoredPropertiesandInstanceVariablesIfyouhaveexperiencewithObjective-C,youmayknowthatitprovidestwowaystostorevaluesandreferencesaspartofaclassinstance.Inadditiontoproperties,youcanuseinstancevariablesasabackingstoreforthevaluesstoredinaproperty.

Swiftunifiestheseconceptsintoasinglepropertydeclaration.ASwiftpropertydoesnothaveacorrespondinginstancevariable,andthebackingstoreforapropertyisnotaccesseddirectly.Thisapproachavoidsconfusionabouthowthevalueisaccessedindifferentcontextsandsimplifiestheproperty’sdeclarationintoasingle,definitivestatement.Allinformationabouttheproperty—includingitsname,type,andmemorymanagementcharacteristics—isdefinedinasinglelocationaspartofthetype’sdefinition.

ComputedProperties

Inadditiontostoredproperties,classes,structures,andenumerationscandefinecomputedproperties,whichdonotactuallystoreavalue.Instead,theyprovideagetterandanoptionalsettertoretrieveandsetotherpropertiesandvaluesindirectly.

1 structPoint{

2 varx=0.0,y=0.0

3 }

4 structSize{

5 varwidth=0.0,height=0.0

6 }

7 structRect{

8 varorigin=Point()

9 varsize=Size()

10 varcenter:Point{

11 get{

12 letcenterX=origin.x+(size.width/2)

13 letcenterY=origin.y+(size.height/2)

14 returnPoint(x:centerX,y:centerY)

15 }

16 set(newCenter){

17 origin.x=newCenter.x-(size.width/2)

18 origin.y=newCenter.y-(size.height/2)

19 }

20 }

21 }

22 varsquare=Rect(origin:Point(x:0.0,y:0.0),

23 size:Size(width:10.0,height:10.0))

24 letinitialSquareCenter=square.center

25 square.center=Point(x:15.0,y:15.0)

26 print("square.originisnowat(\(square.origin.x),\

(square.origin.y))")

27 //Prints"square.originisnowat(10.0,10.0)"

Thisexampledefinesthreestructuresforworkingwithgeometricshapes:

Pointencapsulatesthex-andy-coordinateofapoint.

Sizeencapsulatesawidthandaheight.

Rectdefinesarectanglebyanoriginpointandasize.

TheRectstructurealsoprovidesacomputedpropertycalledcenter.ThecurrentcenterpositionofaRectcanalwaysbedeterminedfromitsoriginandsize,andsoyoudon’tneedtostorethecenterpointasanexplicitPointvalue.Instead,Rectdefinesacustomgetterandsetterforacomputedvariablecalledcenter,toenableyoutoworkwiththerectangle’scenterasifitwerearealstoredproperty.

TheexampleabovecreatesanewRectvariablecalledsquare.Thesquarevariableisinitializedwithanoriginpointof(0,0),andawidthandheightof10.Thissquareisrepresentedbythebluesquareinthediagrambelow.

Thesquarevariable’scenterpropertyisthenaccessedthroughdotsyntax(square.center),whichcausesthegetterforcentertobecalled,toretrievethecurrentpropertyvalue.Ratherthanreturninganexistingvalue,thegetteractuallycalculatesandreturnsanewPointtorepresentthecenterofthesquare.Ascanbeseenabove,thegettercorrectlyreturnsacenterpointof(5,5).

Thecenterpropertyisthensettoanewvalueof(15,15),whichmovesthesquareupandtotheright,tothenewpositionshownbytheorangesquareinthediagrambelow.Settingthecenterpropertycallsthesetterforcenter,whichmodifiesthexandyvaluesofthestoredoriginproperty,andmovesthesquaretoitsnewposition.

ShorthandSetterDeclarationIfacomputedproperty’ssetterdoesn’tdefineanameforthenewvaluetobeset,adefaultnameofnewValueisused.Here’sanalternativeversionoftheRectstructurethattakesadvantageofthisshorthandnotation:

1 structAlternativeRect{

2 varorigin=Point()

3 varsize=Size()

4 varcenter:Point{

5 get{

6 letcenterX=origin.x+(size.width/2)

7 letcenterY=origin.y+(size.height/2)

8 returnPoint(x:centerX,y:centerY)

9 }

10 set{

11 origin.x=newValue.x-(size.width/2)

12 origin.y=newValue.y-(size.height/2)

13 }

14 }

15 }

ShorthandGetterDeclarationIftheentirebodyofagetterisasingleexpression,thegetterimplicitlyreturnsthatexpression.Here’sananotherversionoftheRectstructurethattakesadvantageofthisshorthandnotationandtheshorthandnotationforsetters:

1 structCompactRect{

2 varorigin=Point()

3 varsize=Size()

4 varcenter:Point{

5 get{

6 Point(x:origin.x+(size.width/2),

7 y:origin.y+(size.height/2))

8 }

9 set{

10 origin.x=newValue.x-(size.width/2)

11 origin.y=newValue.y-(size.height/2)

12 }

13 }

14 }

Omittingthereturnfromagetterfollowsthesamerulesasomittingreturnfromafunction,asdescribedinFunctionsWithanImplicitReturn.

Read-OnlyComputedPropertiesAcomputedpropertywithagetterbutnosetterisknownasaread-onlycomputedproperty.Aread-onlycomputedpropertyalwaysreturnsavalue,andcanbeaccessedthroughdotsyntax,butcannotbesettoadifferentvalue.

NOTE

Youmustdeclarecomputedproperties—includingread-onlycomputedproperties—asvariablepropertieswiththevarkeyword,becausetheirvalueisnotfixed.Theletkeywordisonlyusedforconstantproperties,toindicatethattheirvaluescannotbechangedoncetheyaresetaspartofinstanceinitialization.

Youcansimplifythedeclarationofaread-onlycomputedpropertybyremovingthegetkeywordanditsbraces:

1 structCuboid{

2 varwidth=0.0,height=0.0,depth=0.0

3 varvolume:Double{

4 returnwidth*height*depth

5 }

6 }

7 letfourByFiveByTwo=Cuboid(width:4.0,height:5.0,depth:

2.0)

8 print("thevolumeoffourByFiveByTwois\

(fourByFiveByTwo.volume)")

9 //Prints"thevolumeoffourByFiveByTwois40.0"

ThisexampledefinesanewstructurecalledCuboid,whichrepresentsa3Drectangularboxwithwidth,height,anddepthproperties.Thisstructurealsohasaread-onlycomputedpropertycalledvolume,whichcalculatesandreturnsthecurrentvolumeofthecuboid.Itdoesn’tmakesenseforvolumetobesettable,becauseitwouldbeambiguousastowhichvaluesofwidth,height,anddepthshouldbeusedforaparticularvolumevalue.Nonetheless,itisusefulforaCuboidtoprovidearead-onlycomputedpropertytoenableexternaluserstodiscoveritscurrentcalculatedvolume.

PropertyObservers

Propertyobserversobserveandrespondtochangesinaproperty’svalue.Propertyobserversarecalledeverytimeaproperty’svalueisset,evenifthenewvalueisthesameastheproperty’scurrentvalue.

Youcanaddpropertyobserverstoanystoredpropertiesyoudefine,exceptforlazystoredproperties.Youcanalsoaddpropertyobserverstoanyinheritedproperty(whetherstoredorcomputed)byoverridingthepropertywithinasubclass.Youdon’tneedtodefinepropertyobserversfornonoverriddencomputedproperties,becauseyoucanobserveandrespondtochangestotheirvalueinthecomputedproperty’ssetter.PropertyoverridingisdescribedinOverriding.

Youhavetheoptiontodefineeitherorbothoftheseobserversonaproperty:

willSetiscalledjustbeforethevalueisstored.

didSetiscalledimmediatelyafterthenewvalueisstored.

IfyouimplementawillSetobserver,it’spassedthenewpropertyvalueasaconstantparameter.YoucanspecifyanameforthisparameteraspartofyourwillSetimplementation.Ifyoudon’twritetheparameternameandparentheseswithinyourimplementation,theparameterismadeavailablewithadefaultparameternameofnewValue.

Similarly,ifyouimplementadidSetobserver,it’spassedaconstantparametercontainingtheoldpropertyvalue.YoucannametheparameterorusethedefaultparameternameofoldValue.IfyouassignavaluetoapropertywithinitsowndidSetobserver,thenewvaluethatyouassignreplacestheonethatwasjustset.

NOTE

ThewillSetanddidSetobserversofsuperclasspropertiesarecalledwhenapropertyissetinasubclassinitializer,afterthesuperclassinitializerhasbeencalled.Theyarenotcalledwhileaclassissettingitsownproperties,beforethesuperclassinitializerhasbeencalled.

Formoreinformationaboutinitializerdelegation,seeInitializerDelegationforValueTypesandInitializerDelegationforClassTypes.

Here’sanexampleofwillSetanddidSetinaction.TheexamplebelowdefinesanewclasscalledStepCounter,whichtracksthetotalnumberofstepsthatapersontakeswhilewalking.Thisclassmightbeusedwithinputdatafromapedometerorotherstepcountertokeeptrackofaperson’sexerciseduringtheirdailyroutine.

1 classStepCounter{

2 vartotalSteps:Int=0{

3 willSet(newTotalSteps){

4 print("AbouttosettotalStepsto\

(newTotalSteps)")

5 }

6 didSet{

7 iftotalSteps>oldValue{

8 print("Added\(totalSteps-oldValue)steps")

9 }

10 }

11 }

12 }

13 letstepCounter=StepCounter()

14 stepCounter.totalSteps=200

15 //AbouttosettotalStepsto200

16 //Added200steps

17 stepCounter.totalSteps=360

18 //AbouttosettotalStepsto360

19 //Added160steps

20 stepCounter.totalSteps=896

21 //AbouttosettotalStepsto896

22 //Added536steps

TheStepCounterclassdeclaresatotalStepspropertyoftypeInt.Thisisa

storedpropertywithwillSetanddidSetobservers.

ThewillSetanddidSetobserversfortotalStepsarecalledwheneverthepropertyisassignedanewvalue.Thisistrueevenifthenewvalueisthesameasthecurrentvalue.

Thisexample’swillSetobserverusesacustomparameternameofnewTotalStepsfortheupcomingnewvalue.Inthisexample,itsimplyprintsoutthevaluethatisabouttobeset.

ThedidSetobserveriscalledafterthevalueoftotalStepsisupdated.ItcomparesthenewvalueoftotalStepsagainsttheoldvalue.Ifthetotalnumberofstepshasincreased,amessageisprintedtoindicatehowmanynewstepshavebeentaken.ThedidSetobserverdoesnotprovideacustomparameternamefortheoldvalue,andthedefaultnameofoldValueisusedinstead.

NOTE

Ifyoupassapropertythathasobserverstoafunctionasanin-outparameter,thewillSetanddidSetobserversarealwayscalled.Thisisbecauseofthecopy-incopy-outmemorymodelforin-outparameters:Thevalueisalwayswrittenbacktothepropertyattheendofthefunction.Foradetaileddiscussionofthebehaviorofin-outparameters,seeIn-OutParameters.

PropertyWrappers

Apropertywrapperaddsalayerofseparationbetweencodethatmanageshowapropertyisstoredandthecodethatdefinesaproperty.Forexample,ifyouhavepropertiesthatprovidethread-safetychecksorstoretheirunderlyingdatainadatabase,youhavetowritethatcodeoneveryproperty.Whenyouuseapropertywrapper,youwritethemanagementcodeoncewhenyoudefinethewrapper,andthenreusethatmanagementcodebyapplyingittomultipleproperties.

Todefineapropertywrapper,youmakeastructure,enumeration,orclassthatdefinesawrappedValueproperty.Inthecodebelow,theTwelveOrLessstructureensuresthatthevalueitwrapsalwayscontainsanumberlessthanorequalto12.

Ifyouaskittostorealargernumber,itstores12instead.

1 @propertyWrapper

2 structTwelveOrLess{

3 privatevarnumber=0

4 varwrappedValue:Int{

5 get{returnnumber}

6 set{number=min(newValue,12)}

7 }

8 }

Thesetterensuresthatnewvaluesarelessthan12,andthegetterreturnsthestoredvalue.

NOTE

Thedeclarationfornumberintheexampleabovemarksthevariableasprivate,whichensuresnumberisusedonlyintheimplementationofTwelveOrLess.Codethat’swrittenanywhereelseaccessesthevalueusingthegetterandsetterforwrappedValue,andcan’tusenumberdirectly.Forinformationaboutprivate,seeAccessControl.

Youapplyawrappertoapropertybywritingthewrapper’snamebeforethepropertyasanattribute.Here’sastructurethatstoresasmallrectangle,usingthesame(ratherarbitrary)definitionof“small”that’simplementedbytheTwelveOrLesspropertywrapper:

1 structSmallRectangle{

2 @TwelveOrLessvarheight:Int

3 @TwelveOrLessvarwidth:Int

4 }

5

6 varrectangle=SmallRectangle()

7 print(rectangle.height)

8 //Prints"0"

9

10 rectangle.height=10

11 print(rectangle.height)

12 //Prints"10"

13

14 rectangle.height=24

15 print(rectangle.height)

16 //Prints"12"

TheheightandwidthpropertiesgettheirinitialvaluesfromthedefinitionofTwelveOrLess,whichsetsTwelveOrLess.numbertozero.Storingthenumber10intorectangle.heightsucceedsbecauseit’sasmallnumber.Tryingtostore24actuallystoresavalueof12instead,because24istoolargeforthepropertysetter’srule.

Whenyouapplyawrappertoaproperty,thecompilersynthesizescodethatprovidesstorageforthewrapperandcodethatprovidesaccesstothepropertythroughthewrapper.(Thepropertywrapperisresponsibleforstoringthewrappedvalue,sothere’snosynthesizedcodeforthat.)Youcouldwritecodethatusesthebehaviorofapropertywrapper,withouttakingadvantageofthespecialattributesyntax.Forexample,here’saversionofSmallRectanglefromthepreviouscodelistingthatwrapsitspropertiesintheTwelveOrLessstructureexplicitly,insteadofwriting@TwelveOrLessasanattribute:

1 structSmallRectangle{

2 privatevar_height=TwelveOrLess()

3 privatevar_width=TwelveOrLess()

4 varheight:Int{

5 get{return_height.wrappedValue}

6 set{_height.wrappedValue=newValue}

7 }

8 varwidth:Int{

9 get{return_width.wrappedValue}

10 set{_width.wrappedValue=newValue}

11 }

12 }

The_heightand_widthpropertiesstoreaninstanceofthepropertywrapper,TwelveOrLess.ThegetterandsetterforheightandwidthwrapaccesstothewrappedValueproperty.

SettingInitialValuesforWrappedPropertiesThecodeintheexamplesabovesetstheinitialvalueforthewrappedpropertybygivingnumberaninitialvalueinthedefinitionofTwelveOrLess.Codethatusesthispropertywrapper,can’tspecifyadifferentinitialvalueforapropertythat’swrappedbyTwelveOrLess—forexample,thedefinitionofSmallRectanglecan’tgiveheightorwidthinitialvalues.Tosupportsettinganinitialvalueorothercustomization,thepropertywrapperneedstoaddaninitializer.Here’sanexpandedversionofTwelveOrLesscalledSmallNumberthatdefinesinitializersthatsetthewrappedandmaximumvalue:

1 @propertyWrapper

2 structSmallNumber{

3 privatevarmaximum:Int

4 privatevarnumber:Int

5

6 varwrappedValue:Int{

7 get{returnnumber}

8 set{number=min(newValue,maximum)}

9 }

10

11 init(){

12 maximum=12

13 number=0

14 }

15 init(wrappedValue:Int){

16 maximum=12

17 number=min(wrappedValue,maximum)

18 }

19 init(wrappedValue:Int,maximum:Int){

20 self.maximum=maximum

21 number=min(wrappedValue,maximum)

22 }

23 }

ThedefinitionofSmallNumberincludesthreeinitializers—init(),init(wrappedValue:),andinit(wrappedValue:maximum:)—whichtheexamplesbelowusetosetthewrappedvalueandthemaximumvalue.Forinformationaboutinitializationandinitializersyntax,seeInitialization.

Whenyouapplyawrappertoapropertyandyoudon’tspecifyaninitialvalue,Swiftusestheinit()initializertosetupthewrapper.Forexample:

1 structZeroRectangle{

2 @SmallNumbervarheight:Int

3 @SmallNumbervarwidth:Int

4 }

5

6 varzeroRectangle=ZeroRectangle()

7 print(zeroRectangle.height,zeroRectangle.width)

8 //Prints"00"

TheinstancesofSmallNumberthatwrapheightandwidtharecreatedbycallingSmallNumber().Thecodeinsidethatinitializersetstheinitialwrappedvalueandtheinitialmaximumvalue,usingthedefaultvaluesofzeroand12.Thepropertywrapperstillprovidesalloftheinitialvalues,liketheearlierexamplethatusedTwelveOrLessinSmallRectangle.Unlikethatexample,SmallNumberalso

supportswritingthoseinitialvaluesaspartofdeclaringtheproperty.

Whenyouspecifyaninitialvaluefortheproperty,Swiftusestheinit(wrappedValue:)initializertosetupthewrapper.Forexample:

1 structUnitRectangle{

2 @SmallNumbervarheight:Int=1

3 @SmallNumbervarwidth:Int=1

4 }

5

6 varunitRectangle=UnitRectangle()

7 print(unitRectangle.height,unitRectangle.width)

8 //Prints"11"

Whenyouwrite=1onapropertywithawrapper,that’stranslatedintoacalltotheinit(wrappedValue:)initializer.TheinstancesofSmallNumberthatwrapheightandwidtharecreatedbycallingSmallNumber(wrappedValue:1).Theinitializerusesthewrappedvaluethat’sspecifiedhere,anditusesthedefaultmaximumvalueof12.

Whenyouwriteargumentsinparenthesesafterthecustomattribute,Swiftusestheinitializerthatacceptsthoseargumentstosetupthewrapper.Forexample,ifyouprovideaninitialvalueandamaximumvalue,Swiftusestheinit(wrappedValue:maximum:)initializer:

1 structNarrowRectangle{

2 @SmallNumber(wrappedValue:2,maximum:5)varheight:Int

3 @SmallNumber(wrappedValue:3,maximum:4)varwidth:Int

4 }

5

6 varnarrowRectangle=NarrowRectangle()

7 print(narrowRectangle.height,narrowRectangle.width)

8 //Prints"23"

9

10 narrowRectangle.height=100

11 narrowRectangle.width=100

12 print(narrowRectangle.height,narrowRectangle.width)

13 //Prints"54"

TheinstanceofSmallNumberthatwrapsheightiscreatedbycallingSmallNumber(wrappedValue:2,maximum:5),andtheinstancethatwrapswidthiscreatedbycallingSmallNumber(wrappedValue:3,maximum:4).

Byincludingargumentstothepropertywrapper,youcansetuptheinitialstateinthewrapperorpassotheroptionstothewrapperwhenit’screated.Thissyntaxisthemostgeneralwaytouseapropertywrapper.Youcanprovidewhateverargumentsyouneedtotheattribute,andthey’repassedtotheinitializer.

Whenyouincludepropertywrapperarguments,youcanalsospecifyaninitialvalueusingassignment.SwifttreatstheassignmentlikeawrappedValueargumentandusestheinitializerthatacceptstheargumentsyouinclude.Forexample:

1 structMixedRectangle{

2 @SmallNumbervarheight:Int=1

3 @SmallNumber(maximum:9)varwidth:Int=2

4 }

5

6 varmixedRectangle=MixedRectangle()

7 print(mixedRectangle.height)

8 //Prints"1"

9

10 mixedRectangle.height=20

11 print(mixedRectangle.height)

12 //Prints"12"

TheinstanceofSmallNumberthatwrapsheightiscreatedbycallingSmallNumber(wrappedValue:1),whichusesthedefaultmaximumvalueof12.TheinstancethatwrapswidthiscreatedbycallingSmallNumber(wrappedValue:2,maximum:9).

ProjectingaValueFromaPropertyWrapperInadditiontothewrappedvalue,apropertywrappercanexposeadditionalfunctionalitybydefiningaprojectedvalue—forexample,apropertywrapperthatmanagesaccesstoadatabasecanexposeaflushDatabaseConnection()methodonitsprojectedvalue.Thenameoftheprojectedvalueisthesameasthewrappedvalue,exceptitbeginswithadollarsign($).Becauseyourcodecan’tdefinepropertiesthatstartwith$theprojectedvalueneverinterfereswithpropertiesyoudefine.

IntheSmallNumberexampleabove,ifyoutrytosetthepropertytoanumberthat’stoolarge,thepropertywrapperadjuststhenumberbeforestoringit.ThecodebelowaddsaprojectedValuepropertytotheSmallNumberstructuretokeeptrackofwhetherthepropertywrapperadjustedthenewvalueforthepropertybeforestoringthatnewvalue.

1 @propertyWrapper

2 structSmallNumber{

3 privatevarnumber=0

4 varprojectedValue=false

5 varwrappedValue:Int{

6 get{returnnumber}

7 set{

8 ifnewValue>12{

9 number=12

10 projectedValue=true

11 }else{

12 number=newValue

13 projectedValue=false

14 }

15 }

16 }

17 }

18 structSomeStructure{

19 @SmallNumbervarsomeNumber:Int

20 }

21 varsomeStructure=SomeStructure()

22

23 someStructure.someNumber=4

24 print(someStructure.$someNumber)

25 //Prints"false"

26

27 someStructure.someNumber=55

28 print(someStructure.$someNumber)

29 //Prints"true"

Writings.$someNumberaccessesthewrapper’sprojectedvalue.Afterstoringasmallnumberlikefour,thevalueofs.$someNumberisfalse.However,theprojectedvalueistrueaftertryingtostoreanumberthat’stoolarge,like55.

Apropertywrappercanreturnavalueofanytypeasitsprojectedvalue.Inthisexample,thepropertywrapperexposesonlyonepieceofinformation—whetherthenumberwasadjusted—soitexposesthatBooleanvalueasitsprojectedvalue.Awrapperthatneedstoexposemoreinformationcanreturnaninstanceofsomeotherdatatype,oritcanreturnselftoexposetheinstanceofthewrapperasitsprojectedvalue.

Whenyouaccessaprojectedvaluefromcodethat’spartofthetype,likeapropertygetteroraninstancemethod,youcanomitself.beforethepropertyname,justlikeaccessingotherproperties.Thecodeinthefollowingexamplereferstotheprojectedvalueofthewrapperaroundheightandwidthas$height

and$width:

1 enumSize{

2 casesmall,large

3 }

4

5 structSizedRectangle{

6 @SmallNumbervarheight:Int

7 @SmallNumbervarwidth:Int

8

9 mutatingfuncresize(tosize:Size)->Bool{

10 switchsize{

11 case.small:

12 height=10

13 width=20

14 case.large:

15 height=100

16 width=100

17 }

18 return$height||$width

19 }

20 }

Becausepropertywrappersyntaxisjustsyntacticsugarforapropertywithagetterandasetter,accessingheightandwidthbehavesthesameasaccessinganyotherproperty.Forexample,thecodeinresize(to:)accessesheightandwidthusingtheirpropertywrapper.Ifyoucallresize(to:.large),theswitchcasefor.largesetstherectangle’sheightandwidthto100.Thewrapperpreventsthevalueofthosepropertiesfrombeinglargerthan12,anditsetstheprojectedvaluetotrue,torecordthefactthatitadjustedtheirvalues.Attheendofresize(to:),thereturnstatementchecks$heightand$widthtodeterminewhetherthepropertywrapperadjustedeitherheightorwidth.

GlobalandLocalVariables

Thecapabilitiesdescribedaboveforcomputingandobservingpropertiesarealsoavailabletoglobalvariablesandlocalvariables.Globalvariablesarevariablesthataredefinedoutsideofanyfunction,method,closure,ortypecontext.Localvariablesarevariablesthataredefinedwithinafunction,method,orclosurecontext.

Theglobalandlocalvariablesyouhaveencounteredinpreviouschaptershaveallbeenstoredvariables.Storedvariables,likestoredproperties,providestorageforavalueofacertaintypeandallowthatvaluetobesetandretrieved.

However,youcanalsodefinecomputedvariablesanddefineobserversforstoredvariables,ineitheraglobalorlocalscope.Computedvariablescalculatetheirvalue,ratherthanstoringit,andtheyarewritteninthesamewayascomputedproperties.

NOTE

Globalconstantsandvariablesarealwayscomputedlazily,inasimilarmannertoLazyStoredProperties.Unlikelazystoredproperties,globalconstantsandvariablesdonotneedtobemarkedwiththelazymodifier.

Localconstantsandvariablesarenevercomputedlazily.

TypeProperties

Instancepropertiesarepropertiesthatbelongtoaninstanceofaparticulartype.Everytimeyoucreateanewinstanceofthattype,ithasitsownsetofpropertyvalues,separatefromanyotherinstance.

Youcanalsodefinepropertiesthatbelongtothetypeitself,nottoanyoneinstanceofthattype.Therewillonlyeverbeonecopyoftheseproperties,nomatterhowmanyinstancesofthattypeyoucreate.Thesekindsofpropertiesarecalledtypeproperties.

Typepropertiesareusefulfordefiningvaluesthatareuniversaltoallinstances

ofaparticulartype,suchasaconstantpropertythatallinstancescanuse(likeastaticconstantinC),oravariablepropertythatstoresavaluethatisglobaltoallinstancesofthattype(likeastaticvariableinC).

Storedtypepropertiescanbevariablesorconstants.Computedtypepropertiesarealwaysdeclaredasvariableproperties,inthesamewayascomputedinstanceproperties.

NOTE

Unlikestoredinstanceproperties,youmustalwaysgivestoredtypepropertiesadefaultvalue.Thisisbecausethetypeitselfdoesnothaveaninitializerthatcanassignavaluetoastoredtypepropertyatinitializationtime.

Storedtypepropertiesarelazilyinitializedontheirfirstaccess.Theyareguaranteedtobeinitializedonlyonce,evenwhenaccessedbymultiplethreadssimultaneously,andtheydonotneedtobemarkedwiththelazymodifier.

TypePropertySyntaxInCandObjective-C,youdefinestaticconstantsandvariablesassociatedwithatypeasglobalstaticvariables.InSwift,however,typepropertiesarewrittenaspartofthetype’sdefinition,withinthetype’soutercurlybraces,andeachtypepropertyisexplicitlyscopedtothetypeitsupports.

Youdefinetypepropertieswiththestatickeyword.Forcomputedtypepropertiesforclasstypes,youcanusetheclasskeywordinsteadtoallowsubclassestooverridethesuperclass’simplementation.Theexamplebelowshowsthesyntaxforstoredandcomputedtypeproperties:

1 structSomeStructure{

2 staticvarstoredTypeProperty="Somevalue."

3 staticvarcomputedTypeProperty:Int{

4 return1

5 }

6 }

7 enumSomeEnumeration{

8 staticvarstoredTypeProperty="Somevalue."

9 staticvarcomputedTypeProperty:Int{

10 return6

11 }

12 }

13 classSomeClass{

14 staticvarstoredTypeProperty="Somevalue."

15 staticvarcomputedTypeProperty:Int{

16 return27

17 }

18 classvaroverrideableComputedTypeProperty:Int{

19 return107

20 }

21 }

NOTE

Thecomputedtypepropertyexamplesaboveareforread-onlycomputedtypeproperties,butyoucanalsodefineread-writecomputedtypepropertieswiththesamesyntaxasforcomputedinstanceproperties.

QueryingandSettingTypePropertiesTypepropertiesarequeriedandsetwithdotsyntax,justlikeinstanceproperties.However,typepropertiesarequeriedandsetonthetype,notonaninstanceofthattype.Forexample:

1 print(SomeStructure.storedTypeProperty)

2 //Prints"Somevalue."

3 SomeStructure.storedTypeProperty="Anothervalue."

4 print(SomeStructure.storedTypeProperty)

5 //Prints"Anothervalue."

6 print(SomeEnumeration.computedTypeProperty)

7 //Prints"6"

8 print(SomeClass.computedTypeProperty)

9 //Prints"27"

Theexamplesthatfollowusetwostoredtypepropertiesaspartofastructurethatmodelsanaudiolevelmeterforanumberofaudiochannels.Eachchannelhasanintegeraudiolevelbetween0and10inclusive.

Thefigurebelowillustrateshowtwooftheseaudiochannelscanbecombinedtomodelastereoaudiolevelmeter.Whenachannel’saudiolevelis0,noneofthelightsforthatchannelarelit.Whentheaudiolevelis10,allofthelightsforthatchannelarelit.Inthisfigure,theleftchannelhasacurrentlevelof9,andtherightchannelhasacurrentlevelof7:

TheaudiochannelsdescribedabovearerepresentedbyinstancesoftheAudioChannelstructure:

1 structAudioChannel{

2 staticletthresholdLevel=10

3 staticvarmaxInputLevelForAllChannels=0

4 varcurrentLevel:Int=0{

5 didSet{

6 ifcurrentLevel>AudioChannel.thresholdLevel{

7 //capthenewaudioleveltothethreshold

level

8 currentLevel=AudioChannel.thresholdLevel

9 }

10 ifcurrentLevel>

AudioChannel.maxInputLevelForAllChannels{

11 //storethisasthenewoverallmaximuminput

level

12 AudioChannel.maxInputLevelForAllChannels=

currentLevel

13 }

14 }

15 }

16 }

TheAudioChannelstructuredefinestwostoredtypepropertiestosupportitsfunctionality.Thefirst,thresholdLevel,definesthemaximumthresholdvalueanaudiolevelcantake.Thisisaconstantvalueof10forallAudioChannelinstances.Ifanaudiosignalcomesinwithahighervaluethan10,itwillbecappedtothisthresholdvalue(asdescribedbelow).

ThesecondtypepropertyisavariablestoredpropertycalledmaxInputLevelForAllChannels.ThiskeepstrackofthemaximuminputvaluethathasbeenreceivedbyanyAudioChannelinstance.Itstartswithaninitialvalueof0.

TheAudioChannelstructurealsodefinesastoredinstancepropertycalledcurrentLevel,whichrepresentsthechannel’scurrentaudiolevelonascaleof0to10.

ThecurrentLevelpropertyhasadidSetpropertyobservertocheckthevalueofcurrentLevelwheneveritisset.Thisobserverperformstwochecks:

IfthenewvalueofcurrentLevelisgreaterthantheallowedthresholdLevel,thepropertyobservercapscurrentLeveltothresholdLevel.

IfthenewvalueofcurrentLevel(afteranycapping)ishigherthananyvaluepreviouslyreceivedbyanyAudioChannelinstance,thepropertyobserverstoresthenewcurrentLevelvalueinthemaxInputLevelForAllChannelstypeproperty.

NOTE

Inthefirstofthesetwochecks,thedidSetobserversetscurrentLeveltoadifferentvalue.Thisdoesnot,however,causetheobservertobecalledagain.

YoucanusetheAudioChannelstructuretocreatetwonewaudiochannelscalledleftChannelandrightChannel,torepresenttheaudiolevelsofastereosoundsystem:

1 varleftChannel=AudioChannel()

2 varrightChannel=AudioChannel()

IfyousetthecurrentLeveloftheleftchannelto7,youcanseethatthemaxInputLevelForAllChannelstypepropertyisupdatedtoequal7:

1 leftChannel.currentLevel=7

2 print(leftChannel.currentLevel)

3 //Prints"7"

4 print(AudioChannel.maxInputLevelForAllChannels)

5 //Prints"7"

IfyoutrytosetthecurrentLeveloftherightchannelto11,youcanseethattherightchannel’scurrentLevelpropertyiscappedtothemaximumvalueof10,andthemaxInputLevelForAllChannelstypepropertyisupdatedtoequal10:

1 rightChannel.currentLevel=11

2 print(rightChannel.currentLevel)

3 //Prints"10"

4 print(AudioChannel.maxInputLevelForAllChannels)

5 //Prints"10"

Methods

Methodsarefunctionsthatareassociatedwithaparticulartype.Classes,structures,andenumerationscanalldefineinstancemethods,whichencapsulatespecifictasksandfunctionalityforworkingwithaninstanceofagiventype.Classes,structures,andenumerationscanalsodefinetypemethods,whichareassociatedwiththetypeitself.TypemethodsaresimilartoclassmethodsinObjective-C.

ThefactthatstructuresandenumerationscandefinemethodsinSwiftisamajordifferencefromCandObjective-C.InObjective-C,classesaretheonlytypesthatcandefinemethods.InSwift,youcanchoosewhethertodefineaclass,structure,orenumeration,andstillhavetheflexibilitytodefinemethodsonthetypeyoucreate.

InstanceMethods

Instancemethodsarefunctionsthatbelongtoinstancesofaparticularclass,structure,orenumeration.Theysupportthefunctionalityofthoseinstances,eitherbyprovidingwaystoaccessandmodifyinstanceproperties,orbyprovidingfunctionalityrelatedtotheinstance’spurpose.Instancemethodshaveexactlythesamesyntaxasfunctions,asdescribedinFunctions.

Youwriteaninstancemethodwithintheopeningandclosingbracesofthetypeitbelongsto.Aninstancemethodhasimplicitaccesstoallotherinstancemethodsandpropertiesofthattype.Aninstancemethodcanbecalledonlyonaspecificinstanceofthetypeitbelongsto.Itcannotbecalledinisolationwithoutanexistinginstance.

Here’sanexamplethatdefinesasimpleCounterclass,whichcanbeusedtocountthenumberoftimesanactionoccurs:

1 classCounter{

2 varcount=0

3 funcincrement(){

4 count+=1

5 }

6 funcincrement(byamount:Int){

7 count+=amount

8 }

9 funcreset(){

10 count=0

11 }

12 }

TheCounterclassdefinesthreeinstancemethods:

increment()incrementsthecounterby1.

increment(by:Int)incrementsthecounterbyaspecifiedintegeramount.

reset()resetsthecountertozero.

TheCounterclassalsodeclaresavariableproperty,count,tokeeptrackofthecurrentcountervalue.

Youcallinstancemethodswiththesamedotsyntaxasproperties:

1 letcounter=Counter()

2 //theinitialcountervalueis0

3 counter.increment()

4 //thecounter'svalueisnow1

5 counter.increment(by:5)

6 //thecounter'svalueisnow6

7 counter.reset()

8 //thecounter'svalueisnow0

Functionparameterscanhavebothaname(forusewithinthefunction’sbody)andanargumentlabel(forusewhencallingthefunction),asdescribedinFunctionArgumentLabelsandParameterNames.Thesameistrueformethodparameters,becausemethodsarejustfunctionsthatareassociatedwithatype.

TheselfPropertyEveryinstanceofatypehasanimplicitpropertycalledself,whichisexactlyequivalenttotheinstanceitself.Youusetheselfpropertytorefertothecurrentinstancewithinitsowninstancemethods.

Theincrement()methodintheexampleabovecouldhavebeenwrittenlikethis:

1 funcincrement(){

2 self.count+=1

3 }

Inpractice,youdon’tneedtowriteselfinyourcodeveryoften.Ifyoudon’texplicitlywriteself,Swiftassumesthatyouarereferringtoapropertyormethodofthecurrentinstancewheneveryouuseaknownpropertyormethodnamewithinamethod.Thisassumptionisdemonstratedbytheuseofcount(ratherthanself.count)insidethethreeinstancemethodsforCounter.

Themainexceptiontothisruleoccurswhenaparameternameforaninstancemethodhasthesamenameasapropertyofthatinstance.Inthissituation,theparameternametakesprecedence,anditbecomesnecessarytorefertothepropertyinamorequalifiedway.Youusetheselfpropertytodistinguishbetweentheparameternameandthepropertyname.

Here,selfdisambiguatesbetweenamethodparametercalledxandaninstancepropertythatisalsocalledx:

1 structPoint{

2 varx=0.0,y=0.0

3 funcisToTheRightOf(x:Double)->Bool{

4 returnself.x>x

5 }

6 }

7 letsomePoint=Point(x:4.0,y:5.0)

8 ifsomePoint.isToTheRightOf(x:1.0){

9 print("Thispointistotherightofthelinewherex==

1.0")

10 }

11 //Prints"Thispointistotherightofthelinewherex==

1.0"

Withouttheselfprefix,Swiftwouldassumethatbothusesofxreferredtothemethodparametercalledx.

ModifyingValueTypesfromWithinInstanceMethodsStructuresandenumerationsarevaluetypes.Bydefault,thepropertiesofavaluetypecannotbemodifiedfromwithinitsinstancemethods.

However,ifyouneedtomodifythepropertiesofyourstructureorenumerationwithinaparticularmethod,youcanoptintomutatingbehaviorforthatmethod.Themethodcanthenmutate(thatis,change)itspropertiesfromwithinthemethod,andanychangesthatitmakesarewrittenbacktotheoriginalstructurewhenthemethodends.Themethodcanalsoassignacompletelynewinstancetoitsimplicitselfproperty,andthisnewinstancewillreplacetheexistingonewhenthemethodends.

Youcanoptintothisbehaviorbyplacingthemutatingkeywordbeforethefunckeywordforthatmethod:

1 structPoint{

2 varx=0.0,y=0.0

3 mutatingfuncmoveBy(xdeltaX:Double,ydeltaY:Double){

4 x+=deltaX

5 y+=deltaY

6 }

7 }

8 varsomePoint=Point(x:1.0,y:1.0)

9 somePoint.moveBy(x:2.0,y:3.0)

10 print("Thepointisnowat(\(somePoint.x),\(somePoint.y))")

11 //Prints"Thepointisnowat(3.0,4.0)"

ThePointstructureabovedefinesamutatingmoveBy(x:y:)method,whichmovesaPointinstancebyacertainamount.Insteadofreturninganewpoint,thismethodactuallymodifiesthepointonwhichitiscalled.Themutatingkeywordisaddedtoitsdefinitiontoenableittomodifyitsproperties.

Notethatyoucannotcallamutatingmethodonaconstantofstructuretype,becauseitspropertiescannotbechanged,eveniftheyarevariableproperties,asdescribedinStoredPropertiesofConstantStructureInstances:

1 letfixedPoint=Point(x:3.0,y:3.0)

2 fixedPoint.moveBy(x:2.0,y:3.0)

3 //thiswillreportanerror

AssigningtoselfWithinaMutatingMethodMutatingmethodscanassignanentirelynewinstancetotheimplicitselfproperty.ThePointexampleshownabovecouldhavebeenwritteninthefollowingwayinstead:

1 structPoint{

2 varx=0.0,y=0.0

3 mutatingfuncmoveBy(xdeltaX:Double,ydeltaY:Double){

4 self=Point(x:x+deltaX,y:y+deltaY)

5 }

6 }

ThisversionofthemutatingmoveBy(x:y:)methodcreatesanewstructurewhosexandyvaluesaresettothetargetlocation.Theendresultofcallingthisalternativeversionofthemethodwillbeexactlythesameasforcallingtheearlierversion.

Mutatingmethodsforenumerationscansettheimplicitselfparametertobeadifferentcasefromthesameenumeration:

1 enumTriStateSwitch{

2 caseoff,low,high

3 mutatingfuncnext(){

4 switchself{

5 case.off:

6 self=.low

7 case.low:

8 self=.high

9 case.high:

10 self=.off

11 }

12 }

13 }

14 varovenLight=TriStateSwitch.low

15 ovenLight.next()

16 //ovenLightisnowequalto.high

17 ovenLight.next()

18 //ovenLightisnowequalto.off

Thisexampledefinesanenumerationforathree-stateswitch.Theswitchcyclesbetweenthreedifferentpowerstates(off,lowandhigh)everytimeitsnext()

methodiscalled.

TypeMethods

Instancemethods,asdescribedabove,aremethodsthatyoucallonaninstanceofaparticulartype.Youcanalsodefinemethodsthatarecalledonthetypeitself.Thesekindsofmethodsarecalledtypemethods.Youindicatetypemethodsbywritingthestatickeywordbeforethemethod’sfunckeyword.Classescanusetheclasskeywordinstead,toallowsubclassestooverridethesuperclass’simplementationofthatmethod.

NOTE

InObjective-C,youcandefinetype-levelmethodsonlyforObjective-Cclasses.InSwift,youcandefinetype-levelmethodsforallclasses,structures,andenumerations.Eachtypemethodisexplicitlyscopedtothetypeitsupports.

Typemethodsarecalledwithdotsyntax,likeinstancemethods.However,youcalltypemethodsonthetype,notonaninstanceofthattype.Here’showyoucallatypemethodonaclasscalledSomeClass:

1 classSomeClass{

2 classfuncsomeTypeMethod(){

3 //typemethodimplementationgoeshere

4 }

5 }

6 SomeClass.someTypeMethod()

Withinthebodyofatypemethod,theimplicitselfpropertyreferstothetypeitself,ratherthananinstanceofthattype.Thismeansthatyoucanuseselftodisambiguatebetweentypepropertiesandtypemethodparameters,justasyoudoforinstancepropertiesandinstancemethodparameters.

Moregenerally,anyunqualifiedmethodandpropertynamesthatyouusewithin

thebodyofatypemethodwillrefertoothertype-levelmethodsandproperties.Atypemethodcancallanothertypemethodwiththeothermethod’sname,withoutneedingtoprefixitwiththetypename.Similarly,typemethodsonstructuresandenumerationscanaccesstypepropertiesbyusingthetypeproperty’snamewithoutatypenameprefix.

TheexamplebelowdefinesastructurecalledLevelTracker,whichtracksaplayer’sprogressthroughthedifferentlevelsorstagesofagame.Itisasingle-playergame,butcanstoreinformationformultipleplayersonasingledevice.

Allofthegame’slevels(apartfromlevelone)arelockedwhenthegameisfirstplayed.Everytimeaplayerfinishesalevel,thatlevelisunlockedforallplayersonthedevice.TheLevelTrackerstructureusestypepropertiesandmethodstokeeptrackofwhichlevelsofthegamehavebeenunlocked.Italsotracksthecurrentlevelforanindividualplayer.

1 structLevelTracker{

2 staticvarhighestUnlockedLevel=1

3 varcurrentLevel=1

4

5 staticfuncunlock(_level:Int){

6 iflevel>highestUnlockedLevel{highestUnlockedLevel

=level}

7 }

8

9 staticfuncisUnlocked(_level:Int)->Bool{

10 returnlevel<=highestUnlockedLevel

11 }

12

13 @discardableResult

14 mutatingfuncadvance(tolevel:Int)->Bool{

15 ifLevelTracker.isUnlocked(level){

16 currentLevel=level

17 returntrue

18 }else{

19 returnfalse

20 }

21 }

22 }

TheLevelTrackerstructurekeepstrackofthehighestlevelthatanyplayerhasunlocked.ThisvalueisstoredinatypepropertycalledhighestUnlockedLevel.

LevelTrackeralsodefinestwotypefunctionstoworkwiththehighestUnlockedLevelproperty.Thefirstisatypefunctioncalledunlock(_:),whichupdatesthevalueofhighestUnlockedLevelwheneveranewlevelisunlocked.ThesecondisaconveniencetypefunctioncalledisUnlocked(_:),whichreturnstrueifaparticularlevelnumberisalreadyunlocked.(NotethatthesetypemethodscanaccessthehighestUnlockedLeveltypepropertywithoutyourneedingtowriteitasLevelTracker.highestUnlockedLevel.)

Inadditiontoitstypepropertyandtypemethods,LevelTrackertracksanindividualplayer’sprogressthroughthegame.ItusesaninstancepropertycalledcurrentLeveltotrackthelevelthataplayeriscurrentlyplaying.

TohelpmanagethecurrentLevelproperty,LevelTrackerdefinesaninstancemethodcalledadvance(to:).BeforeupdatingcurrentLevel,thismethodcheckswhethertherequestednewlevelisalreadyunlocked.Theadvance(to:)methodreturnsaBooleanvaluetoindicatewhetherornotitwasactuallyabletosetcurrentLevel.Becauseit’snotnecessarilyamistakeforcodethatcallstheadvance(to:)methodtoignorethereturnvalue,thisfunctionismarkedwiththe@discardableResultattribute.Formoreinformationaboutthisattribute,seeAttributes.

TheLevelTrackerstructureisusedwiththePlayerclass,shownbelow,totrackandupdatetheprogressofanindividualplayer:

1 classPlayer{

2 vartracker=LevelTracker()

3 letplayerName:String

4 funccomplete(level:Int){

5 LevelTracker.unlock(level+1)

6 tracker.advance(to:level+1)

7 }

8 init(name:String){

9 playerName=name

10 }

11 }

ThePlayerclasscreatesanewinstanceofLevelTrackertotrackthatplayer’sprogress.Italsoprovidesamethodcalledcomplete(level:),whichiscalledwheneveraplayercompletesaparticularlevel.Thismethodunlocksthenextlevelforallplayersandupdatestheplayer’sprogresstomovethemtothenextlevel.(TheBooleanreturnvalueofadvance(to:)isignored,becausethelevelisknowntohavebeenunlockedbythecalltoLevelTracker.unlock(_:)onthepreviousline.)

YoucancreateaninstanceofthePlayerclassforanewplayer,andseewhathappenswhentheplayercompleteslevelone:

1 varplayer=Player(name:"Argyrios")

2 player.complete(level:1)

3 print("highestunlockedlevelisnow\

(LevelTracker.highestUnlockedLevel)")

4 //Prints"highestunlockedlevelisnow2"

Ifyoucreateasecondplayer,whomyoutrytomovetoalevelthatisnotyetunlockedbyanyplayerinthegame,theattempttosettheplayer’scurrentlevelfails:

1 player=Player(name:"Beto")

2 ifplayer.tracker.advance(to:6){

3 print("playerisnowonlevel6")

4 }else{

5 print("level6hasnotyetbeenunlocked")

6 }

7 //Prints"level6hasnotyetbeenunlocked"

Subscripts

Classes,structures,andenumerationscandefinesubscripts,whichareshortcutsforaccessingthememberelementsofacollection,list,orsequence.Youusesubscriptstosetandretrievevaluesbyindexwithoutneedingseparatemethodsforsettingandretrieval.Forexample,youaccesselementsinanArrayinstanceassomeArray[index]andelementsinaDictionaryinstanceassomeDictionary[key].

Youcandefinemultiplesubscriptsforasingletype,andtheappropriatesubscriptoverloadtouseisselectedbasedonthetypeofindexvalueyoupasstothesubscript.Subscriptsarenotlimitedtoasingledimension,andyoucandefinesubscriptswithmultipleinputparameterstosuityourcustomtype’sneeds.

SubscriptSyntax

Subscriptsenableyoutoqueryinstancesofatypebywritingoneormorevaluesinsquarebracketsaftertheinstancename.Theirsyntaxissimilartobothinstancemethodsyntaxandcomputedpropertysyntax.Youwritesubscriptdefinitionswiththesubscriptkeyword,andspecifyoneormoreinputparametersandareturntype,inthesamewayasinstancemethods.Unlikeinstancemethods,subscriptscanberead-writeorread-only.Thisbehavioriscommunicatedbyagetterandsetterinthesamewayasforcomputedproperties:

1 subscript(index:Int)->Int{

2 get{

3 //Returnanappropriatesubscriptvaluehere.

4 }

5 set(newValue){

6 //Performasuitablesettingactionhere.

7 }

8 }

ThetypeofnewValueisthesameasthereturnvalueofthesubscript.Aswithcomputedproperties,youcanchoosenottospecifythesetter’s(newValue)parameter.AdefaultparametercallednewValueisprovidedtoyoursetterifyoudonotprovideoneyourself.

Aswithread-onlycomputedproperties,youcansimplifythedeclarationofaread-onlysubscriptbyremovingthegetkeywordanditsbraces:

1 subscript(index:Int)->Int{

2 //Returnanappropriatesubscriptvaluehere.

3 }

Here’sanexampleofaread-onlysubscriptimplementation,whichdefinesaTimesTablestructuretorepresentann-times-tableofintegers:

1 structTimesTable{

2 letmultiplier:Int

3 subscript(index:Int)->Int{

4 returnmultiplier*index

5 }

6 }

7 letthreeTimesTable=TimesTable(multiplier:3)

8 print("sixtimesthreeis\(threeTimesTable[6])")

9 //Prints"sixtimesthreeis18"

Inthisexample,anewinstanceofTimesTableiscreatedtorepresentthethree-times-table.Thisisindicatedbypassingavalueof3tothestructure’sinitializerasthevaluetousefortheinstance’smultiplierparameter.

YoucanquerythethreeTimesTableinstancebycallingitssubscript,asshowninthecalltothreeTimesTable[6].Thisrequeststhesixthentryinthethree-times-

table,whichreturnsavalueof18,or3times6.

NOTE

Ann-times-tableisbasedonafixedmathematicalrule.ItisnotappropriatetosetthreeTimesTable[someIndex]toanewvalue,andsothesubscriptforTimesTableisdefinedasaread-onlysubscript.

SubscriptUsage

Theexactmeaningof“subscript”dependsonthecontextinwhichitisused.Subscriptsaretypicallyusedasashortcutforaccessingthememberelementsinacollection,list,orsequence.Youarefreetoimplementsubscriptsinthemostappropriatewayforyourparticularclassorstructure’sfunctionality.

Forexample,Swift’sDictionarytypeimplementsasubscripttosetandretrievethevaluesstoredinaDictionaryinstance.Youcansetavalueinadictionarybyprovidingakeyofthedictionary’skeytypewithinsubscriptbrackets,andassigningavalueofthedictionary’svaluetypetothesubscript:

1 varnumberOfLegs=["spider":8,"ant":6,"cat":4]

2 numberOfLegs["bird"]=2

TheexampleabovedefinesavariablecallednumberOfLegsandinitializesitwithadictionaryliteralcontainingthreekey-valuepairs.ThetypeofthenumberOfLegsdictionaryisinferredtobe[String:Int].Aftercreatingthedictionary,thisexampleusessubscriptassignmenttoaddaStringkeyof"bird"andanIntvalueof2tothedictionary.

FormoreinformationaboutDictionarysubscripting,seeAccessingandModifyingaDictionary.

NOTE

Swift’sDictionarytypeimplementsitskey-valuesubscriptingasasubscriptthattakesandreturnsanoptionaltype.ForthenumberOfLegsdictionaryabove,thekey-valuesubscripttakesandreturnsavalueoftypeInt?,or“optionalint”.TheDictionarytypeusesanoptional

subscripttypetomodelthefactthatnoteverykeywillhaveavalue,andtogiveawaytodeleteavalueforakeybyassigninganilvalueforthatkey.

SubscriptOptions

Subscriptscantakeanynumberofinputparameters,andtheseinputparameterscanbeofanytype.Subscriptscanalsoreturnanytype.Subscriptscanusevariadicparameters,buttheycan’tusein-outparametersorprovidedefaultparametervalues.

Aclassorstructurecanprovideasmanysubscriptimplementationsasitneeds,andtheappropriatesubscripttobeusedwillbeinferredbasedonthetypesofthevalueorvaluesthatarecontainedwithinthesubscriptbracketsatthepointthatthesubscriptisused.Thisdefinitionofmultiplesubscriptsisknownassubscriptoverloading.

Whileitismostcommonforasubscripttotakeasingleparameter,youcanalsodefineasubscriptwithmultipleparametersifitisappropriateforyourtype.ThefollowingexampledefinesaMatrixstructure,whichrepresentsatwo-dimensionalmatrixofDoublevalues.TheMatrixstructure’ssubscripttakestwointegerparameters:

1 structMatrix{

2 letrows:Int,columns:Int

3 vargrid:[Double]

4 init(rows:Int,columns:Int){

5 self.rows=rows

6 self.columns=columns

7 grid=Array(repeating:0.0,count:rows*columns)

8 }

9 funcindexIsValid(row:Int,column:Int)->Bool{

10 returnrow>=0&&row<rows&&column>=0&&column

<columns

11 }

12 subscript(row:Int,column:Int)->Double{

13 get{

14 assert(indexIsValid(row:row,column:column),

"Indexoutofrange")

15 returngrid[(row*columns)+column]

16 }

17 set{

18 assert(indexIsValid(row:row,column:column),

"Indexoutofrange")

19 grid[(row*columns)+column]=newValue

20 }

21 }

22 }

Matrixprovidesaninitializerthattakestwoparameterscalledrowsandcolumns,andcreatesanarraythatislargeenoughtostorerows*columnsvaluesoftypeDouble.Eachpositioninthematrixisgivenaninitialvalueof0.0.Toachievethis,thearray’ssize,andaninitialcellvalueof0.0,arepassedtoanarrayinitializerthatcreatesandinitializesanewarrayofthecorrectsize.ThisinitializerisdescribedinmoredetailinCreatinganArraywithaDefaultValue.

YoucanconstructanewMatrixinstancebypassinganappropriaterowandcolumncounttoitsinitializer:

varmatrix=Matrix(rows:2,columns:2)

TheexampleabovecreatesanewMatrixinstancewithtworowsandtwocolumns.ThegridarrayforthisMatrixinstanceiseffectivelyaflattenedversionofthematrix,asreadfromtoplefttobottomright:

Valuesinthematrixcanbesetbypassingrowandcolumnvaluesintothesubscript,separatedbyacomma:

1 matrix[0,1]=1.5

2 matrix[1,0]=3.2

Thesetwostatementscallthesubscript’ssettertosetavalueof1.5inthetoprightpositionofthematrix(whererowis0andcolumnis1),and3.2inthebottomleftposition(whererowis1andcolumnis0):

TheMatrixsubscript’sgetterandsetterbothcontainanassertiontocheckthatthesubscript’srowandcolumnvaluesarevalid.Toassistwiththeseassertions,MatrixincludesaconveniencemethodcalledindexIsValid(row:column:),whichcheckswhethertherequestedrowandcolumnareinsidetheboundsofthematrix:

1 funcindexIsValid(row:Int,column:Int)->Bool{

2 returnrow>=0&&row<rows&&column>=0&&column<

columns

3 }

Anassertionistriggeredifyoutrytoaccessasubscriptthatisoutsideofthematrixbounds:

1 letsomeValue=matrix[2,2]

2 //Thistriggersanassert,because[2,2]isoutsideofthe

matrixbounds.

TypeSubscripts

Instancesubscripts,asdescribedabove,aresubscriptsthatyoucallonaninstanceofaparticulartype.Youcanalsodefinesubscriptsthatarecalledonthetypeitself.Thiskindofsubscriptiscalledatypesubscript.Youindicateatypesubscriptbywritingthestatickeywordbeforethesubscriptkeyword.Classescanusetheclasskeywordinstead,toallowsubclassestooverridethesuperclass’simplementationofthatsubscript.Theexamplebelowshowshowyoudefineandcallatypesubscript:

1 enumPlanet:Int{

2 casemercury=1,venus,earth,mars,jupiter,saturn,

uranus,neptune

3 staticsubscript(n:Int)->Planet{

4 returnPlanet(rawValue:n)!

5 }

6 }

7 letmars=Planet[4]

8 print(mars)

Inheritance

Aclasscaninheritmethods,properties,andothercharacteristicsfromanotherclass.Whenoneclassinheritsfromanother,theinheritingclassisknownasasubclass,andtheclassitinheritsfromisknownasitssuperclass.InheritanceisafundamentalbehaviorthatdifferentiatesclassesfromothertypesinSwift.

ClassesinSwiftcancallandaccessmethods,properties,andsubscriptsbelongingtotheirsuperclassandcanprovidetheirownoverridingversionsofthosemethods,properties,andsubscriptstorefineormodifytheirbehavior.Swifthelpstoensureyouroverridesarecorrectbycheckingthattheoverridedefinitionhasamatchingsuperclassdefinition.

Classescanalsoaddpropertyobserverstoinheritedpropertiesinordertobenotifiedwhenthevalueofapropertychanges.Propertyobserverscanbeaddedtoanyproperty,regardlessofwhetheritwasoriginallydefinedasastoredorcomputedproperty.

DefiningaBaseClass

Anyclassthatdoesnotinheritfromanotherclassisknownasabaseclass.

NOTE

Swiftclassesdonotinheritfromauniversalbaseclass.Classesyoudefinewithoutspecifyingasuperclassautomaticallybecomebaseclassesforyoutobuildupon.

TheexamplebelowdefinesabaseclasscalledVehicle.ThisbaseclassdefinesastoredpropertycalledcurrentSpeed,withadefaultvalueof0.0(inferringapropertytypeofDouble).ThecurrentSpeedproperty’svalueisusedbyaread-onlycomputedStringpropertycalleddescriptiontocreateadescriptionofthevehicle.

TheVehiclebaseclassalsodefinesamethodcalledmakeNoise.Thismethod

doesnotactuallydoanythingforabaseVehicleinstance,butwillbecustomizedbysubclassesofVehiclelateron:

1 classVehicle{

2 varcurrentSpeed=0.0

3 vardescription:String{

4 return"travelingat\(currentSpeed)milesperhour"

5 }

6 funcmakeNoise(){

7 //donothing-anarbitraryvehicledoesn't

necessarilymakeanoise

8 }

9 }

YoucreateanewinstanceofVehiclewithinitializersyntax,whichiswrittenasatypenamefollowedbyemptyparentheses:

letsomeVehicle=Vehicle()

HavingcreatedanewVehicleinstance,youcanaccessitsdescriptionpropertytoprintahuman-readabledescriptionofthevehicle’scurrentspeed:

1 print("Vehicle:\(someVehicle.description)")

2 //Vehicle:travelingat0.0milesperhour

TheVehicleclassdefinescommoncharacteristicsforanarbitraryvehicle,butisnotmuchuseinitself.Tomakeitmoreuseful,youneedtorefineittodescribemorespecifickindsofvehicles.

Subclassing

Subclassingistheactofbasinganewclassonanexistingclass.Thesubclass

inheritscharacteristicsfromtheexistingclass,whichyoucanthenrefine.Youcanalsoaddnewcharacteristicstothesubclass.

Toindicatethatasubclasshasasuperclass,writethesubclassnamebeforethesuperclassname,separatedbyacolon:

1 classSomeSubclass:SomeSuperclass{

2 //subclassdefinitiongoeshere

3 }

ThefollowingexampledefinesasubclasscalledBicycle,withasuperclassofVehicle:

1 classBicycle:Vehicle{

2 varhasBasket=false

3 }

ThenewBicycleclassautomaticallygainsallofthecharacteristicsofVehicle,suchasitscurrentSpeedanddescriptionpropertiesanditsmakeNoise()method.

Inadditiontothecharacteristicsitinherits,theBicycleclassdefinesanewstoredproperty,hasBasket,withadefaultvalueoffalse(inferringatypeofBoolfortheproperty).

Bydefault,anynewBicycleinstanceyoucreatewillnothaveabasket.YoucansetthehasBasketpropertytotrueforaparticularBicycleinstanceafterthatinstanceiscreated:

1 letbicycle=Bicycle()

2 bicycle.hasBasket=true

YoucanalsomodifytheinheritedcurrentSpeedpropertyofaBicycleinstance,andquerytheinstance’sinheriteddescriptionproperty:

1 bicycle.currentSpeed=15.0

2 print("Bicycle:\(bicycle.description)")

3 //Bicycle:travelingat15.0milesperhour

Subclassescanthemselvesbesubclassed.ThenextexamplecreatesasubclassofBicycleforatwo-seaterbicycleknownasa“tandem”:

1 classTandem:Bicycle{

2 varcurrentNumberOfPassengers=0

3 }

TandeminheritsallofthepropertiesandmethodsfromBicycle,whichinturninheritsallofthepropertiesandmethodsfromVehicle.TheTandemsubclassalsoaddsanewstoredpropertycalledcurrentNumberOfPassengers,withadefaultvalueof0.

IfyoucreateaninstanceofTandem,youcanworkwithanyofitsnewandinheritedproperties,andquerytheread-onlydescriptionpropertyitinheritsfromVehicle:

1 lettandem=Tandem()

2 tandem.hasBasket=true

3 tandem.currentNumberOfPassengers=2

4 tandem.currentSpeed=22.0

5 print("Tandem:\(tandem.description)")

6 //Tandem:travelingat22.0milesperhour

Overriding

Asubclasscanprovideitsowncustomimplementationofaninstancemethod,typemethod,instanceproperty,typeproperty,orsubscriptthatitwouldotherwiseinheritfromasuperclass.Thisisknownasoverriding.

Tooverrideacharacteristicthatwouldotherwisebeinherited,youprefixyouroverridingdefinitionwiththeoverridekeyword.Doingsoclarifiesthatyouintendtoprovideanoverrideandhavenotprovidedamatchingdefinitionbymistake.Overridingbyaccidentcancauseunexpectedbehavior,andanyoverrideswithouttheoverridekeywordarediagnosedasanerrorwhenyourcodeiscompiled.

TheoverridekeywordalsopromptstheSwiftcompilertocheckthatyouroverridingclass’ssuperclass(oroneofitsparents)hasadeclarationthatmatchestheoneyouprovidedfortheoverride.Thischeckensuresthatyouroverridingdefinitioniscorrect.

AccessingSuperclassMethods,Properties,andSubscriptsWhenyouprovideamethod,property,orsubscriptoverrideforasubclass,itissometimesusefultousetheexistingsuperclassimplementationaspartofyouroverride.Forexample,youcanrefinethebehaviorofthatexistingimplementation,orstoreamodifiedvalueinanexistinginheritedvariable.

Wherethisisappropriate,youaccessthesuperclassversionofamethod,property,orsubscriptbyusingthesuperprefix:

AnoverriddenmethodnamedsomeMethod()cancallthesuperclassversionofsomeMethod()bycallingsuper.someMethod()withintheoverridingmethodimplementation.

AnoverriddenpropertycalledsomePropertycanaccessthesuperclassversionofsomePropertyassuper.somePropertywithintheoverridinggetterorsetterimplementation.

AnoverriddensubscriptforsomeIndexcanaccessthesuperclassversionofthesamesubscriptassuper[someIndex]fromwithintheoverridingsubscriptimplementation.

OverridingMethods

Youcanoverrideaninheritedinstanceortypemethodtoprovideatailoredoralternativeimplementationofthemethodwithinyoursubclass.

ThefollowingexampledefinesanewsubclassofVehiclecalledTrain,whichoverridesthemakeNoise()methodthatTraininheritsfromVehicle:

1 classTrain:Vehicle{

2 overridefuncmakeNoise(){

3 print("ChooChoo")

4 }

5 }

IfyoucreateanewinstanceofTrainandcallitsmakeNoise()method,youcanseethattheTrainsubclassversionofthemethodiscalled:

1 lettrain=Train()

2 train.makeNoise()

3 //Prints"ChooChoo"

OverridingPropertiesYoucanoverrideaninheritedinstanceortypepropertytoprovideyourowncustomgetterandsetterforthatproperty,ortoaddpropertyobserverstoenabletheoverridingpropertytoobservewhentheunderlyingpropertyvaluechanges.

OverridingPropertyGettersandSetters

Youcanprovideacustomgetter(andsetter,ifappropriate)tooverrideanyinheritedproperty,regardlessofwhethertheinheritedpropertyisimplementedasastoredorcomputedpropertyatsource.Thestoredorcomputednatureofaninheritedpropertyisnotknownbyasubclass—itonlyknowsthattheinheritedpropertyhasacertainnameandtype.Youmustalwaysstateboththenameandthetypeofthepropertyyouareoverriding,toenablethecompilertocheckthatyouroverridematchesasuperclasspropertywiththesamenameandtype.

Youcanpresentaninheritedread-onlypropertyasaread-writepropertybyprovidingbothagetterandasetterinyoursubclasspropertyoverride.Youcannot,however,presentaninheritedread-writepropertyasaread-onlyproperty.

NOTE

Ifyouprovideasetteraspartofapropertyoverride,youmustalsoprovideagetterforthatoverride.Ifyoudon’twanttomodifytheinheritedproperty’svaluewithintheoverridinggetter,youcansimplypassthroughtheinheritedvaluebyreturningsuper.somePropertyfromthegetter,wheresomePropertyisthenameofthepropertyyouareoverriding.

ThefollowingexampledefinesanewclasscalledCar,whichisasubclassofVehicle.TheCarclassintroducesanewstoredpropertycalledgear,withadefaultintegervalueof1.TheCarclassalsooverridesthedescriptionpropertyitinheritsfromVehicle,toprovideacustomdescriptionthatincludesthecurrentgear:

1 classCar:Vehicle{

2 vargear=1

3 overridevardescription:String{

4 returnsuper.description+"ingear\(gear)"

5 }

6 }

Theoverrideofthedescriptionpropertystartsbycallingsuper.description,whichreturnstheVehicleclass’sdescriptionproperty.TheCarclass’sversionofdescriptionthenaddssomeextratextontotheendofthisdescriptiontoprovideinformationaboutthecurrentgear.

IfyoucreateaninstanceoftheCarclassandsetitsgearandcurrentSpeedproperties,youcanseethatitsdescriptionpropertyreturnsthetailoreddescriptiondefinedwithintheCarclass:

1 letcar=Car()

2 car.currentSpeed=25.0

3 car.gear=3

4 print("Car:\(car.description)")

5 //Car:travelingat25.0milesperhouringear3

OverridingPropertyObservers

Youcanusepropertyoverridingtoaddpropertyobserverstoaninheritedproperty.Thisenablesyoutobenotifiedwhenthevalueofaninheritedpropertychanges,regardlessofhowthatpropertywasoriginallyimplemented.Formoreinformationonpropertyobservers,seePropertyObservers.

NOTE

Youcannotaddpropertyobserverstoinheritedconstantstoredpropertiesorinheritedread-onlycomputedproperties.Thevalueofthesepropertiescannotbeset,andsoitisnotappropriatetoprovideawillSetordidSetimplementationaspartofanoverride.

Notealsothatyoucannotprovidebothanoverridingsetterandanoverridingpropertyobserverforthesameproperty.Ifyouwanttoobservechangestoaproperty’svalue,andyouarealreadyprovidingacustomsetterforthatproperty,youcansimplyobserveanyvaluechangesfromwithinthecustomsetter.

ThefollowingexampledefinesanewclasscalledAutomaticCar,whichisasubclassofCar.TheAutomaticCarclassrepresentsacarwithanautomaticgearbox,whichautomaticallyselectsanappropriategeartousebasedonthecurrentspeed:

1 classAutomaticCar:Car{

2 overridevarcurrentSpeed:Double{

3 didSet{

4 gear=Int(currentSpeed/10.0)+1

5 }

6 }

7 }

WheneveryousetthecurrentSpeedpropertyofanAutomaticCarinstance,theproperty’sdidSetobserversetstheinstance’sgearpropertytoanappropriate

choiceofgearforthenewspeed.Specifically,thepropertyobserverchoosesagearthatisthenewcurrentSpeedvaluedividedby10,roundeddowntothenearestinteger,plus1.Aspeedof35.0producesagearof4:

1 letautomatic=AutomaticCar()

2 automatic.currentSpeed=35.0

3 print("AutomaticCar:\(automatic.description)")

4 //AutomaticCar:travelingat35.0milesperhouringear4

PreventingOverrides

Youcanpreventamethod,property,orsubscriptfrombeingoverriddenbymarkingitasfinal.Dothisbywritingthefinalmodifierbeforethemethod,property,orsubscript’sintroducerkeyword(suchasfinalvar,finalfunc,finalclassfunc,andfinalsubscript).

Anyattempttooverrideafinalmethod,property,orsubscriptinasubclassisreportedasacompile-timeerror.Methods,properties,orsubscriptsthatyouaddtoaclassinanextensioncanalsobemarkedasfinalwithintheextension’sdefinition.

Youcanmarkanentireclassasfinalbywritingthefinalmodifierbeforetheclasskeywordinitsclassdefinition(finalclass).Anyattempttosubclassafinalclassisreportedasacompile-timeerror.

Initialization

Initializationistheprocessofpreparinganinstanceofaclass,structure,orenumerationforuse.Thisprocessinvolvessettinganinitialvalueforeachstoredpropertyonthatinstanceandperforminganyothersetuporinitializationthatisrequiredbeforethenewinstanceisreadyforuse.

Youimplementthisinitializationprocessbydefininginitializers,whicharelikespecialmethodsthatcanbecalledtocreateanewinstanceofaparticulartype.UnlikeObjective-Cinitializers,Swiftinitializersdonotreturnavalue.Theirprimaryroleistoensurethatnewinstancesofatypearecorrectlyinitializedbeforetheyareusedforthefirsttime.

Instancesofclasstypescanalsoimplementadeinitializer,whichperformsanycustomcleanupjustbeforeaninstanceofthatclassisdeallocated.Formoreinformationaboutdeinitializers,seeDeinitialization.

SettingInitialValuesforStoredProperties

Classesandstructuresmustsetalloftheirstoredpropertiestoanappropriateinitialvaluebythetimeaninstanceofthatclassorstructureiscreated.Storedpropertiescannotbeleftinanindeterminatestate.

Youcansetaninitialvalueforastoredpropertywithinaninitializer,orbyassigningadefaultpropertyvalueaspartoftheproperty’sdefinition.Theseactionsaredescribedinthefollowingsections.

NOTE

Whenyouassignadefaultvaluetoastoredproperty,orsetitsinitialvaluewithinaninitializer,thevalueofthatpropertyissetdirectly,withoutcallinganypropertyobservers.

InitializersInitializersarecalledtocreateanewinstanceofaparticulartype.Initssimplestform,aninitializerislikeaninstancemethodwithnoparameters,writtenusingtheinitkeyword:

1 init(){

2 //performsomeinitializationhere

3 }

TheexamplebelowdefinesanewstructurecalledFahrenheittostoretemperaturesexpressedintheFahrenheitscale.TheFahrenheitstructurehasonestoredproperty,temperature,whichisoftypeDouble:

1 structFahrenheit{

2 vartemperature:Double

3 init(){

4 temperature=32.0

5 }

6 }

7 varf=Fahrenheit()

8 print("Thedefaulttemperatureis\(f.temperature)°

Fahrenheit")

9 //Prints"Thedefaulttemperatureis32.0°Fahrenheit"

Thestructuredefinesasingleinitializer,init,withnoparameters,whichinitializesthestoredtemperaturewithavalueof32.0(thefreezingpointofwaterindegreesFahrenheit).

DefaultPropertyValuesYoucansettheinitialvalueofastoredpropertyfromwithinaninitializer,asshownabove.Alternatively,specifyadefaultpropertyvalueaspartoftheproperty’sdeclaration.Youspecifyadefaultpropertyvaluebyassigningan

initialvaluetothepropertywhenitisdefined.

NOTE

Ifapropertyalwaystakesthesameinitialvalue,provideadefaultvalueratherthansettingavaluewithinaninitializer.Theendresultisthesame,butthedefaultvaluetiestheproperty’sinitializationmorecloselytoitsdeclaration.Itmakesforshorter,clearerinitializersandenablesyoutoinferthetypeofthepropertyfromitsdefaultvalue.Thedefaultvaluealsomakesiteasierforyoutotakeadvantageofdefaultinitializersandinitializerinheritance,asdescribedlaterinthischapter.

YoucanwritetheFahrenheitstructurefromaboveinasimplerformbyprovidingadefaultvalueforitstemperaturepropertyatthepointthatthepropertyisdeclared:

1 structFahrenheit{

2 vartemperature=32.0

3 }

CustomizingInitialization

Youcancustomizetheinitializationprocesswithinputparametersandoptionalpropertytypes,orbyassigningconstantpropertiesduringinitialization,asdescribedinthefollowingsections.

InitializationParametersYoucanprovideinitializationparametersaspartofaninitializer’sdefinition,todefinethetypesandnamesofvaluesthatcustomizetheinitializationprocess.Initializationparametershavethesamecapabilitiesandsyntaxasfunctionandmethodparameters.

ThefollowingexampledefinesastructurecalledCelsius,whichstorestemperaturesexpressedindegreesCelsius.TheCelsiusstructureimplementstwocustominitializerscalledinit(fromFahrenheit:)andinit(fromKelvin:),whichinitializeanewinstanceofthestructurewithavaluefromadifferent

temperaturescale:

1 structCelsius{

2 vartemperatureInCelsius:Double

3 init(fromFahrenheitfahrenheit:Double){

4 temperatureInCelsius=(fahrenheit-32.0)/1.8

5 }

6 init(fromKelvinkelvin:Double){

7 temperatureInCelsius=kelvin-273.15

8 }

9 }

10 letboilingPointOfWater=Celsius(fromFahrenheit:212.0)

11 //boilingPointOfWater.temperatureInCelsiusis100.0

12 letfreezingPointOfWater=Celsius(fromKelvin:273.15)

13 //freezingPointOfWater.temperatureInCelsiusis0.0

ThefirstinitializerhasasingleinitializationparameterwithanargumentlabeloffromFahrenheitandaparameternameoffahrenheit.ThesecondinitializerhasasingleinitializationparameterwithanargumentlabeloffromKelvinandaparameternameofkelvin.BothinitializersconverttheirsingleargumentintothecorrespondingCelsiusvalueandstorethisvalueinapropertycalledtemperatureInCelsius.

ParameterNamesandArgumentLabelsAswithfunctionandmethodparameters,initializationparameterscanhavebothaparameternameforusewithintheinitializer’sbodyandanargumentlabelforusewhencallingtheinitializer.

However,initializersdonothaveanidentifyingfunctionnamebeforetheirparenthesesinthewaythatfunctionsandmethodsdo.Therefore,thenamesandtypesofaninitializer’sparametersplayaparticularlyimportantroleinidentifyingwhichinitializershouldbecalled.Becauseofthis,Swiftprovidesan

automaticargumentlabelforeveryparameterinaninitializerifyoudon’tprovideone.

ThefollowingexampledefinesastructurecalledColor,withthreeconstantpropertiescalledred,green,andblue.Thesepropertiesstoreavaluebetween0.0and1.0toindicatetheamountofred,green,andblueinthecolor.

ColorprovidesaninitializerwiththreeappropriatelynamedparametersoftypeDoubleforitsred,green,andbluecomponents.Coloralsoprovidesasecondinitializerwithasinglewhiteparameter,whichisusedtoprovidethesamevalueforallthreecolorcomponents.

1 structColor{

2 letred,green,blue:Double

3 init(red:Double,green:Double,blue:Double){

4 self.red=red

5 self.green=green

6 self.blue=blue

7 }

8 init(white:Double){

9 red=white

10 green=white

11 blue=white

12 }

13 }

BothinitializerscanbeusedtocreateanewColorinstance,byprovidingnamedvaluesforeachinitializerparameter:

1 letmagenta=Color(red:1.0,green:0.0,blue:1.0)

2 lethalfGray=Color(white:0.5)

Notethatitisnotpossibletocalltheseinitializerswithoutusingargumentlabels.Argumentlabelsmustalwaysbeusedinaninitializeriftheyaredefined,

andomittingthemisacompile-timeerror:

1 letveryGreen=Color(0.0,1.0,0.0)

2 //thisreportsacompile-timeerror-argumentlabelsare

required

InitializerParametersWithoutArgumentLabelsIfyoudonotwanttouseanargumentlabelforaninitializerparameter,writeanunderscore(_)insteadofanexplicitargumentlabelforthatparametertooverridethedefaultbehavior.

Here’sanexpandedversionoftheCelsiusexamplefromInitializationParametersabove,withanadditionalinitializertocreateanewCelsiusinstancefromaDoublevaluethatisalreadyintheCelsiusscale:

1 structCelsius{

2 vartemperatureInCelsius:Double

3 init(fromFahrenheitfahrenheit:Double){

4 temperatureInCelsius=(fahrenheit-32.0)/1.8

5 }

6 init(fromKelvinkelvin:Double){

7 temperatureInCelsius=kelvin-273.15

8 }

9 init(_celsius:Double){

10 temperatureInCelsius=celsius

11 }

12 }

13 letbodyTemperature=Celsius(37.0)

14 //bodyTemperature.temperatureInCelsiusis37.0

TheinitializercallCelsius(37.0)isclearinitsintentwithouttheneedforan

argumentlabel.Itisthereforeappropriatetowritethisinitializerasinit(_celsius:Double)sothatitcanbecalledbyprovidinganunnamedDoublevalue.

OptionalPropertyTypesIfyourcustomtypehasastoredpropertythatislogicallyallowedtohave“novalue”—perhapsbecauseitsvaluecannotbesetduringinitialization,orbecauseitisallowedtohave“novalue”atsomelaterpoint—declarethepropertywithanoptionaltype.Propertiesofoptionaltypeareautomaticallyinitializedwithavalueofnil,indicatingthatthepropertyisdeliberatelyintendedtohave“novalueyet”duringinitialization.

ThefollowingexampledefinesaclasscalledSurveyQuestion,withanoptionalStringpropertycalledresponse:

1 classSurveyQuestion{

2 vartext:String

3 varresponse:String?

4 init(text:String){

5 self.text=text

6 }

7 funcask(){

8 print(text)

9 }

10 }

11 letcheeseQuestion=SurveyQuestion(text:"Doyoulike

cheese?")

12 cheeseQuestion.ask()

13 //Prints"Doyoulikecheese?"

14 cheeseQuestion.response="Yes,Idolikecheese."

Theresponsetoasurveyquestioncannotbeknownuntilitisasked,andsotheresponsepropertyisdeclaredwithatypeofString?,or“optionalString”.Itis

automaticallyassignedadefaultvalueofnil,meaning“nostringyet”,whenanewinstanceofSurveyQuestionisinitialized.

AssigningConstantPropertiesDuringInitializationYoucanassignavaluetoaconstantpropertyatanypointduringinitialization,aslongasitissettoadefinitevaluebythetimeinitializationfinishes.Onceaconstantpropertyisassignedavalue,itcan’tbefurthermodified.

NOTE

Forclassinstances,aconstantpropertycanbemodifiedduringinitializationonlybytheclassthatintroducesit.Itcannotbemodifiedbyasubclass.

YoucanrevisetheSurveyQuestionexamplefromabovetouseaconstantpropertyratherthanavariablepropertyforthetextpropertyofthequestion,toindicatethatthequestiondoesnotchangeonceaninstanceofSurveyQuestioniscreated.Eventhoughthetextpropertyisnowaconstant,itcanstillbesetwithintheclass’sinitializer:

1 classSurveyQuestion{

2 lettext:String

3 varresponse:String?

4 init(text:String){

5 self.text=text

6 }

7 funcask(){

8 print(text)

9 }

10 }

11 letbeetsQuestion=SurveyQuestion(text:"Howaboutbeets?")

12 beetsQuestion.ask()

13 //Prints"Howaboutbeets?"

14 beetsQuestion.response="Ialsolikebeets.(Butnotwith

cheese.)"

DefaultInitializers

Swiftprovidesadefaultinitializerforanystructureorclassthatprovidesdefaultvaluesforallofitspropertiesanddoesnotprovideatleastoneinitializeritself.Thedefaultinitializersimplycreatesanewinstancewithallofitspropertiessettotheirdefaultvalues.

ThisexampledefinesaclasscalledShoppingListItem,whichencapsulatesthename,quantity,andpurchasestateofaniteminashoppinglist:

1 classShoppingListItem{

2 varname:String?

3 varquantity=1

4 varpurchased=false

5 }

6 varitem=ShoppingListItem()

BecauseallpropertiesoftheShoppingListItemclasshavedefaultvalues,andbecauseitisabaseclasswithnosuperclass,ShoppingListItemautomaticallygainsadefaultinitializerimplementationthatcreatesanewinstancewithallofitspropertiessettotheirdefaultvalues.(ThenamepropertyisanoptionalStringproperty,andsoitautomaticallyreceivesadefaultvalueofnil,eventhoughthisvalueisnotwritteninthecode.)TheexampleaboveusesthedefaultinitializerfortheShoppingListItemclasstocreateanewinstanceoftheclasswithinitializersyntax,writtenasShoppingListItem(),andassignsthisnewinstancetoavariablecalleditem.

MemberwiseInitializersforStructureTypes

Structuretypesautomaticallyreceiveamemberwiseinitializeriftheydon’tdefineanyoftheirowncustominitializers.Unlikeadefaultinitializer,thestructurereceivesamemberwiseinitializerevenifithasstoredpropertiesthatdon’thavedefaultvalues.

Thememberwiseinitializerisashorthandwaytoinitializethememberpropertiesofnewstructureinstances.Initialvaluesforthepropertiesofthenewinstancecanbepassedtothememberwiseinitializerbyname.

TheexamplebelowdefinesastructurecalledSizewithtwopropertiescalledwidthandheight.BothpropertiesareinferredtobeoftypeDoublebyassigningadefaultvalueof0.0.

TheSizestructureautomaticallyreceivesaninit(width:height:)memberwiseinitializer,whichyoucanusetoinitializeanewSizeinstance:

1 structSize{

2 varwidth=0.0,height=0.0

3 }

4 lettwoByTwo=Size(width:2.0,height:2.0)

Whenyoucallamemberwiseinitializer,youcanomitvaluesforanypropertiesthathavedefaultvalues.Intheexampleabove,theSizestructurehasadefaultvalueforbothitsheightandwidthproperties.Youcanomiteitherpropertyorbothproperties,andtheinitializerusesthedefaultvalueforanythingyouomit—forexample:

1 letzeroByTwo=Size(height:2.0)

2 print(zeroByTwo.width,zeroByTwo.height)

3 //Prints"0.02.0"

4

5 letzeroByZero=Size()

6 print(zeroByZero.width,zeroByZero.height)

7 //Prints"0.00.0"

InitializerDelegationforValueTypes

Initializerscancallotherinitializerstoperformpartofaninstance’sinitialization.Thisprocess,knownasinitializerdelegation,avoidsduplicatingcodeacrossmultipleinitializers.

Therulesforhowinitializerdelegationworks,andforwhatformsofdelegationareallowed,aredifferentforvaluetypesandclasstypes.Valuetypes(structuresandenumerations)donotsupportinheritance,andsotheirinitializerdelegationprocessisrelativelysimple,becausetheycanonlydelegatetoanotherinitializerthattheyprovidethemselves.Classes,however,caninheritfromotherclasses,asdescribedinInheritance.Thismeansthatclasseshaveadditionalresponsibilitiesforensuringthatallstoredpropertiestheyinheritareassignedasuitablevalueduringinitialization.TheseresponsibilitiesaredescribedinClassInheritanceandInitializationbelow.

Forvaluetypes,youuseself.inittorefertootherinitializersfromthesamevaluetypewhenwritingyourowncustominitializers.Youcancallself.initonlyfromwithinaninitializer.

Notethatifyoudefineacustominitializerforavaluetype,youwillnolongerhaveaccesstothedefaultinitializer(orthememberwiseinitializer,ifitisastructure)forthattype.Thisconstraintpreventsasituationinwhichadditionalessentialsetupprovidedinamorecomplexinitializerisaccidentallycircumventedbysomeoneusingoneoftheautomaticinitializers.

NOTE

Ifyouwantyourcustomvaluetypetobeinitializablewiththedefaultinitializerandmemberwiseinitializer,andalsowithyourowncustominitializers,writeyourcustominitializersinanextensionratherthanaspartofthevaluetype’soriginalimplementation.Formoreinformation,seeExtensions.

ThefollowingexampledefinesacustomRectstructuretorepresentageometricrectangle.TheexamplerequirestwosupportingstructurescalledSizeandPoint,bothofwhichprovidedefaultvaluesof0.0foralloftheirproperties:

1 structSize{

2 varwidth=0.0,height=0.0

3 }

4 structPoint{

5 varx=0.0,y=0.0

6 }

YoucaninitializetheRectstructurebelowinoneofthreeways—byusingitsdefaultzero-initializedoriginandsizepropertyvalues,byprovidingaspecificoriginpointandsize,orbyprovidingaspecificcenterpointandsize.TheseinitializationoptionsarerepresentedbythreecustominitializersthatarepartoftheRectstructure’sdefinition:

1 structRect{

2 varorigin=Point()

3 varsize=Size()

4 init(){}

5 init(origin:Point,size:Size){

6 self.origin=origin

7 self.size=size

8 }

9 init(center:Point,size:Size){

10 letoriginX=center.x-(size.width/2)

11 letoriginY=center.y-(size.height/2)

12 self.init(origin:Point(x:originX,y:originY),size:

size)

13 }

14 }

ThefirstRectinitializer,init(),isfunctionallythesameasthedefaultinitializerthatthestructurewouldhavereceivedifitdidnothaveitsowncustominitializers.Thisinitializerhasanemptybody,representedbyanemptypairofcurlybraces{}.CallingthisinitializerreturnsaRectinstancewhoseoriginandsizepropertiesarebothinitializedwiththedefaultvaluesofPoint(x:0.0,y:

0.0)andSize(width:0.0,height:0.0)fromtheirpropertydefinitions:

1 letbasicRect=Rect()

2 //basicRect'soriginis(0.0,0.0)anditssizeis(0.0,0.0)

ThesecondRectinitializer,init(origin:size:),isfunctionallythesameasthememberwiseinitializerthatthestructurewouldhavereceivedifitdidnothaveitsowncustominitializers.Thisinitializersimplyassignstheoriginandsizeargumentvaluestotheappropriatestoredproperties:

1 letoriginRect=Rect(origin:Point(x:2.0,y:2.0),

2 size:Size(width:5.0,height:5.0))

3 //originRect'soriginis(2.0,2.0)anditssizeis(5.0,5.0)

ThethirdRectinitializer,init(center:size:),isslightlymorecomplex.Itstartsbycalculatinganappropriateoriginpointbasedonacenterpointandasizevalue.Itthencalls(ordelegates)totheinit(origin:size:)initializer,whichstorestheneworiginandsizevaluesintheappropriateproperties:

1 letcenterRect=Rect(center:Point(x:4.0,y:4.0),

2 size:Size(width:3.0,height:3.0))

3 //centerRect'soriginis(2.5,2.5)anditssizeis(3.0,3.0)

Theinit(center:size:)initializercouldhaveassignedthenewvaluesoforiginandsizetotheappropriatepropertiesitself.However,itismoreconvenient(andclearerinintent)fortheinit(center:size:)initializertotakeadvantageofanexistinginitializerthatalreadyprovidesexactlythatfunctionality.

NOTE

Foranalternativewaytowritethisexamplewithoutdefiningtheinit()andinit(origin:size:)initializersyourself,seeExtensions.

ClassInheritanceandInitialization

Allofaclass’sstoredproperties—includinganypropertiestheclassinheritsfromitssuperclass—mustbeassignedaninitialvalueduringinitialization.

Swiftdefinestwokindsofinitializersforclasstypestohelpensureallstoredpropertiesreceiveaninitialvalue.Theseareknownasdesignatedinitializersandconvenienceinitializers.

DesignatedInitializersandConvenienceInitializersDesignatedinitializersaretheprimaryinitializersforaclass.Adesignatedinitializerfullyinitializesallpropertiesintroducedbythatclassandcallsanappropriatesuperclassinitializertocontinuetheinitializationprocessupthesuperclasschain.

Classestendtohaveveryfewdesignatedinitializers,anditisquitecommonforaclasstohaveonlyone.Designatedinitializersare“funnel”pointsthroughwhichinitializationtakesplace,andthroughwhichtheinitializationprocesscontinuesupthesuperclasschain.

Everyclassmusthaveatleastonedesignatedinitializer.Insomecases,thisrequirementissatisfiedbyinheritingoneormoredesignatedinitializersfromasuperclass,asdescribedinAutomaticInitializerInheritancebelow.

Convenienceinitializersaresecondary,supportinginitializersforaclass.Youcandefineaconvenienceinitializertocalladesignatedinitializerfromthesameclassastheconvenienceinitializerwithsomeofthedesignatedinitializer’sparameterssettodefaultvalues.Youcanalsodefineaconvenienceinitializertocreateaninstanceofthatclassforaspecificusecaseorinputvaluetype.

Youdonothavetoprovideconvenienceinitializersifyourclassdoesnotrequirethem.Createconvenienceinitializerswheneverashortcuttoacommoninitializationpatternwillsavetimeormakeinitializationoftheclassclearerinintent.

SyntaxforDesignatedandConvenienceInitializersDesignatedinitializersforclassesarewritteninthesamewayassimpleinitializersforvaluetypes:

init( parameters ){

statements

}

Convenienceinitializersarewritteninthesamestyle,butwiththeconveniencemodifierplacedbeforetheinitkeyword,separatedbyaspace:

convenienceinit( parameters ){

statements

}

InitializerDelegationforClassTypesTosimplifytherelationshipsbetweendesignatedandconvenienceinitializers,Swiftappliesthefollowingthreerulesfordelegationcallsbetweeninitializers:

Rule1

Adesignatedinitializermustcalladesignatedinitializerfromitsimmediatesuperclass.

Rule2

Aconvenienceinitializermustcallanotherinitializerfromthesameclass.

Rule3

Aconvenienceinitializermustultimatelycalladesignatedinitializer.

Asimplewaytorememberthisis:

Designatedinitializersmustalwaysdelegateup.

Convenienceinitializersmustalwaysdelegateacross.

Theserulesareillustratedinthefigurebelow:

Here,thesuperclasshasasingledesignatedinitializerandtwoconvenienceinitializers.Oneconvenienceinitializercallsanotherconvenienceinitializer,whichinturncallsthesingledesignatedinitializer.Thissatisfiesrules2and3fromabove.Thesuperclassdoesnotitselfhaveafurthersuperclass,andsorule1doesnotapply.

Thesubclassinthisfigurehastwodesignatedinitializersandoneconvenienceinitializer.Theconvenienceinitializermustcalloneofthetwodesignatedinitializers,becauseitcanonlycallanotherinitializerfromthesameclass.Thissatisfiesrules2and3fromabove.Bothdesignatedinitializersmustcallthesingledesignatedinitializerfromthesuperclass,tosatisfyrule1fromabove.

NOTE

Theserulesdon’taffecthowusersofyourclassescreateinstancesofeachclass.Anyinitializerinthediagramabovecanbeusedtocreateafullyinitializedinstanceoftheclasstheybelongto.Therulesonlyaffecthowyouwritetheimplementationoftheclass’sinitializers.

Thefigurebelowshowsamorecomplexclasshierarchyforfourclasses.Itillustrateshowthedesignatedinitializersinthishierarchyactas“funnel”pointsforclassinitialization,simplifyingtheinterrelationshipsamongclassesinthechain:

Two-PhaseInitializationClassinitializationinSwiftisatwo-phaseprocess.Inthefirstphase,eachstoredpropertyisassignedaninitialvaluebytheclassthatintroducedit.Oncetheinitialstateforeverystoredpropertyhasbeendetermined,thesecondphasebegins,andeachclassisgiventheopportunitytocustomizeitsstoredpropertiesfurtherbeforethenewinstanceisconsideredreadyforuse.

Theuseofatwo-phaseinitializationprocessmakesinitializationsafe,whilestillgivingcompleteflexibilitytoeachclassinaclasshierarchy.Two-phaseinitializationpreventspropertyvaluesfrombeingaccessedbeforetheyareinitialized,andpreventspropertyvaluesfrombeingsettoadifferentvaluebyanotherinitializerunexpectedly.

NOTE

Swift’stwo-phaseinitializationprocessissimilartoinitializationinObjective-C.Themaindifferenceisthatduringphase1,Objective-Cassignszeroornullvalues(suchas0ornil)toevery

property.Swift’sinitializationflowismoreflexibleinthatitletsyousetcustominitialvalues,andcancopewithtypesforwhich0ornilisnotavaliddefaultvalue.

Swift’scompilerperformsfourhelpfulsafety-checkstomakesurethattwo-phaseinitializationiscompletedwithouterror:

Safetycheck1

Adesignatedinitializermustensurethatallofthepropertiesintroducedbyitsclassareinitializedbeforeitdelegatesuptoasuperclassinitializer.

Asmentionedabove,thememoryforanobjectisonlyconsideredfullyinitializedoncetheinitialstateofallofitsstoredpropertiesisknown.Inorderforthisruletobesatisfied,adesignatedinitializermustmakesurethatallofitsownpropertiesareinitializedbeforeithandsoffupthechain.

Safetycheck2

Adesignatedinitializermustdelegateuptoasuperclassinitializerbeforeassigningavaluetoaninheritedproperty.Ifitdoesn’t,thenewvaluethedesignatedinitializerassignswillbeoverwrittenbythesuperclassaspartofitsowninitialization.

Safetycheck3

Aconvenienceinitializermustdelegatetoanotherinitializerbeforeassigningavaluetoanyproperty(includingpropertiesdefinedbythesameclass).Ifitdoesn’t,thenewvaluetheconvenienceinitializerassignswillbeoverwrittenbyitsownclass’sdesignatedinitializer.

Safetycheck4

Aninitializercannotcallanyinstancemethods,readthevaluesofanyinstanceproperties,orrefertoselfasavalueuntilafterthefirstphaseofinitializationiscomplete.

Theclassinstanceisnotfullyvaliduntilthefirstphaseends.Propertiescanonlybeaccessed,andmethodscanonlybecalled,oncetheclassinstanceisknowntobevalidattheendofthefirstphase.

Here’showtwo-phaseinitializationplaysout,basedonthefoursafetychecksabove:

Phase1

Adesignatedorconvenienceinitializeriscalledonaclass.

Memoryforanewinstanceofthatclassisallocated.Thememoryisnotyetinitialized.

Adesignatedinitializerforthatclassconfirmsthatallstoredpropertiesintroducedbythatclasshaveavalue.Thememoryforthesestoredpropertiesisnowinitialized.

Thedesignatedinitializerhandsofftoasuperclassinitializertoperformthesametaskforitsownstoredproperties.

Thiscontinuesuptheclassinheritancechainuntilthetopofthechainisreached.

Oncethetopofthechainisreached,andthefinalclassinthechainhasensuredthatallofitsstoredpropertieshaveavalue,theinstance’smemoryisconsideredtobefullyinitialized,andphase1iscomplete.

Phase2

Workingbackdownfromthetopofthechain,eachdesignatedinitializerinthechainhastheoptiontocustomizetheinstancefurther.Initializersarenowabletoaccessselfandcanmodifyitsproperties,callitsinstancemethods,andsoon.

Finally,anyconvenienceinitializersinthechainhavetheoptiontocustomizetheinstanceandtoworkwithself.

Here’showphase1looksforaninitializationcallforahypotheticalsubclassandsuperclass:

Inthisexample,initializationbeginswithacalltoaconvenienceinitializeronthesubclass.Thisconvenienceinitializercannotyetmodifyanyproperties.Itdelegatesacrosstoadesignatedinitializerfromthesameclass.

Thedesignatedinitializermakessurethatallofthesubclass’spropertieshaveavalue,aspersafetycheck1.Itthencallsadesignatedinitializeronitssuperclasstocontinuetheinitializationupthechain.

Thesuperclass’sdesignatedinitializermakessurethatallofthesuperclasspropertieshaveavalue.Therearenofurthersuperclassestoinitialize,andsonofurtherdelegationisneeded.

Assoonasallpropertiesofthesuperclasshaveaninitialvalue,itsmemoryisconsideredfullyinitialized,andphase1iscomplete.

Here’showphase2looksforthesameinitializationcall:

Thesuperclass’sdesignatedinitializernowhasanopportunitytocustomizethe

instancefurther(althoughitdoesnothaveto).

Oncethesuperclass’sdesignatedinitializerisfinished,thesubclass’sdesignatedinitializercanperformadditionalcustomization(althoughagain,itdoesnothaveto).

Finally,oncethesubclass’sdesignatedinitializerisfinished,theconvenienceinitializerthatwasoriginallycalledcanperformadditionalcustomization.

InitializerInheritanceandOverridingUnlikesubclassesinObjective-C,Swiftsubclassesdonotinherittheirsuperclassinitializersbydefault.Swift’sapproachpreventsasituationinwhichasimpleinitializerfromasuperclassisinheritedbyamorespecializedsubclassandisusedtocreateanewinstanceofthesubclassthatisnotfullyorcorrectlyinitialized.

NOTE

Superclassinitializersareinheritedincertaincircumstances,butonlywhenitissafeandappropriatetodoso.Formoreinformation,seeAutomaticInitializerInheritancebelow.

Ifyouwantacustomsubclasstopresentoneormoreofthesameinitializersasitssuperclass,youcanprovideacustomimplementationofthoseinitializerswithinthesubclass.

Whenyouwriteasubclassinitializerthatmatchesasuperclassdesignatedinitializer,youareeffectivelyprovidinganoverrideofthatdesignatedinitializer.Therefore,youmustwritetheoverridemodifierbeforethesubclass’sinitializerdefinition.Thisistrueevenifyouareoverridinganautomaticallyprovideddefaultinitializer,asdescribedinDefaultInitializers.

Aswithanoverriddenproperty,methodorsubscript,thepresenceoftheoverridemodifierpromptsSwifttocheckthatthesuperclasshasamatchingdesignatedinitializertobeoverridden,andvalidatesthattheparametersforyouroverridinginitializerhavebeenspecifiedasintended.

NOTE

Youalwayswritetheoverridemodifierwhenoverridingasuperclassdesignatedinitializer,evenifyoursubclass’simplementationoftheinitializerisaconvenienceinitializer.

Conversely,ifyouwriteasubclassinitializerthatmatchesasuperclassconvenienceinitializer,thatsuperclassconvenienceinitializercanneverbecalleddirectlybyyoursubclass,aspertherulesdescribedaboveinInitializerDelegationforClassTypes.Therefore,yoursubclassisnot(strictlyspeaking)providinganoverrideofthesuperclassinitializer.Asaresult,youdonotwritetheoverridemodifierwhenprovidingamatchingimplementationofasuperclassconvenienceinitializer.

TheexamplebelowdefinesabaseclasscalledVehicle.ThisbaseclassdeclaresastoredpropertycallednumberOfWheels,withadefaultIntvalueof0.ThenumberOfWheelspropertyisusedbyacomputedpropertycalleddescriptiontocreateaStringdescriptionofthevehicle’scharacteristics:

1 classVehicle{

2 varnumberOfWheels=0

3 vardescription:String{

4 return"\(numberOfWheels)wheel(s)"

5 }

6 }

TheVehicleclassprovidesadefaultvalueforitsonlystoredproperty,anddoesnotprovideanycustominitializersitself.Asaresult,itautomaticallyreceivesadefaultinitializer,asdescribedinDefaultInitializers.Thedefaultinitializer(whenavailable)isalwaysadesignatedinitializerforaclass,andcanbeusedtocreateanewVehicleinstancewithanumberOfWheelsof0:

1 letvehicle=Vehicle()

2 print("Vehicle:\(vehicle.description)")

3 //Vehicle:0wheel(s)

ThenextexampledefinesasubclassofVehiclecalledBicycle:

1 classBicycle:Vehicle{

2 overrideinit(){

3 super.init()

4 numberOfWheels=2

5 }

6 }

TheBicyclesubclassdefinesacustomdesignatedinitializer,init().ThisdesignatedinitializermatchesadesignatedinitializerfromthesuperclassofBicycle,andsotheBicycleversionofthisinitializerismarkedwiththeoverridemodifier.

Theinit()initializerforBicyclestartsbycallingsuper.init(),whichcallsthedefaultinitializerfortheBicycleclass’ssuperclass,Vehicle.ThisensuresthatthenumberOfWheelsinheritedpropertyisinitializedbyVehiclebeforeBicyclehastheopportunitytomodifytheproperty.Aftercallingsuper.init(),theoriginalvalueofnumberOfWheelsisreplacedwithanewvalueof2.

IfyoucreateaninstanceofBicycle,youcancallitsinheriteddescriptioncomputedpropertytoseehowitsnumberOfWheelspropertyhasbeenupdated:

1 letbicycle=Bicycle()

2 print("Bicycle:\(bicycle.description)")

3 //Bicycle:2wheel(s)

Ifasubclassinitializerperformsnocustomizationinphase2oftheinitializationprocess,andthesuperclasshasazero-argumentdesignatedinitializer,youcanomitacalltosuper.init()afterassigningvaluestoallofthesubclass’sstoredproperties.

ThisexampledefinesanothersubclassofVehicle,calledHoverboard.Initsinitializer,theHoverboardclasssetsonlyitscolorproperty.Insteadofmakinganexplicitcalltosuper.init(),thisinitializerreliesonanimplicitcalltoits

superclass’sinitializertocompletetheprocess.

1 classHoverboard:Vehicle{

2 varcolor:String

3 init(color:String){

4 self.color=color

5 //super.init()implicitlycalledhere

6 }

7 overridevardescription:String{

8 return"\(super.description)inabeautiful\(color)"

9 }

10 }

AninstanceofHoverboardusesthedefaultnumberofwheelssuppliedbytheVehicleinitializer.

1 lethoverboard=Hoverboard(color:"silver")

2 print("Hoverboard:\(hoverboard.description)")

3 //Hoverboard:0wheel(s)inabeautifulsilver

NOTE

Subclassescanmodifyinheritedvariablepropertiesduringinitialization,butcannotmodifyinheritedconstantproperties.

AutomaticInitializerInheritanceAsmentionedabove,subclassesdonotinherittheirsuperclassinitializersbydefault.However,superclassinitializersareautomaticallyinheritedifcertainconditionsaremet.Inpractice,thismeansthatyoudonotneedtowriteinitializeroverridesinmanycommonscenarios,andcaninherityoursuperclassinitializerswithminimaleffortwheneveritissafetodoso.

Assumingthatyouprovidedefaultvaluesforanynewpropertiesyouintroduce

inasubclass,thefollowingtworulesapply:

Rule1

Ifyoursubclassdoesn’tdefineanydesignatedinitializers,itautomaticallyinheritsallofitssuperclassdesignatedinitializers.

Rule2

Ifyoursubclassprovidesanimplementationofallofitssuperclassdesignatedinitializers—eitherbyinheritingthemasperrule1,orbyprovidingacustomimplementationaspartofitsdefinition—thenitautomaticallyinheritsallofthesuperclassconvenienceinitializers.

Theserulesapplyevenifyoursubclassaddsfurtherconvenienceinitializers.

NOTE

Asubclasscanimplementasuperclassdesignatedinitializerasasubclassconvenienceinitializeraspartofsatisfyingrule2.

DesignatedandConvenienceInitializersinActionThefollowingexampleshowsdesignatedinitializers,convenienceinitializers,andautomaticinitializerinheritanceinaction.ThisexampledefinesahierarchyofthreeclassescalledFood,RecipeIngredient,andShoppingListItem,anddemonstrateshowtheirinitializersinteract.

ThebaseclassinthehierarchyiscalledFood,whichisasimpleclasstoencapsulatethenameofafoodstuff.TheFoodclassintroducesasingleStringpropertycallednameandprovidestwoinitializersforcreatingFoodinstances:

1 classFood{

2 varname:String

3 init(name:String){

4 self.name=name

5 }

6 convenienceinit(){

7 self.init(name:"[Unnamed]")

8 }

9 }

ThefigurebelowshowstheinitializerchainfortheFoodclass:

Classesdonothaveadefaultmemberwiseinitializer,andsotheFoodclassprovidesadesignatedinitializerthattakesasingleargumentcalledname.ThisinitializercanbeusedtocreateanewFoodinstancewithaspecificname:

1 letnamedMeat=Food(name:"Bacon")

2 //namedMeat'snameis"Bacon"

Theinit(name:String)initializerfromtheFoodclassisprovidedasadesignatedinitializer,becauseitensuresthatallstoredpropertiesofanewFoodinstancearefullyinitialized.TheFoodclassdoesnothaveasuperclass,andsotheinit(name:String)initializerdoesnotneedtocallsuper.init()tocompleteitsinitialization.

TheFoodclassalsoprovidesaconvenienceinitializer,init(),withnoarguments.Theinit()initializerprovidesadefaultplaceholdernameforanewfoodbydelegatingacrosstotheFoodclass’sinit(name:String)withanamevalueof[Unnamed]:

1 letmysteryMeat=Food()

2 //mysteryMeat'snameis"[Unnamed]"

ThesecondclassinthehierarchyisasubclassofFoodcalledRecipeIngredient.TheRecipeIngredientclassmodelsaningredientinacookingrecipe.ItintroducesanIntpropertycalledquantity(inadditiontothenamepropertyitinheritsfromFood)anddefinestwoinitializersforcreatingRecipeIngredientinstances:

1 classRecipeIngredient:Food{

2 varquantity:Int

3 init(name:String,quantity:Int){

4 self.quantity=quantity

5 super.init(name:name)

6 }

7 overrideconvenienceinit(name:String){

8 self.init(name:name,quantity:1)

9 }

10 }

ThefigurebelowshowstheinitializerchainfortheRecipeIngredientclass:

TheRecipeIngredientclasshasasingledesignatedinitializer,init(name:String,quantity:Int),whichcanbeusedtopopulateallofthepropertiesofanewRecipeIngredientinstance.Thisinitializerstartsbyassigningthepassed

quantityargumenttothequantityproperty,whichistheonlynewpropertyintroducedbyRecipeIngredient.Afterdoingso,theinitializerdelegatesuptotheinit(name:String)initializeroftheFoodclass.Thisprocesssatisfiessafetycheck1fromTwo-PhaseInitializationabove.

RecipeIngredientalsodefinesaconvenienceinitializer,init(name:String),whichisusedtocreateaRecipeIngredientinstancebynamealone.Thisconvenienceinitializerassumesaquantityof1foranyRecipeIngredientinstancethatiscreatedwithoutanexplicitquantity.ThedefinitionofthisconvenienceinitializermakesRecipeIngredientinstancesquickerandmoreconvenienttocreate,andavoidscodeduplicationwhencreatingseveralsingle-quantityRecipeIngredientinstances.Thisconvenienceinitializersimplydelegatesacrosstotheclass’sdesignatedinitializer,passinginaquantityvalueof1.

Theinit(name:String)convenienceinitializerprovidedbyRecipeIngredienttakesthesameparametersastheinit(name:String)designatedinitializerfromFood.Becausethisconvenienceinitializeroverridesadesignatedinitializerfromitssuperclass,itmustbemarkedwiththeoverridemodifier(asdescribedinInitializerInheritanceandOverriding).

EventhoughRecipeIngredientprovidestheinit(name:String)initializerasaconvenienceinitializer,RecipeIngredienthasnonethelessprovidedanimplementationofallofitssuperclass’sdesignatedinitializers.Therefore,RecipeIngredientautomaticallyinheritsallofitssuperclass’sconvenienceinitializerstoo.

Inthisexample,thesuperclassforRecipeIngredientisFood,whichhasasingleconvenienceinitializercalledinit().ThisinitializeristhereforeinheritedbyRecipeIngredient.Theinheritedversionofinit()functionsinexactlythesamewayastheFoodversion,exceptthatitdelegatestotheRecipeIngredientversionofinit(name:String)ratherthantheFoodversion.

AllthreeoftheseinitializerscanbeusedtocreatenewRecipeIngredientinstances:

1 letoneMysteryItem=RecipeIngredient()

2 letoneBacon=RecipeIngredient(name:"Bacon")

3 letsixEggs=RecipeIngredient(name:"Eggs",quantity:6)

ThethirdandfinalclassinthehierarchyisasubclassofRecipeIngredientcalledShoppingListItem.TheShoppingListItemclassmodelsarecipeingredientasitappearsinashoppinglist.

Everyitemintheshoppingliststartsoutas“unpurchased”.Torepresentthisfact,ShoppingListItemintroducesaBooleanpropertycalledpurchased,withadefaultvalueoffalse.ShoppingListItemalsoaddsacomputeddescriptionproperty,whichprovidesatextualdescriptionofaShoppingListIteminstance:

1 classShoppingListItem:RecipeIngredient{

2 varpurchased=false

3 vardescription:String{

4 varoutput="\(quantity)x\(name)"

5 output+=purchased?"✔":"✘"

6 returnoutput

7 }

8 }

NOTE

ShoppingListItemdoesnotdefineaninitializertoprovideaninitialvalueforpurchased,becauseitemsinashoppinglist(asmodeledhere)alwaysstartoutunpurchased.

Becauseitprovidesadefaultvalueforallofthepropertiesitintroducesanddoesnotdefineanyinitializersitself,ShoppingListItemautomaticallyinheritsallofthedesignatedandconvenienceinitializersfromitssuperclass.

Thefigurebelowshowstheoverallinitializerchainforallthreeclasses:

YoucanuseallthreeoftheinheritedinitializerstocreateanewShoppingListIteminstance:

1 varbreakfastList=[

2 ShoppingListItem(),

3 ShoppingListItem(name:"Bacon"),

4 ShoppingListItem(name:"Eggs",quantity:6),

5 ]

6 breakfastList[0].name="Orangejuice"

7 breakfastList[0].purchased=true

8 foriteminbreakfastList{

9 print(item.description)

10 }

11 //1xOrangejuice✔

12 //1xBacon✘

13 //6xEggs✘

Here,anewarraycalledbreakfastListiscreatedfromanarrayliteralcontainingthreenewShoppingListIteminstances.Thetypeofthearrayisinferredtobe[ShoppingListItem].Afterthearrayiscreated,thenameoftheShoppingListItematthestartofthearrayischangedfrom"[Unnamed]"to"Orangejuice"anditismarkedashavingbeenpurchased.Printingthedescriptionofeachiteminthearrayshowsthattheirdefaultstateshavebeensetasexpected.

FailableInitializers

Itissometimesusefultodefineaclass,structure,orenumerationforwhichinitializationcanfail.Thisfailuremightbetriggeredbyinvalidinitializationparametervalues,theabsenceofarequiredexternalresource,orsomeotherconditionthatpreventsinitializationfromsucceeding.

Tocopewithinitializationconditionsthatcanfail,defineoneormorefailableinitializersaspartofaclass,structure,orenumerationdefinition.Youwriteafailableinitializerbyplacingaquestionmarkaftertheinitkeyword(init?).

NOTE

Youcannotdefineafailableandanonfailableinitializerwiththesameparametertypesandnames.

Afailableinitializercreatesanoptionalvalueofthetypeitinitializes.Youwritereturnnilwithinafailableinitializertoindicateapointatwhichinitializationfailurecanbetriggered.

NOTE

Strictlyspeaking,initializersdonotreturnavalue.Rather,theirroleistoensurethatselfisfullyandcorrectlyinitializedbythetimethatinitializationends.Althoughyouwritereturnniltotriggeraninitializationfailure,youdonotusethereturnkeywordtoindicateinitializationsuccess.

Forinstance,failableinitializersareimplementedfornumerictypeconversions.

Toensureconversionbetweennumerictypesmaintainsthevalueexactly,usetheinit(exactly:)initializer.Ifthetypeconversioncannotmaintainthevalue,theinitializerfails.

1 letwholeNumber:Double=12345.0

2 letpi=3.14159

3

4 ifletvalueMaintained=Int(exactly:wholeNumber){

5 print("\(wholeNumber)conversiontoIntmaintainsvalueof

\(valueMaintained)")

6 }

7 //Prints"12345.0conversiontoIntmaintainsvalueof12345"

8

9 letvalueChanged=Int(exactly:pi)

10 //valueChangedisoftypeInt?,notInt

11

12 ifvalueChanged==nil{

13 print("\(pi)conversiontoIntdoesnotmaintainvalue")

14 }

15 //Prints"3.14159conversiontoIntdoesnotmaintainvalue"

TheexamplebelowdefinesastructurecalledAnimal,withaconstantStringpropertycalledspecies.TheAnimalstructurealsodefinesafailableinitializerwithasingleparametercalledspecies.Thisinitializerchecksifthespeciesvaluepassedtotheinitializerisanemptystring.Ifanemptystringisfound,aninitializationfailureistriggered.Otherwise,thespeciesproperty’svalueisset,andinitializationsucceeds:

1 structAnimal{

2 letspecies:String

3 init?(species:String){

4 ifspecies.isEmpty{returnnil}

5 self.species=species

6 }

7 }

YoucanusethisfailableinitializertotrytoinitializeanewAnimalinstanceandtocheckifinitializationsucceeded:

1 letsomeCreature=Animal(species:"Giraffe")

2 //someCreatureisoftypeAnimal?,notAnimal

3

4 ifletgiraffe=someCreature{

5 print("Ananimalwasinitializedwithaspeciesof\

(giraffe.species)")

6 }

7 //Prints"AnanimalwasinitializedwithaspeciesofGiraffe"

Ifyoupassanemptystringvaluetothefailableinitializer’sspeciesparameter,theinitializertriggersaninitializationfailure:

1 letanonymousCreature=Animal(species:"")

2 //anonymousCreatureisoftypeAnimal?,notAnimal

3

4 ifanonymousCreature==nil{

5 print("Theanonymouscreaturecouldnotbeinitialized")

6 }

7 //Prints"Theanonymouscreaturecouldnotbeinitialized"

NOTE

Checkingforanemptystringvalue(suchas""ratherthan"Giraffe")isnotthesameascheckingforniltoindicatetheabsenceofanoptionalStringvalue.Intheexampleabove,anemptystring("")isavalid,non-optionalString.However,itisnotappropriateforananimaltohaveanemptystringasthevalueofitsspeciesproperty.Tomodelthisrestriction,thefailableinitializertriggersaninitializationfailureifanemptystringisfound.

FailableInitializersforEnumerationsYoucanuseafailableinitializertoselectanappropriateenumerationcasebasedononeormoreparameters.Theinitializercanthenfailiftheprovidedparametersdonotmatchanappropriateenumerationcase.

TheexamplebelowdefinesanenumerationcalledTemperatureUnit,withthreepossiblestates(kelvin,celsius,andfahrenheit).AfailableinitializerisusedtofindanappropriateenumerationcaseforaCharactervaluerepresentingatemperaturesymbol:

1 enumTemperatureUnit{

2 casekelvin,celsius,fahrenheit

3 init?(symbol:Character){

4 switchsymbol{

5 case"K":

6 self=.kelvin

7 case"C":

8 self=.celsius

9 case"F":

10 self=.fahrenheit

11 default:

12 returnnil

13 }

14 }

15 }

Youcanusethisfailableinitializertochooseanappropriateenumerationcaseforthethreepossiblestatesandtocauseinitializationtofailiftheparameterdoesnotmatchoneofthesestates:

1 letfahrenheitUnit=TemperatureUnit(symbol:"F")

2 iffahrenheitUnit!=nil{

3 print("Thisisadefinedtemperatureunit,so

initializationsucceeded.")

4 }

5 //Prints"Thisisadefinedtemperatureunit,so

initializationsucceeded."

6

7 letunknownUnit=TemperatureUnit(symbol:"X")

8 ifunknownUnit==nil{

9 print("Thisisnotadefinedtemperatureunit,so

initializationfailed.")

10 }

11 //Prints"Thisisnotadefinedtemperatureunit,so

initializationfailed."

FailableInitializersforEnumerationswithRawValuesEnumerationswithrawvaluesautomaticallyreceiveafailableinitializer,init?(rawValue:),thattakesaparametercalledrawValueoftheappropriateraw-valuetypeandselectsamatchingenumerationcaseifoneisfound,ortriggersaninitializationfailureifnomatchingvalueexists.

YoucanrewritetheTemperatureUnitexamplefromabovetouserawvaluesoftypeCharacterandtotakeadvantageoftheinit?(rawValue:)initializer:

1 enumTemperatureUnit:Character{

2 casekelvin="K",celsius="C",fahrenheit="F"

3 }

4

5 letfahrenheitUnit=TemperatureUnit(rawValue:"F")

6 iffahrenheitUnit!=nil{

7 print("Thisisadefinedtemperatureunit,so

initializationsucceeded.")

8 }

9 //Prints"Thisisadefinedtemperatureunit,so

initializationsucceeded."

10

11 letunknownUnit=TemperatureUnit(rawValue:"X")

12 ifunknownUnit==nil{

13 print("Thisisnotadefinedtemperatureunit,so

initializationfailed.")

14 }

15 //Prints"Thisisnotadefinedtemperatureunit,so

initializationfailed."

PropagationofInitializationFailureAfailableinitializerofaclass,structure,orenumerationcandelegateacrosstoanotherfailableinitializerfromthesameclass,structure,orenumeration.Similarly,asubclassfailableinitializercandelegateuptoasuperclassfailableinitializer.

Ineithercase,ifyoudelegatetoanotherinitializerthatcausesinitializationtofail,theentireinitializationprocessfailsimmediately,andnofurtherinitializationcodeisexecuted.

NOTE

Afailableinitializercanalsodelegatetoanonfailableinitializer.Usethisapproachifyouneedtoaddapotentialfailurestatetoanexistinginitializationprocessthatdoesnototherwisefail.

TheexamplebelowdefinesasubclassofProductcalledCartItem.TheCartItemclassmodelsaniteminanonlineshoppingcart.CartItemintroducesastoredconstantpropertycalledquantityandensuresthatthispropertyalwayshasavalueofatleast1:

1 classProduct{

2 letname:String

3 init?(name:String){

4 ifname.isEmpty{returnnil}

5 self.name=name

6 }

7 }

8

9 classCartItem:Product{

10 letquantity:Int

11 init?(name:String,quantity:Int){

12 ifquantity<1{returnnil}

13 self.quantity=quantity

14 super.init(name:name)

15 }

16 }

ThefailableinitializerforCartItemstartsbyvalidatingthatithasreceivedaquantityvalueof1ormore.Ifthequantityisinvalid,theentireinitializationprocessfailsimmediatelyandnofurtherinitializationcodeisexecuted.Likewise,thefailableinitializerforProductchecksthenamevalue,andtheinitializerprocessfailsimmediatelyifnameistheemptystring.

IfyoucreateaCartIteminstancewithanonemptynameandaquantityof1ormore,initializationsucceeds:

1 iflettwoSocks=CartItem(name:"sock",quantity:2){

2 print("Item:\(twoSocks.name),quantity:\

(twoSocks.quantity)")

3 }

4 //Prints"Item:sock,quantity:2"

IfyoutrytocreateaCartIteminstancewithaquantityvalueof0,theCartItem

initializercausesinitializationtofail:

1 ifletzeroShirts=CartItem(name:"shirt",quantity:0){

2 print("Item:\(zeroShirts.name),quantity:\

(zeroShirts.quantity)")

3 }else{

4 print("Unabletoinitializezeroshirts")

5 }

6 //Prints"Unabletoinitializezeroshirts"

Similarly,ifyoutrytocreateaCartIteminstancewithanemptynamevalue,thesuperclassProductinitializercausesinitializationtofail:

1 ifletoneUnnamed=CartItem(name:"",quantity:1){

2 print("Item:\(oneUnnamed.name),quantity:\

(oneUnnamed.quantity)")

3 }else{

4 print("Unabletoinitializeoneunnamedproduct")

5 }

6 //Prints"Unabletoinitializeoneunnamedproduct"

OverridingaFailableInitializerYoucanoverrideasuperclassfailableinitializerinasubclass,justlikeanyotherinitializer.Alternatively,youcanoverrideasuperclassfailableinitializerwithasubclassnonfailableinitializer.Thisenablesyoutodefineasubclassforwhichinitializationcannotfail,eventhoughinitializationofthesuperclassisallowedtofail.

Notethatifyouoverrideafailablesuperclassinitializerwithanonfailablesubclassinitializer,theonlywaytodelegateuptothesuperclassinitializeristoforce-unwraptheresultofthefailablesuperclassinitializer.

NOTE

Youcanoverrideafailableinitializerwithanonfailableinitializerbutnottheotherwayaround.

TheexamplebelowdefinesaclasscalledDocument.Thisclassmodelsadocumentthatcanbeinitializedwithanamepropertythatiseitheranonemptystringvalueornil,butcannotbeanemptystring:

1 classDocument{

2 varname:String?

3 //thisinitializercreatesadocumentwithanilname

value

4 init(){}

5 //thisinitializercreatesadocumentwithanonemptyname

value

6 init?(name:String){

7 ifname.isEmpty{returnnil}

8 self.name=name

9 }

10 }

ThenextexampledefinesasubclassofDocumentcalledAutomaticallyNamedDocument.TheAutomaticallyNamedDocumentsubclassoverridesbothofthedesignatedinitializersintroducedbyDocument.TheseoverridesensurethatanAutomaticallyNamedDocumentinstancehasaninitialnamevalueof"[Untitled]"iftheinstanceisinitializedwithoutaname,orifanemptystringispassedtotheinit(name:)initializer:

1 classAutomaticallyNamedDocument:Document{

2 overrideinit(){

3 super.init()

4 self.name="[Untitled]"

5 }

6 overrideinit(name:String){

7 super.init()

8 ifname.isEmpty{

9 self.name="[Untitled]"

10 }else{

11 self.name=name

12 }

13 }

14 }

TheAutomaticallyNamedDocumentoverridesitssuperclass’sfailableinit?(name:)initializerwithanonfailableinit(name:)initializer.BecauseAutomaticallyNamedDocumentcopeswiththeemptystringcaseinadifferentwaythanitssuperclass,itsinitializerdoesnotneedtofail,andsoitprovidesanonfailableversionoftheinitializerinstead.

Youcanuseforcedunwrappinginaninitializertocallafailableinitializerfromthesuperclassaspartoftheimplementationofasubclass’snonfailableinitializer.Forexample,theUntitledDocumentsubclassbelowisalwaysnamed"[Untitled]",anditusesthefailableinit(name:)initializerfromitssuperclassduringinitialization.

1 classUntitledDocument:Document{

2 overrideinit(){

3 super.init(name:"[Untitled]")!

4 }

5 }

Inthiscase,iftheinit(name:)initializerofthesuperclasswereevercalledwithanemptystringasthename,theforcedunwrappingoperationwouldresultinaruntimeerror.However,becauseit’scalledwithastringconstant,youcanseethattheinitializerwon’tfail,sonoruntimeerrorcanoccurinthiscase.

Theinit!FailableInitializerYoutypicallydefineafailableinitializerthatcreatesanoptionalinstanceoftheappropriatetypebyplacingaquestionmarkaftertheinitkeyword(init?).Alternatively,youcandefineafailableinitializerthatcreatesanimplicitlyunwrappedoptionalinstanceoftheappropriatetype.Dothisbyplacinganexclamationmarkaftertheinitkeyword(init!)insteadofaquestionmark.

Youcandelegatefrominit?toinit!andviceversa,andyoucanoverrideinit?withinit!andviceversa.Youcanalsodelegatefrominittoinit!,althoughdoingsowilltriggeranassertioniftheinit!initializercausesinitializationtofail.

RequiredInitializers

Writetherequiredmodifierbeforethedefinitionofaclassinitializertoindicatethateverysubclassoftheclassmustimplementthatinitializer:

1 classSomeClass{

2 requiredinit(){

3 //initializerimplementationgoeshere

4 }

5 }

Youmustalsowritetherequiredmodifierbeforeeverysubclassimplementationofarequiredinitializer,toindicatethattheinitializerrequirementappliestofurthersubclassesinthechain.Youdonotwritetheoverridemodifierwhenoverridingarequireddesignatedinitializer:

1 classSomeSubclass:SomeClass{

2 requiredinit(){

3 //subclassimplementationoftherequiredinitializer

goeshere

4 }

5 }

NOTE

Youdonothavetoprovideanexplicitimplementationofarequiredinitializerifyoucansatisfytherequirementwithaninheritedinitializer.

SettingaDefaultPropertyValuewithaClosureorFunction

Ifastoredproperty’sdefaultvaluerequiressomecustomizationorsetup,youcanuseaclosureorglobalfunctiontoprovideacustomizeddefaultvalueforthatproperty.Wheneveranewinstanceofthetypethatthepropertybelongstoisinitialized,theclosureorfunctioniscalled,anditsreturnvalueisassignedastheproperty’sdefaultvalue.

Thesekindsofclosuresorfunctionstypicallycreateatemporaryvalueofthesametypeastheproperty,tailorthatvaluetorepresentthedesiredinitialstate,andthenreturnthattemporaryvaluetobeusedastheproperty’sdefaultvalue.

Here’saskeletonoutlineofhowaclosurecanbeusedtoprovideadefaultpropertyvalue:

1 classSomeClass{

2 letsomeProperty:SomeType={

3 //createadefaultvalueforsomePropertyinsidethis

closure

4 //someValuemustbeofthesametypeasSomeType

5 returnsomeValue

6 }()

7 }

Notethattheclosure’sendcurlybraceisfollowedbyanemptypairof

parentheses.ThistellsSwifttoexecutetheclosureimmediately.Ifyouomittheseparentheses,youaretryingtoassigntheclosureitselftotheproperty,andnotthereturnvalueoftheclosure.

NOTE

Ifyouuseaclosuretoinitializeaproperty,rememberthattherestoftheinstancehasnotyetbeeninitializedatthepointthattheclosureisexecuted.Thismeansthatyoucannotaccessanyotherpropertyvaluesfromwithinyourclosure,evenifthosepropertieshavedefaultvalues.Youalsocannotusetheimplicitselfproperty,orcallanyoftheinstance’smethods.

TheexamplebelowdefinesastructurecalledChessboard,whichmodelsaboardforthegameofchess.Chessisplayedonan8x8board,withalternatingblackandwhitesquares.

Torepresentthisgameboard,theChessboardstructurehasasinglepropertycalledboardColors,whichisanarrayof64Boolvalues.Avalueoftrueinthearrayrepresentsablacksquareandavalueoffalserepresentsawhitesquare.Thefirstiteminthearrayrepresentsthetopleftsquareontheboardandthelastiteminthearrayrepresentsthebottomrightsquareontheboard.

TheboardColorsarrayisinitializedwithaclosuretosetupitscolorvalues:

1 structChessboard{

2 letboardColors:[Bool]={

3 vartemporaryBoard=[Bool]()

4 varisBlack=false

5 foriin1...8{

6 forjin1...8{

7 temporaryBoard.append(isBlack)

8 isBlack=!isBlack

9 }

10 isBlack=!isBlack

11 }

12 returntemporaryBoard

13 }()

14 funcsquareIsBlackAt(row:Int,column:Int)->Bool{

15 returnboardColors[(row*8)+column]

16 }

17 }

WheneveranewChessboardinstanceiscreated,theclosureisexecuted,andthedefaultvalueofboardColorsiscalculatedandreturned.TheclosureintheexampleabovecalculatesandsetstheappropriatecolorforeachsquareontheboardinatemporaryarraycalledtemporaryBoard,andreturnsthistemporaryarrayastheclosure’sreturnvalueonceitssetupiscomplete.ThereturnedarrayvalueisstoredinboardColorsandcanbequeriedwiththesquareIsBlackAt(row:column:)utilityfunction:

1 letboard=Chessboard()

2 print(board.squareIsBlackAt(row:0,column:1))

3 //Prints"true"

4 print(board.squareIsBlackAt(row:7,column:7))

5 //Prints"false"

Deinitialization

Adeinitializeriscalledimmediatelybeforeaclassinstanceisdeallocated.Youwritedeinitializerswiththedeinitkeyword,similartohowinitializersarewrittenwiththeinitkeyword.Deinitializersareonlyavailableonclasstypes.

HowDeinitializationWorks

Swiftautomaticallydeallocatesyourinstanceswhentheyarenolongerneeded,tofreeupresources.Swifthandlesthememorymanagementofinstancesthroughautomaticreferencecounting(ARC),asdescribedinAutomaticReferenceCounting.Typicallyyoudon’tneedtoperformmanualcleanupwhenyourinstancesaredeallocated.However,whenyouareworkingwithyourownresources,youmightneedtoperformsomeadditionalcleanupyourself.Forexample,ifyoucreateacustomclasstoopenafileandwritesomedatatoit,youmightneedtoclosethefilebeforetheclassinstanceisdeallocated.

Classdefinitionscanhaveatmostonedeinitializerperclass.Thedeinitializerdoesnottakeanyparametersandiswrittenwithoutparentheses:

1 deinit{

2 //performthedeinitialization

3 }

Deinitializersarecalledautomatically,justbeforeinstancedeallocationtakesplace.Youarenotallowedtocalladeinitializeryourself.Superclassdeinitializersareinheritedbytheirsubclasses,andthesuperclassdeinitializeriscalledautomaticallyattheendofasubclassdeinitializerimplementation.Superclassdeinitializersarealwayscalled,evenifasubclassdoesnotprovideitsowndeinitializer.

Becauseaninstanceisnotdeallocateduntilafteritsdeinitializeriscalled,adeinitializercanaccessallpropertiesoftheinstanceitiscalledonandcan

modifyitsbehaviorbasedonthoseproperties(suchaslookingupthenameofafilethatneedstobeclosed).

DeinitializersinAction

Here’sanexampleofadeinitializerinaction.Thisexampledefinestwonewtypes,BankandPlayer,forasimplegame.TheBankclassmanagesamade-upcurrency,whichcanneverhavemorethan10,000coinsincirculation.TherecanonlyeverbeoneBankinthegame,andsotheBankisimplementedasaclasswithtypepropertiesandmethodstostoreandmanageitscurrentstate:

1 classBank{

2 staticvarcoinsInBank=10_000

3 staticfuncdistribute(coinsnumberOfCoinsRequested:Int)-

>Int{

4 letnumberOfCoinsToVend=min(numberOfCoinsRequested,

coinsInBank)

5 coinsInBank-=numberOfCoinsToVend

6 returnnumberOfCoinsToVend

7 }

8 staticfuncreceive(coins:Int){

9 coinsInBank+=coins

10 }

11 }

BankkeepstrackofthecurrentnumberofcoinsitholdswithitscoinsInBankproperty.Italsoofferstwomethods—distribute(coins:)andreceive(coins:)—tohandlethedistributionandcollectionofcoins.

Thedistribute(coins:)methodchecksthatthereareenoughcoinsinthebankbeforedistributingthem.Iftherearenotenoughcoins,Bankreturnsasmallernumberthanthenumberthatwasrequested(andreturnszeroifnocoinsareleft

inthebank).Itreturnsanintegervaluetoindicatetheactualnumberofcoinsthatwereprovided.

Thereceive(coins:)methodsimplyaddsthereceivednumberofcoinsbackintothebank’scoinstore.

ThePlayerclassdescribesaplayerinthegame.Eachplayerhasacertainnumberofcoinsstoredintheirpurseatanytime.Thisisrepresentedbytheplayer’scoinsInPurseproperty:

1 classPlayer{

2 varcoinsInPurse:Int

3 init(coins:Int){

4 coinsInPurse=Bank.distribute(coins:coins)

5 }

6 funcwin(coins:Int){

7 coinsInPurse+=Bank.distribute(coins:coins)

8 }

9 deinit{

10 Bank.receive(coins:coinsInPurse)

11 }

12 }

EachPlayerinstanceisinitializedwithastartingallowanceofaspecifiednumberofcoinsfromthebankduringinitialization,althoughaPlayerinstancemayreceivefewerthanthatnumberifnotenoughcoinsareavailable.

ThePlayerclassdefinesawin(coins:)method,whichretrievesacertainnumberofcoinsfromthebankandaddsthemtotheplayer’spurse.ThePlayerclassalsoimplementsadeinitializer,whichiscalledjustbeforeaPlayerinstanceisdeallocated.Here,thedeinitializersimplyreturnsalloftheplayer’scoinstothebank:

1 varplayerOne:Player?=Player(coins:100)

2 print("Anewplayerhasjoinedthegamewith\

(playerOne!.coinsInPurse)coins")

3 //Prints"Anewplayerhasjoinedthegamewith100coins"

4 print("Therearenow\(Bank.coinsInBank)coinsleftinthe

bank")

5 //Prints"Therearenow9900coinsleftinthebank"

AnewPlayerinstanceiscreated,witharequestfor100coinsiftheyareavailable.ThisPlayerinstanceisstoredinanoptionalPlayervariablecalledplayerOne.Anoptionalvariableisusedhere,becauseplayerscanleavethegameatanypoint.Theoptionalletsyoutrackwhetherthereiscurrentlyaplayerinthegame.

BecauseplayerOneisanoptional,itisqualifiedwithanexclamationmark(!)whenitscoinsInPursepropertyisaccessedtoprintitsdefaultnumberofcoins,andwheneveritswin(coins:)methodiscalled:

1 playerOne!.win(coins:2_000)

2 print("PlayerOnewon2000coins&nowhas\

(playerOne!.coinsInPurse)coins")

3 //Prints"PlayerOnewon2000coins&nowhas2100coins"

4 print("Thebanknowonlyhas\(Bank.coinsInBank)coinsleft")

5 //Prints"Thebanknowonlyhas7900coinsleft"

Here,theplayerhaswon2,000coins.Theplayer’spursenowcontains2,100coins,andthebankhasonly7,900coinsleft.

1 playerOne=nil

2 print("PlayerOnehasleftthegame")

3 //Prints"PlayerOnehasleftthegame"

4 print("Thebanknowhas\(Bank.coinsInBank)coins")

5 //Prints"Thebanknowhas10000coins"

Theplayerhasnowleftthegame.ThisisindicatedbysettingtheoptionalplayerOnevariabletonil,meaning“noPlayerinstance.”Atthepointthatthishappens,theplayerOnevariable’sreferencetothePlayerinstanceisbroken.NootherpropertiesorvariablesarestillreferringtothePlayerinstance,andsoitisdeallocatedinordertofreeupitsmemory.Justbeforethishappens,itsdeinitializeriscalledautomatically,anditscoinsarereturnedtothebank.

OptionalChaining

Optionalchainingisaprocessforqueryingandcallingproperties,methods,andsubscriptsonanoptionalthatmightcurrentlybenil.Iftheoptionalcontainsavalue,theproperty,method,orsubscriptcallsucceeds;iftheoptionalisnil,theproperty,method,orsubscriptcallreturnsnil.Multiplequeriescanbechainedtogether,andtheentirechainfailsgracefullyifanylinkinthechainisnil.

NOTE

OptionalchaininginSwiftissimilartomessagingnilinObjective-C,butinawaythatworksforanytype,andthatcanbecheckedforsuccessorfailure.

OptionalChainingasanAlternativetoForcedUnwrapping

Youspecifyoptionalchainingbyplacingaquestionmark(?)aftertheoptionalvalueonwhichyouwishtocallaproperty,methodorsubscriptiftheoptionalisnon-nil.Thisisverysimilartoplacinganexclamationmark(!)afteranoptionalvaluetoforcetheunwrappingofitsvalue.Themaindifferenceisthatoptionalchainingfailsgracefullywhentheoptionalisnil,whereasforcedunwrappingtriggersaruntimeerrorwhentheoptionalisnil.

Toreflectthefactthatoptionalchainingcanbecalledonanilvalue,theresultofanoptionalchainingcallisalwaysanoptionalvalue,eveniftheproperty,method,orsubscriptyouarequeryingreturnsanon-optionalvalue.Youcanusethisoptionalreturnvaluetocheckwhethertheoptionalchainingcallwassuccessful(thereturnedoptionalcontainsavalue),ordidnotsucceedduetoanilvalueinthechain(thereturnedoptionalvalueisnil).

Specifically,theresultofanoptionalchainingcallisofthesametypeastheexpectedreturnvalue,butwrappedinanoptional.ApropertythatnormallyreturnsanIntwillreturnanInt?whenaccessedthroughoptionalchaining.

Thenextseveralcodesnippetsdemonstratehowoptionalchainingdiffersfrom

forcedunwrappingandenablesyoutocheckforsuccess.

First,twoclassescalledPersonandResidencearedefined:

1 classPerson{

2 varresidence:Residence?

3 }

4

5 classResidence{

6 varnumberOfRooms=1

7 }

ResidenceinstanceshaveasingleIntpropertycallednumberOfRooms,withadefaultvalueof1.PersoninstanceshaveanoptionalresidencepropertyoftypeResidence?.

IfyoucreateanewPersoninstance,itsresidencepropertyisdefaultinitializedtonil,byvirtueofbeingoptional.Inthecodebelow,johnhasaresidencepropertyvalueofnil:

letjohn=Person()

IfyoutrytoaccessthenumberOfRoomspropertyofthisperson’sresidence,byplacinganexclamationmarkafterresidencetoforcetheunwrappingofitsvalue,youtriggeraruntimeerror,becausethereisnoresidencevaluetounwrap:

1 letroomCount=john.residence!.numberOfRooms

2 //thistriggersaruntimeerror

Thecodeabovesucceedswhenjohn.residencehasanon-nilvalueandwillsetroomCounttoanIntvaluecontainingtheappropriatenumberofrooms.However,thiscodealwaystriggersaruntimeerrorwhenresidenceisnil,asillustratedabove.

OptionalchainingprovidesanalternativewaytoaccessthevalueofnumberOfRooms.Touseoptionalchaining,useaquestionmarkinplaceoftheexclamationmark:

1 ifletroomCount=john.residence?.numberOfRooms{

2 print("John'sresidencehas\(roomCount)room(s).")

3 }else{

4 print("Unabletoretrievethenumberofrooms.")

5 }

6 //Prints"Unabletoretrievethenumberofrooms."

ThistellsSwiftto“chain”ontheoptionalresidencepropertyandtoretrievethevalueofnumberOfRoomsifresidenceexists.

BecausetheattempttoaccessnumberOfRoomshasthepotentialtofail,theoptionalchainingattemptreturnsavalueoftypeInt?,or“optionalInt”.Whenresidenceisnil,asintheexampleabove,thisoptionalIntwillalsobenil,toreflectthefactthatitwasnotpossibletoaccessnumberOfRooms.TheoptionalIntisaccessedthroughoptionalbindingtounwraptheintegerandassignthenon-optionalvaluetotheroomCountvariable.

NotethatthisistrueeventhoughnumberOfRoomsisanon-optionalInt.ThefactthatitisqueriedthroughanoptionalchainmeansthatthecalltonumberOfRoomswillalwaysreturnanInt?insteadofanInt.

YoucanassignaResidenceinstancetojohn.residence,sothatitnolongerhasanilvalue:

john.residence=Residence()

john.residencenowcontainsanactualResidenceinstance,ratherthannil.IfyoutrytoaccessnumberOfRoomswiththesameoptionalchainingasbefore,itwillnowreturnanInt?thatcontainsthedefaultnumberOfRoomsvalueof1:

1 ifletroomCount=john.residence?.numberOfRooms{

2 print("John'sresidencehas\(roomCount)room(s).")

3 }else{

4 print("Unabletoretrievethenumberofrooms.")

5 }

6 //Prints"John'sresidencehas1room(s)."

DefiningModelClassesforOptionalChaining

Youcanuseoptionalchainingwithcallstoproperties,methods,andsubscriptsthataremorethanoneleveldeep.Thisenablesyoutodrilldownintosubpropertieswithincomplexmodelsofinterrelatedtypes,andtocheckwhetheritispossibletoaccessproperties,methods,andsubscriptsonthosesubproperties.

Thecodesnippetsbelowdefinefourmodelclassesforuseinseveralsubsequentexamples,includingexamplesofmultileveloptionalchaining.TheseclassesexpanduponthePersonandResidencemodelfromabovebyaddingaRoomandAddressclass,withassociatedproperties,methods,andsubscripts.

ThePersonclassisdefinedinthesamewayasbefore:

1 classPerson{

2 varresidence:Residence?

3 }

TheResidenceclassismorecomplexthanbefore.Thistime,theResidenceclassdefinesavariablepropertycalledrooms,whichisinitializedwithanemptyarrayoftype[Room]:

1 classResidence{

2 varrooms=[Room]()

3 varnumberOfRooms:Int{

4 returnrooms.count

5 }

6 subscript(i:Int)->Room{

7 get{

8 returnrooms[i]

9 }

10 set{

11 rooms[i]=newValue

12 }

13 }

14 funcprintNumberOfRooms(){

15 print("Thenumberofroomsis\(numberOfRooms)")

16 }

17 varaddress:Address?

18 }

BecausethisversionofResidencestoresanarrayofRoominstances,itsnumberOfRoomspropertyisimplementedasacomputedproperty,notastoredproperty.ThecomputednumberOfRoomspropertysimplyreturnsthevalueofthecountpropertyfromtheroomsarray.

Asashortcuttoaccessingitsroomsarray,thisversionofResidenceprovidesaread-writesubscriptthatprovidesaccesstotheroomattherequestedindexintheroomsarray.

ThisversionofResidencealsoprovidesamethodcalledprintNumberOfRooms,whichsimplyprintsthenumberofroomsintheresidence.

Finally,Residencedefinesanoptionalpropertycalledaddress,withatypeofAddress?.TheAddressclasstypeforthispropertyisdefinedbelow.

TheRoomclassusedfortheroomsarrayisasimpleclasswithonepropertycalledname,andaninitializertosetthatpropertytoasuitableroomname:

1 classRoom{

2 letname:String

3 init(name:String){self.name=name}

4 }

ThefinalclassinthismodeliscalledAddress.ThisclasshasthreeoptionalpropertiesoftypeString?.Thefirsttwoproperties,buildingNameandbuildingNumber,arealternativewaystoidentifyaparticularbuildingaspartofanaddress.Thethirdproperty,street,isusedtonamethestreetforthataddress:

1 classAddress{

2 varbuildingName:String?

3 varbuildingNumber:String?

4 varstreet:String?

5 funcbuildingIdentifier()->String?{

6 ifletbuildingNumber=buildingNumber,letstreet=

street{

7 return"\(buildingNumber)\(street)"

8 }elseifbuildingName!=nil{

9 returnbuildingName

10 }else{

11 returnnil

12 }

13 }

14 }

TheAddressclassalsoprovidesamethodcalledbuildingIdentifier(),whichhasareturntypeofString?.ThismethodchecksthepropertiesoftheaddressandreturnsbuildingNameifithasavalue,orbuildingNumberconcatenatedwithstreetifbothhavevalues,ornilotherwise.

AccessingPropertiesThroughOptionalChaining

AsdemonstratedinOptionalChainingasanAlternativetoForcedUnwrapping,youcanuseoptionalchainingtoaccessapropertyonanoptionalvalue,andtocheckifthatpropertyaccessissuccessful.

UsetheclassesdefinedabovetocreateanewPersoninstance,andtrytoaccessitsnumberOfRoomspropertyasbefore:

1 letjohn=Person()

2 ifletroomCount=john.residence?.numberOfRooms{

3 print("John'sresidencehas\(roomCount)room(s).")

4 }else{

5 print("Unabletoretrievethenumberofrooms.")

6 }

7 //Prints"Unabletoretrievethenumberofrooms."

Becausejohn.residenceisnil,thisoptionalchainingcallfailsinthesamewayasbefore.

Youcanalsoattempttosetaproperty’svaluethroughoptionalchaining:

1 letsomeAddress=Address()

2 someAddress.buildingNumber="29"

3 someAddress.street="AcaciaRoad"

4 john.residence?.address=someAddress

Inthisexample,theattempttosettheaddresspropertyofjohn.residencewillfail,becausejohn.residenceiscurrentlynil.

Theassignmentispartoftheoptionalchaining,whichmeansnoneofthecodeontheright-handsideofthe=operatorisevaluated.Inthepreviousexample,it’snoteasytoseethatsomeAddressisneverevaluated,becauseaccessingaconstantdoesn’thaveanysideeffects.Thelistingbelowdoesthesameassignment,butitusesafunctiontocreatetheaddress.Thefunctionprints“Functionwascalled”

beforereturningavalue,whichletsyouseewhethertheright-handsideofthe=operatorwasevaluated.

1 funccreateAddress()->Address{

2 print("Functionwascalled.")

3

4 letsomeAddress=Address()

5 someAddress.buildingNumber="29"

6 someAddress.street="AcaciaRoad"

7

8 returnsomeAddress

9 }

10 john.residence?.address=createAddress()

YoucantellthatthecreateAddress()functionisn’tcalled,becausenothingisprinted.

CallingMethodsThroughOptionalChaining

Youcanuseoptionalchainingtocallamethodonanoptionalvalue,andtocheckwhetherthatmethodcallissuccessful.Youcandothisevenifthatmethoddoesnotdefineareturnvalue.

TheprintNumberOfRooms()methodontheResidenceclassprintsthecurrentvalueofnumberOfRooms.Here’showthemethodlooks:

1 funcprintNumberOfRooms(){

2 print("Thenumberofroomsis\(numberOfRooms)")

3 }

Thismethoddoesnotspecifyareturntype.However,functionsandmethodswithnoreturntypehaveanimplicitreturntypeofVoid,asdescribedin

FunctionsWithoutReturnValues.Thismeansthattheyreturnavalueof(),oranemptytuple.

Ifyoucallthismethodonanoptionalvaluewithoptionalchaining,themethod’sreturntypewillbeVoid?,notVoid,becausereturnvaluesarealwaysofanoptionaltypewhencalledthroughoptionalchaining.ThisenablesyoutouseanifstatementtocheckwhetheritwaspossibletocalltheprintNumberOfRooms()method,eventhoughthemethoddoesnotitselfdefineareturnvalue.ComparethereturnvaluefromtheprintNumberOfRoomscallagainstniltoseeifthemethodcallwassuccessful:

1 ifjohn.residence?.printNumberOfRooms()!=nil{

2 print("Itwaspossibletoprintthenumberofrooms.")

3 }else{

4 print("Itwasnotpossibletoprintthenumberofrooms.")

5 }

6 //Prints"Itwasnotpossibletoprintthenumberofrooms."

Thesameistrueifyouattempttosetapropertythroughoptionalchaining.TheexampleaboveinAccessingPropertiesThroughOptionalChainingattemptstosetanaddressvalueforjohn.residence,eventhoughtheresidencepropertyisnil.AnyattempttosetapropertythroughoptionalchainingreturnsavalueoftypeVoid?,whichenablesyoutocompareagainstniltoseeifthepropertywassetsuccessfully:

1 if(john.residence?.address=someAddress)!=nil{

2 print("Itwaspossibletosettheaddress.")

3 }else{

4 print("Itwasnotpossibletosettheaddress.")

5 }

6 //Prints"Itwasnotpossibletosettheaddress."

AccessingSubscriptsThroughOptionalChaining

Youcanuseoptionalchainingtotrytoretrieveandsetavaluefromasubscriptonanoptionalvalue,andtocheckwhetherthatsubscriptcallissuccessful.

NOTE

Whenyouaccessasubscriptonanoptionalvaluethroughoptionalchaining,youplacethequestionmarkbeforethesubscript’sbrackets,notafter.Theoptionalchainingquestionmarkalwaysfollowsimmediatelyafterthepartoftheexpressionthatisoptional.

Theexamplebelowtriestoretrievethenameofthefirstroomintheroomsarrayofthejohn.residencepropertyusingthesubscriptdefinedontheResidenceclass.Becausejohn.residenceiscurrentlynil,thesubscriptcallfails:

1 ifletfirstRoomName=john.residence?[0].name{

2 print("Thefirstroomnameis\(firstRoomName).")

3 }else{

4 print("Unabletoretrievethefirstroomname.")

5 }

6 //Prints"Unabletoretrievethefirstroomname."

Theoptionalchainingquestionmarkinthissubscriptcallisplacedimmediatelyafterjohn.residence,beforethesubscriptbrackets,becausejohn.residenceistheoptionalvalueonwhichoptionalchainingisbeingattempted.

Similarly,youcantrytosetanewvaluethroughasubscriptwithoptionalchaining:

john.residence?[0]=Room(name:"Bathroom")

Thissubscriptsettingattemptalsofails,becauseresidenceiscurrentlynil.

IfyoucreateandassignanactualResidenceinstancetojohn.residence,withoneormoreRoominstancesinitsroomsarray,youcanusetheResidencesubscripttoaccesstheactualitemsintheroomsarraythroughoptionalchaining:

1 letjohnsHouse=Residence()

2 johnsHouse.rooms.append(Room(name:"LivingRoom"))

3 johnsHouse.rooms.append(Room(name:"Kitchen"))

4 john.residence=johnsHouse

5

6 ifletfirstRoomName=john.residence?[0].name{

7 print("Thefirstroomnameis\(firstRoomName).")

8 }else{

9 print("Unabletoretrievethefirstroomname.")

10 }

11 //Prints"ThefirstroomnameisLivingRoom."

AccessingSubscriptsofOptionalTypeIfasubscriptreturnsavalueofoptionaltype—suchasthekeysubscriptofSwift’sDictionarytype—placeaquestionmarkafterthesubscript’sclosingbrackettochainonitsoptionalreturnvalue:

1 vartestScores=["Dave":[86,82,84],"Bev":[79,94,81]]

2 testScores["Dave"]?[0]=91

3 testScores["Bev"]?[0]+=1

4 testScores["Brian"]?[0]=72

5 //the"Dave"arrayisnow[91,82,84]andthe"Bev"arrayis

now[80,94,81]

TheexampleabovedefinesadictionarycalledtestScores,whichcontainstwokey-valuepairsthatmapaStringkeytoanarrayofIntvalues.Theexampleusesoptionalchainingtosetthefirstiteminthe"Dave"arrayto91;toincrementthefirstiteminthe"Bev"arrayby1;andtotrytosetthefirstiteminanarrayforakeyof"Brian".Thefirsttwocallssucceed,becausethetestScoresdictionarycontainskeysfor"Dave"and"Bev".Thethirdcallfails,becausethetestScoresdictionarydoesnotcontainakeyfor"Brian".

LinkingMultipleLevelsofChaining

Youcanlinktogethermultiplelevelsofoptionalchainingtodrilldowntoproperties,methods,andsubscriptsdeeperwithinamodel.However,multiplelevelsofoptionalchainingdonotaddmorelevelsofoptionalitytothereturnedvalue.

Toputitanotherway:

Ifthetypeyouaretryingtoretrieveisnotoptional,itwillbecomeoptionalbecauseoftheoptionalchaining.

Ifthetypeyouaretryingtoretrieveisalreadyoptional,itwillnotbecomemoreoptionalbecauseofthechaining.

Therefore:

IfyoutrytoretrieveanIntvaluethroughoptionalchaining,anInt?isalwaysreturned,nomatterhowmanylevelsofchainingareused.

Similarly,ifyoutrytoretrieveanInt?valuethroughoptionalchaining,anInt?isalwaysreturned,nomatterhowmanylevelsofchainingareused.

Theexamplebelowtriestoaccessthestreetpropertyoftheaddresspropertyoftheresidencepropertyofjohn.Therearetwolevelsofoptionalchaininginusehere,tochainthroughtheresidenceandaddressproperties,bothofwhichareofoptionaltype:

1 ifletjohnsStreet=john.residence?.address?.street{

2 print("John'sstreetnameis\(johnsStreet).")

3 }else{

4 print("Unabletoretrievetheaddress.")

5 }

6 //Prints"Unabletoretrievetheaddress."

Thevalueofjohn.residencecurrentlycontainsavalidResidenceinstance.However,thevalueofjohn.residence.addressiscurrentlynil.Becauseofthis,

thecalltojohn.residence?.address?.streetfails.

Notethatintheexampleabove,youaretryingtoretrievethevalueofthestreetproperty.ThetypeofthispropertyisString?.Thereturnvalueofjohn.residence?.address?.streetisthereforealsoString?,eventhoughtwolevelsofoptionalchainingareappliedinadditiontotheunderlyingoptionaltypeoftheproperty.

IfyousetanactualAddressinstanceasthevalueforjohn.residence.address,andsetanactualvaluefortheaddress’sstreetproperty,youcanaccessthevalueofthestreetpropertythroughmultileveloptionalchaining:

1 letjohnsAddress=Address()

2 johnsAddress.buildingName="TheLarches"

3 johnsAddress.street="LaurelStreet"

4 john.residence?.address=johnsAddress

5

6 ifletjohnsStreet=john.residence?.address?.street{

7 print("John'sstreetnameis\(johnsStreet).")

8 }else{

9 print("Unabletoretrievetheaddress.")

10 }

11 //Prints"John'sstreetnameisLaurelStreet."

Inthisexample,theattempttosettheaddresspropertyofjohn.residencewillsucceed,becausethevalueofjohn.residencecurrentlycontainsavalidResidenceinstance.

ChainingonMethodswithOptionalReturnValues

Thepreviousexampleshowshowtoretrievethevalueofapropertyofoptionaltypethroughoptionalchaining.Youcanalsouseoptionalchainingtocalla

methodthatreturnsavalueofoptionaltype,andtochainonthatmethod’sreturnvalueifneeded.

TheexamplebelowcallstheAddressclass’sbuildingIdentifier()methodthroughoptionalchaining.ThismethodreturnsavalueoftypeString?.Asdescribedabove,theultimatereturntypeofthismethodcallafteroptionalchainingisalsoString?:

1 ifletbuildingIdentifier=

john.residence?.address?.buildingIdentifier(){

2 print("John'sbuildingidentifieris\

(buildingIdentifier).")

3 }

4 //Prints"John'sbuildingidentifierisTheLarches."

Ifyouwanttoperformfurtheroptionalchainingonthismethod’sreturnvalue,placetheoptionalchainingquestionmarkafterthemethod’sparentheses:

1 ifletbeginsWithThe=

2

john.residence?.address?.buildingIdentifier()?.hasPrefix("The"

{

3 ifbeginsWithThe{

4 print("John'sbuildingidentifierbeginswith

\"The\".")

5 }else{

6 print("John'sbuildingidentifierdoesnotbeginwith

\"The\".")

7 }

8 }

9 //Prints"John'sbuildingidentifierbeginswith"The"."

NOTE

Intheexampleabove,youplacetheoptionalchainingquestionmarkaftertheparentheses,becausetheoptionalvalueyouarechainingonisthebuildingIdentifier()method’sreturnvalue,andnotthebuildingIdentifier()methoditself.

ErrorHandling

Errorhandlingistheprocessofrespondingtoandrecoveringfromerrorconditionsinyourprogram.Swiftprovidesfirst-classsupportforthrowing,catching,propagating,andmanipulatingrecoverableerrorsatruntime.

Someoperationsaren’tguaranteedtoalwayscompleteexecutionorproduceausefuloutput.Optionalsareusedtorepresenttheabsenceofavalue,butwhenanoperationfails,it’softenusefultounderstandwhatcausedthefailure,sothatyourcodecanrespondaccordingly.

Asanexample,considerthetaskofreadingandprocessingdatafromafileondisk.Thereareanumberofwaysthistaskcanfail,includingthefilenotexistingatthespecifiedpath,thefilenothavingreadpermissions,orthefilenotbeingencodedinacompatibleformat.Distinguishingamongthesedifferentsituationsallowsaprogramtoresolvesomeerrorsandtocommunicatetotheuseranyerrorsitcan’tresolve.

NOTE

ErrorhandlinginSwiftinteroperateswitherrorhandlingpatternsthatusetheNSErrorclassinCocoaandObjective-C.Formoreinformationaboutthisclass,seeHandlingCocoaErrorsinSwift.

RepresentingandThrowingErrors

InSwift,errorsarerepresentedbyvaluesoftypesthatconformtotheErrorprotocol.Thisemptyprotocolindicatesthatatypecanbeusedforerrorhandling.

Swiftenumerationsareparticularlywellsuitedtomodelingagroupofrelatederrorconditions,withassociatedvaluesallowingforadditionalinformationaboutthenatureofanerrortobecommunicated.Forexample,here’showyoumightrepresenttheerrorconditionsofoperatingavendingmachineinsideagame:

1 enumVendingMachineError:Error{

2 caseinvalidSelection

3 caseinsufficientFunds(coinsNeeded:Int)

4 caseoutOfStock

5 }

Throwinganerrorletsyouindicatethatsomethingunexpectedhappenedandthenormalflowofexecutioncan’tcontinue.Youuseathrowstatementtothrowanerror.Forexample,thefollowingcodethrowsanerrortoindicatethatfiveadditionalcoinsareneededbythevendingmachine:

throwVendingMachineError.insufficientFunds(coinsNeeded:5)

HandlingErrors

Whenanerroristhrown,somesurroundingpieceofcodemustberesponsibleforhandlingtheerror—forexample,bycorrectingtheproblem,tryinganalternativeapproach,orinformingtheuserofthefailure.

TherearefourwaystohandleerrorsinSwift.Youcanpropagatetheerrorfromafunctiontothecodethatcallsthatfunction,handletheerrorusingado-catchstatement,handletheerrorasanoptionalvalue,orassertthattheerrorwillnotoccur.Eachapproachisdescribedinasectionbelow.

Whenafunctionthrowsanerror,itchangestheflowofyourprogram,soit’simportantthatyoucanquicklyidentifyplacesinyourcodethatcanthrowerrors.Toidentifytheseplacesinyourcode,writethetrykeyword—orthetry?ortry!variation—beforeapieceofcodethatcallsafunction,method,orinitializerthatcanthrowanerror.Thesekeywordsaredescribedinthesectionsbelow.

NOTE

ErrorhandlinginSwiftresemblesexceptionhandlinginotherlanguages,withtheuseofthetry,catchandthrowkeywords.Unlikeexceptionhandlinginmanylanguages—includingObjective-C—errorhandlinginSwiftdoesnotinvolveunwindingthecallstack,aprocessthatcanbe

computationallyexpensive.Assuch,theperformancecharacteristicsofathrowstatementarecomparabletothoseofareturnstatement.

PropagatingErrorsUsingThrowingFunctionsToindicatethatafunction,method,orinitializercanthrowanerror,youwritethethrowskeywordinthefunction’sdeclarationafteritsparameters.Afunctionmarkedwiththrowsiscalledathrowingfunction.Ifthefunctionspecifiesareturntype,youwritethethrowskeywordbeforethereturnarrow(->).

1 funccanThrowErrors()throws->String

2

3 funccannotThrowErrors()->String

Athrowingfunctionpropagateserrorsthatarethrowninsideofittothescopefromwhichit’scalled.

NOTE

Onlythrowingfunctionscanpropagateerrors.Anyerrorsthrowninsideanonthrowingfunctionmustbehandledinsidethefunction.

Intheexamplebelow,theVendingMachineclasshasavend(itemNamed:)methodthatthrowsanappropriateVendingMachineErroriftherequesteditemisnotavailable,isoutofstock,orhasacostthatexceedsthecurrentdepositedamount:

1 structItem{

2 varprice:Int

3 varcount:Int

4 }

5

6 classVendingMachine{

7 varinventory=[

8 "CandyBar":Item(price:12,count:7),

9 "Chips":Item(price:10,count:4),

10 "Pretzels":Item(price:7,count:11)

11 ]

12 varcoinsDeposited=0

13

14 funcvend(itemNamedname:String)throws{

15 guardletitem=inventory[name]else{

16 throwVendingMachineError.invalidSelection

17 }

18

19 guarditem.count>0else{

20 throwVendingMachineError.outOfStock

21 }

22

23 guarditem.price<=coinsDepositedelse{

24 throw

VendingMachineError.insufficientFunds(coinsNeeded:

item.price-coinsDeposited)

25 }

26

27 coinsDeposited-=item.price

28

29 varnewItem=item

30 newItem.count-=1

31 inventory[name]=newItem

32

33 print("Dispensing\(name)")

34 }

35 }

Theimplementationofthevend(itemNamed:)methodusesguardstatementstoexitthemethodearlyandthrowappropriateerrorsifanyoftherequirementsforpurchasingasnackaren’tmet.Becauseathrowstatementimmediatelytransfersprogramcontrol,anitemwillbevendedonlyifalloftheserequirementsaremet.

Becausethevend(itemNamed:)methodpropagatesanyerrorsitthrows,anycodethatcallsthismethodmusteitherhandletheerrors—usingado-catchstatement,try?,ortry!—orcontinuetopropagatethem.Forexample,thebuyFavoriteSnack(person:vendingMachine:)intheexamplebelowisalsoathrowingfunction,andanyerrorsthatthevend(itemNamed:)methodthrowswillpropagateuptothepointwherethebuyFavoriteSnack(person:vendingMachine:)functioniscalled.

1 letfavoriteSnacks=[

2 "Alice":"Chips",

3 "Bob":"Licorice",

4 "Eve":"Pretzels",

5 ]

6 funcbuyFavoriteSnack(person:String,vendingMachine:

VendingMachine)throws{

7 letsnackName=favoriteSnacks[person]??"CandyBar"

8 tryvendingMachine.vend(itemNamed:snackName)

9 }

Inthisexample,thebuyFavoriteSnack(person:vendingMachine:)functionlooksupagivenperson’sfavoritesnackandtriestobuyitforthembycallingthevend(itemNamed:)method.Becausethevend(itemNamed:)methodcanthrowanerror,it’scalledwiththetrykeywordinfrontofit.

Throwinginitializerscanpropagateerrorsinthesamewayasthrowingfunctions.Forexample,theinitializerforthePurchasedSnackstructureinthelistingbelowcallsathrowingfunctionaspartoftheinitializationprocess,andithandlesanyerrorsthatitencountersbypropagatingthemtoitscaller.

1 structPurchasedSnack{

2 letname:String

3 init(name:String,vendingMachine:VendingMachine)throws{

4 tryvendingMachine.vend(itemNamed:name)

5 self.name=name

6 }

7 }

HandlingErrorsUsingDo-CatchYouuseado-catchstatementtohandleerrorsbyrunningablockofcode.Ifanerroristhrownbythecodeinthedoclause,itismatchedagainstthecatchclausestodeterminewhichoneofthemcanhandletheerror.

Hereisthegeneralformofado-catchstatement:

do{

try expression

statements

}catch pattern1 {

statements

}catch pattern2 where condition {

statements

}catch{

statements

}

Youwriteapatternaftercatchtoindicatewhaterrorsthatclausecanhandle.Ifacatchclausedoesn’thaveapattern,theclausematchesanyerrorandbindstheerrortoalocalconstantnamederror.Formoreinformationaboutpatternmatching,seePatterns.

Forexample,thefollowingcodematchesagainstallthreecasesofthe

VendingMachineErrorenumeration.

1 varvendingMachine=VendingMachine()

2 vendingMachine.coinsDeposited=8

3 do{

4 trybuyFavoriteSnack(person:"Alice",vendingMachine:

vendingMachine)

5 print("Success!Yum.")

6 }catchVendingMachineError.invalidSelection{

7 print("InvalidSelection.")

8 }catchVendingMachineError.outOfStock{

9 print("OutofStock.")

10 }catchVendingMachineError.insufficientFunds(letcoinsNeeded)

{

11 print("Insufficientfunds.Pleaseinsertanadditional\

(coinsNeeded)coins.")

12 }catch{

13 print("Unexpectederror:\(error).")

14 }

15 //Prints"Insufficientfunds.Pleaseinsertanadditional2

coins."

Intheaboveexample,thebuyFavoriteSnack(person:vendingMachine:)functioniscalledinatryexpression,becauseitcanthrowanerror.Ifanerroristhrown,executionimmediatelytransferstothecatchclauses,whichdecidewhethertoallowpropagationtocontinue.Ifnopatternismatched,theerrorgetscaughtbythefinalcatchclauseandisboundtoalocalerrorconstant.Ifnoerroristhrown,theremainingstatementsinthedostatementareexecuted.

Thecatchclausesdon’thavetohandleeverypossibleerrorthatthecodeinthedoclausecanthrow.Ifnoneofthecatchclauseshandletheerror,theerrorpropagatestothesurroundingscope.However,thepropagatederrormustbe

handledbysomesurroundingscope.Inanonthrowingfunction,anenclosingdo-catchclausemusthandletheerror.Inathrowingfunction,eitheranenclosingdo-catchclauseorthecallermusthandletheerror.Iftheerrorpropagatestothetop-levelscopewithoutbeinghandled,you’llgetaruntimeerror.

Forexample,theaboveexamplecanbewrittensoanyerrorthatisn’taVendingMachineErrorisinsteadcaughtbythecallingfunction:

1 funcnourish(withitem:String)throws{

2 do{

3 tryvendingMachine.vend(itemNamed:item)

4 }catchisVendingMachineError{

5 print("Invalidselection,outofstock,ornotenough

money.")

6 }

7 }

8

9 do{

10 trynourish(with:"Beet-FlavoredChips")

11 }catch{

12 print("Unexpectednon-vending-machine-relatederror:\

(error)")

13 }

14 //Prints"Invalidselection,outofstock,ornotenough

money."

Inthenourish(with:)function,ifvend(itemNamed:)throwsanerrorthat’soneofthecasesoftheVendingMachineErrorenumeration,nourish(with:)handlestheerrorbyprintingamessage.Otherwise,nourish(with:)propagatestheerrortoitscallsite.Theerroristhencaughtbythegeneralcatchclause.

ConvertingErrorstoOptionalValuesYouusetry?tohandleanerrorbyconvertingittoanoptionalvalue.Ifanerroristhrownwhileevaluatingthetry?expression,thevalueoftheexpressionisnil.Forexample,inthefollowingcodexandyhavethesamevalueandbehavior:

1 funcsomeThrowingFunction()throws->Int{

2 //...

3 }

4

5 letx=try?someThrowingFunction()

6

7 lety:Int?

8 do{

9 y=trysomeThrowingFunction()

10 }catch{

11 y=nil

12 }

IfsomeThrowingFunction()throwsanerror,thevalueofxandyisnil.Otherwise,thevalueofxandyisthevaluethatthefunctionreturned.NotethatxandyareanoptionalofwhatevertypesomeThrowingFunction()returns.Herethefunctionreturnsaninteger,soxandyareoptionalintegers.

Usingtry?letsyouwriteconciseerrorhandlingcodewhenyouwanttohandleallerrorsinthesameway.Forexample,thefollowingcodeusesseveralapproachestofetchdata,orreturnsnilifalloftheapproachesfail.

1 funcfetchData()->Data?{

2 ifletdata=try?fetchDataFromDisk(){returndata}

3 ifletdata=try?fetchDataFromServer(){returndata}

4 returnnil

5 }

DisablingErrorPropagationSometimesyouknowathrowingfunctionormethodwon’t,infact,throwanerroratruntime.Onthoseoccasions,youcanwritetry!beforetheexpressiontodisableerrorpropagationandwrapthecallinaruntimeassertionthatnoerrorwillbethrown.Ifanerroractuallyisthrown,you’llgetaruntimeerror.

Forexample,thefollowingcodeusesaloadImage(atPath:)function,whichloadstheimageresourceatagivenpathorthrowsanerroriftheimagecan’tbeloaded.Inthiscase,becausetheimageisshippedwiththeapplication,noerrorwillbethrownatruntime,soitisappropriatetodisableerrorpropagation.

letphoto=try!loadImage(atPath:"./Resources/John

Appleseed.jpg")

SpecifyingCleanupActions

Youuseadeferstatementtoexecuteasetofstatementsjustbeforecodeexecutionleavesthecurrentblockofcode.Thisstatementletsyoudoanynecessarycleanupthatshouldbeperformedregardlessofhowexecutionleavesthecurrentblockofcode—whetheritleavesbecauseanerrorwasthrownorbecauseofastatementsuchasreturnorbreak.Forexample,youcanuseadeferstatementtoensurethatfiledescriptorsareclosedandmanuallyallocatedmemoryisfreed.

Adeferstatementdefersexecutionuntilthecurrentscopeisexited.Thisstatementconsistsofthedeferkeywordandthestatementstobeexecutedlater.Thedeferredstatementsmaynotcontainanycodethatwouldtransfercontroloutofthestatements,suchasabreakorareturnstatement,orbythrowinganerror.Deferredactionsareexecutedinthereverseoftheorderthatthey’rewritteninyoursourcecode.Thatis,thecodeinthefirstdeferstatementexecuteslast,thecodeintheseconddeferstatementexecutessecondtolast,andsoon.Thelastdeferstatementinsourcecodeorderexecutesfirst.

1 funcprocessFile(filename:String)throws{

2 ifexists(filename){

3 letfile=open(filename)

4 defer{

5 close(file)

6 }

7 whileletline=tryfile.readline(){

8 //Workwiththefile.

9 }

10 //close(file)iscalledhere,attheendofthe

scope.

11 }

12 }

Theaboveexampleusesadeferstatementtoensurethattheopen(_:)functionhasacorrespondingcalltoclose(_:).

NOTE

Youcanuseadeferstatementevenwhennoerrorhandlingcodeisinvolved.

TypeCasting

Typecastingisawaytocheckthetypeofaninstance,ortotreatthatinstanceasadifferentsuperclassorsubclassfromsomewhereelseinitsownclasshierarchy.

TypecastinginSwiftisimplementedwiththeisandasoperators.Thesetwooperatorsprovideasimpleandexpressivewaytocheckthetypeofavalueorcastavaluetoadifferenttype.

Youcanalsousetypecastingtocheckwhetheratypeconformstoaprotocol,asdescribedinCheckingforProtocolConformance.

DefiningaClassHierarchyforTypeCasting

Youcanusetypecastingwithahierarchyofclassesandsubclassestocheckthetypeofaparticularclassinstanceandtocastthatinstancetoanotherclasswithinthesamehierarchy.Thethreecodesnippetsbelowdefineahierarchyofclassesandanarraycontaininginstancesofthoseclasses,foruseinanexampleoftypecasting.

ThefirstsnippetdefinesanewbaseclasscalledMediaItem.Thisclassprovidesbasicfunctionalityforanykindofitemthatappearsinadigitalmedialibrary.Specifically,itdeclaresanamepropertyoftypeString,andaninitnameinitializer.(Itisassumedthatallmediaitems,includingallmoviesandsongs,willhaveaname.)

1 classMediaItem{

2 varname:String

3 init(name:String){

4 self.name=name

5 }

6 }

ThenextsnippetdefinestwosubclassesofMediaItem.Thefirstsubclass,Movie,encapsulatesadditionalinformationaboutamovieorfilm.ItaddsadirectorpropertyontopofthebaseMediaItemclass,withacorrespondinginitializer.Thesecondsubclass,Song,addsanartistpropertyandinitializerontopofthebaseclass:

1 classMovie:MediaItem{

2 vardirector:String

3 init(name:String,director:String){

4 self.director=director

5 super.init(name:name)

6 }

7 }

8

9 classSong:MediaItem{

10 varartist:String

11 init(name:String,artist:String){

12 self.artist=artist

13 super.init(name:name)

14 }

15 }

Thefinalsnippetcreatesaconstantarraycalledlibrary,whichcontainstwoMovieinstancesandthreeSonginstances.Thetypeofthelibraryarrayisinferredbyinitializingitwiththecontentsofanarrayliteral.Swift’stypecheckerisabletodeducethatMovieandSonghaveacommonsuperclassofMediaItem,andsoitinfersatypeof[MediaItem]forthelibraryarray:

1 letlibrary=[

2 Movie(name:"Casablanca",director:"MichaelCurtiz"),

3 Song(name:"BlueSuedeShoes",artist:"ElvisPresley"),

4 Movie(name:"CitizenKane",director:"OrsonWelles"),

5 Song(name:"TheOneAndOnly",artist:"ChesneyHawkes"),

6 Song(name:"NeverGonnaGiveYouUp",artist:"Rick

Astley")

7 ]

8 //thetypeof"library"isinferredtobe[MediaItem]

TheitemsstoredinlibraryarestillMovieandSonginstancesbehindthescenes.However,ifyouiterateoverthecontentsofthisarray,theitemsyoureceivebackaretypedasMediaItem,andnotasMovieorSong.Inordertoworkwiththemastheirnativetype,youneedtochecktheirtype,ordowncastthemtoadifferenttype,asdescribedbelow.

CheckingType

Usethetypecheckoperator(is)tocheckwhetheraninstanceisofacertainsubclasstype.Thetypecheckoperatorreturnstrueiftheinstanceisofthatsubclasstypeandfalseifitisnot.

Theexamplebelowdefinestwovariables,movieCountandsongCount,whichcountthenumberofMovieandSonginstancesinthelibraryarray:

1 varmovieCount=0

2 varsongCount=0

3

4 foriteminlibrary{

5 ifitemisMovie{

6 movieCount+=1

7 }elseifitemisSong{

8 songCount+=1

9 }

10 }

11

12 print("Medialibrarycontains\(movieCount)moviesand\

(songCount)songs")

13 //Prints"Medialibrarycontains2moviesand3songs"

Thisexampleiteratesthroughallitemsinthelibraryarray.Oneachpass,thefor-inloopsetstheitemconstanttothenextMediaIteminthearray.

itemisMoviereturnstrueifthecurrentMediaItemisaMovieinstanceandfalseifitisnot.Similarly,itemisSongcheckswhethertheitemisaSonginstance.Attheendofthefor-inloop,thevaluesofmovieCountandsongCountcontainacountofhowmanyMediaIteminstanceswerefoundofeachtype.

Downcasting

Aconstantorvariableofacertainclasstypemayactuallyrefertoaninstanceofasubclassbehindthescenes.Whereyoubelievethisisthecase,youcantrytodowncasttothesubclasstypewithatypecastoperator(as?oras!).

Becausedowncastingcanfail,thetypecastoperatorcomesintwodifferentforms.Theconditionalform,as?,returnsanoptionalvalueofthetypeyouaretryingtodowncastto.Theforcedform,as!,attemptsthedowncastandforce-unwrapstheresultasasinglecompoundaction.

Usetheconditionalformofthetypecastoperator(as?)whenyouarenotsureifthedowncastwillsucceed.Thisformoftheoperatorwillalwaysreturnanoptionalvalue,andthevaluewillbenilifthedowncastwasnotpossible.Thisenablesyoutocheckforasuccessfuldowncast.

Usetheforcedformofthetypecastoperator(as!)onlywhenyouaresurethatthedowncastwillalwayssucceed.Thisformoftheoperatorwilltriggeraruntimeerrorifyoutrytodowncasttoanincorrectclasstype.

TheexamplebelowiteratesovereachMediaIteminlibrary,andprintsanappropriatedescriptionforeachitem.Todothis,itneedstoaccesseachitemasatrueMovieorSong,andnotjustasaMediaItem.ThisisnecessaryinorderforittobeabletoaccessthedirectororartistpropertyofaMovieorSongforuseinthedescription.

Inthisexample,eachiteminthearraymightbeaMovie,oritmightbeaSong.Youdon’tknowinadvancewhichactualclasstouseforeachitem,andsoitisappropriatetousetheconditionalformofthetypecastoperator(as?)tocheckthedowncasteachtimethroughtheloop:

1 foriteminlibrary{

2 ifletmovie=itemas?Movie{

3 print("Movie:\(movie.name),dir.\(movie.director)")

4 }elseifletsong=itemas?Song{

5 print("Song:\(song.name),by\(song.artist)")

6 }

7 }

8

9 //Movie:Casablanca,dir.MichaelCurtiz

10 //Song:BlueSuedeShoes,byElvisPresley

11 //Movie:CitizenKane,dir.OrsonWelles

12 //Song:TheOneAndOnly,byChesneyHawkes

13 //Song:NeverGonnaGiveYouUp,byRickAstley

TheexamplestartsbytryingtodowncastthecurrentitemasaMovie.BecauseitemisaMediaIteminstance,it’spossiblethatitmightbeaMovie;equally,it’salsopossiblethatitmightbeaSong,orevenjustabaseMediaItem.Becauseofthisuncertainty,theas?formofthetypecastoperatorreturnsanoptionalvaluewhenattemptingtodowncasttoasubclasstype.Theresultofitemas?MovieisoftypeMovie?,or“optionalMovie”.

DowncastingtoMoviefailswhenappliedtotheSonginstancesinthelibraryarray.Tocopewiththis,theexampleaboveusesoptionalbindingtocheck

whethertheoptionalMovieactuallycontainsavalue(thatis,tofindoutwhetherthedowncastsucceeded.)Thisoptionalbindingiswritten“ifletmovie=itemas?Movie”,whichcanbereadas:

“TrytoaccessitemasaMovie.Ifthisissuccessful,setanewtemporaryconstantcalledmovietothevaluestoredinthereturnedoptionalMovie.”

Ifthedowncastingsucceeds,thepropertiesofmoviearethenusedtoprintadescriptionforthatMovieinstance,includingthenameofitsdirector.AsimilarprincipleisusedtocheckforSonginstances,andtoprintanappropriatedescription(includingartistname)wheneveraSongisfoundinthelibrary.

NOTE

Castingdoesnotactuallymodifytheinstanceorchangeitsvalues.Theunderlyinginstanceremainsthesame;itissimplytreatedandaccessedasaninstanceofthetypetowhichithasbeencast.

TypeCastingforAnyandAnyObject

Swiftprovidestwospecialtypesforworkingwithnonspecifictypes:

Anycanrepresentaninstanceofanytypeatall,includingfunctiontypes.

AnyObjectcanrepresentaninstanceofanyclasstype.

UseAnyandAnyObjectonlywhenyouexplicitlyneedthebehaviorandcapabilitiestheyprovide.Itisalwaysbettertobespecificaboutthetypesyouexpecttoworkwithinyourcode.

Here’sanexampleofusingAnytoworkwithamixofdifferenttypes,includingfunctiontypesandnonclasstypes.Theexamplecreatesanarraycalledthings,whichcanstorevaluesoftypeAny:

1 varthings=[Any]()

2

3 things.append(0)

4 things.append(0.0)

5 things.append(42)

6 things.append(3.14159)

7 things.append("hello")

8 things.append((3.0,5.0))

9 things.append(Movie(name:"Ghostbusters",director:"Ivan

Reitman"))

10 things.append({(name:String)->Stringin"Hello,\(name)"

})

ThethingsarraycontainstwoIntvalues,twoDoublevalues,aStringvalue,atupleoftype(Double,Double),themovie“Ghostbusters”,andaclosureexpressionthattakesaStringvalueandreturnsanotherStringvalue.

TodiscoverthespecifictypeofaconstantorvariablethatisknownonlytobeoftypeAnyorAnyObject,youcanuseanisoraspatterninaswitchstatement’scases.Theexamplebelowiteratesovertheitemsinthethingsarrayandqueriesthetypeofeachitemwithaswitchstatement.Severaloftheswitchstatement’scasesbindtheirmatchedvaluetoaconstantofthespecifiedtypetoenableitsvaluetobeprinted:

1 forthinginthings{

2 switchthing{

3 case0asInt:

4 print("zeroasanInt")

5 case0asDouble:

6 print("zeroasaDouble")

7 caseletsomeIntasInt:

8 print("anintegervalueof\(someInt)")

9 caseletsomeDoubleasDoublewheresomeDouble>0:

10 print("apositivedoublevalueof\(someDouble)")

11 caseisDouble:

12 print("someotherdoublevaluethatIdon'twantto

print")

13 caseletsomeStringasString:

14 print("astringvalueof\"\(someString)\"")

15 caselet(x,y)as(Double,Double):

16 print("an(x,y)pointat\(x),\(y)")

17 caseletmovieasMovie:

18 print("amoviecalled\(movie.name),dir.\

(movie.director)")

19 caseletstringConverteras(String)->String:

20 print(stringConverter("Michael"))

21 default:

22 print("somethingelse")

23 }

24 }

25

26 //zeroasanInt

27 //zeroasaDouble

28 //anintegervalueof42

29 //apositivedoublevalueof3.14159

30 //astringvalueof"hello"

31 //an(x,y)pointat3.0,5.0

32 //amoviecalledGhostbusters,dir.IvanReitman

33 //Hello,Michael

NOTE

TheAnytyperepresentsvaluesofanytype,includingoptionaltypes.SwiftgivesyouawarningifyouuseanoptionalvaluewhereavalueoftypeAnyisexpected.IfyoureallydoneedtouseanoptionalvalueasanAnyvalue,youcanusetheasoperatortoexplicitlycasttheoptionaltoAny,asshownbelow.

1 letoptionalNumber:Int?=3

2 things.append(optionalNumber)//Warning

3 things.append(optionalNumberasAny)//Nowarning

NestedTypes

Enumerationsareoftencreatedtosupportaspecificclassorstructure’sfunctionality.Similarly,itcanbeconvenienttodefineutilityclassesandstructurespurelyforusewithinthecontextofamorecomplextype.Toaccomplishthis,Swiftenablesyoutodefinenestedtypes,wherebyyounestsupportingenumerations,classes,andstructureswithinthedefinitionofthetypetheysupport.

Tonestatypewithinanothertype,writeitsdefinitionwithintheouterbracesofthetypeitsupports.Typescanbenestedtoasmanylevelsasarerequired.

NestedTypesinAction

TheexamplebelowdefinesastructurecalledBlackjackCard,whichmodelsaplayingcardasusedinthegameofBlackjack.TheBlackjackCardstructurecontainstwonestedenumerationtypescalledSuitandRank.

InBlackjack,theAcecardshaveavalueofeitheroneoreleven.ThisfeatureisrepresentedbyastructurecalledValues,whichisnestedwithintheRankenumeration:

1 structBlackjackCard{

2

3 //nestedSuitenumeration

4 enumSuit:Character{

5 casespades="♠",hearts="♡",diamonds="♢",clubs

="♣"

6 }

7

8 //nestedRankenumeration

9 enumRank:Int{

10 casetwo=2,three,four,five,six,seven,eight,

nine,ten

11 casejack,queen,king,ace

12 structValues{

13 letfirst:Int,second:Int?

14 }

15 varvalues:Values{

16 switchself{

17 case.ace:

18 returnValues(first:1,second:11)

19 case.jack,.queen,.king:

20 returnValues(first:10,second:nil)

21 default:

22 returnValues(first:self.rawValue,second:

nil)

23 }

24 }

25 }

26

27 //BlackjackCardpropertiesandmethods

28 letrank:Rank,suit:Suit

29 vardescription:String{

30 varoutput="suitis\(suit.rawValue),"

31 output+="valueis\(rank.values.first)"

32 ifletsecond=rank.values.second{

33 output+="or\(second)"

34 }

35 returnoutput

36 }

37 }

TheSuitenumerationdescribesthefourcommonplayingcardsuits,togetherwitharawCharactervaluetorepresenttheirsymbol.

TheRankenumerationdescribesthethirteenpossibleplayingcardranks,togetherwitharawIntvaluetorepresenttheirfacevalue.(ThisrawIntvalueisnotusedfortheJack,Queen,King,andAcecards.)

Asmentionedabove,theRankenumerationdefinesafurthernestedstructureofitsown,calledValues.Thisstructureencapsulatesthefactthatmostcardshaveonevalue,buttheAcecardhastwovalues.TheValuesstructuredefinestwopropertiestorepresentthis:

first,oftypeInt

second,oftypeInt?,or“optionalInt”

Rankalsodefinesacomputedproperty,values,whichreturnsaninstanceoftheValuesstructure.ThiscomputedpropertyconsiderstherankofthecardandinitializesanewValuesinstancewithappropriatevaluesbasedonitsrank.Itusesspecialvaluesforjack,queen,king,andace.Forthenumericcards,itusestherank’srawIntvalue.

TheBlackjackCardstructureitselfhastwoproperties—rankandsuit.Italsodefinesacomputedpropertycalleddescription,whichusesthevaluesstoredinrankandsuittobuildadescriptionofthenameandvalueofthecard.Thedescriptionpropertyusesoptionalbindingtocheckwhetherthereisasecondvaluetodisplay,andifso,insertsadditionaldescriptiondetailforthatsecondvalue.

BecauseBlackjackCardisastructurewithnocustominitializers,ithasanimplicitmemberwiseinitializer,asdescribedinMemberwiseInitializersforStructureTypes.YoucanusethisinitializertoinitializeanewconstantcalledtheAceOfSpades:

1 lettheAceOfSpades=BlackjackCard(rank:.ace,suit:.spades)

2 print("theAceOfSpades:\(theAceOfSpades.description)")

3 //Prints"theAceOfSpades:suitis♠,valueis1or11"

EventhoughRankandSuitarenestedwithinBlackjackCard,theirtypecanbeinferredfromcontext,andsotheinitializationofthisinstanceisabletorefertotheenumerationcasesbytheircasenames(.aceand.spades)alone.Intheexampleabove,thedescriptionpropertycorrectlyreportsthattheAceofSpadeshasavalueof1or11.

ReferringtoNestedTypes

Touseanestedtypeoutsideofitsdefinitioncontext,prefixitsnamewiththenameofthetypeitisnestedwithin:

1 letheartsSymbol=BlackjackCard.Suit.hearts.rawValue

2 //heartsSymbolis"♡"

Fortheexampleabove,thisenablesthenamesofSuit,Rank,andValuestobekeptdeliberatelyshort,becausetheirnamesarenaturallyqualifiedbythecontextinwhichtheyaredefined.

Extensions

Extensionsaddnewfunctionalitytoanexistingclass,structure,enumeration,orprotocoltype.Thisincludestheabilitytoextendtypesforwhichyoudonothaveaccesstotheoriginalsourcecode(knownasretroactivemodeling).ExtensionsaresimilartocategoriesinObjective-C.(UnlikeObjective-Ccategories,Swiftextensionsdonothavenames.)

ExtensionsinSwiftcan:

Addcomputedinstancepropertiesandcomputedtypeproperties

Defineinstancemethodsandtypemethods

Providenewinitializers

Definesubscripts

Defineandusenewnestedtypes

Makeanexistingtypeconformtoaprotocol

InSwift,youcanevenextendaprotocoltoprovideimplementationsofitsrequirementsoraddadditionalfunctionalitythatconformingtypescantakeadvantageof.Formoredetails,seeProtocolExtensions.

NOTE

Extensionscanaddnewfunctionalitytoatype,buttheycannotoverrideexistingfunctionality.

ExtensionSyntax

Declareextensionswiththeextensionkeyword:

1 extensionSomeType{

2 //newfunctionalitytoaddtoSomeTypegoeshere

3 }

Anextensioncanextendanexistingtypetomakeitadoptoneormoreprotocols.Toaddprotocolconformance,youwritetheprotocolnamesthesamewayasyouwritethemforaclassorstructure:

1 extensionSomeType:SomeProtocol,AnotherProtocol{

2 //implementationofprotocolrequirementsgoeshere

3 }

AddingprotocolconformanceinthiswayisdescribedinAddingProtocolConformancewithanExtension.

Anextensioncanbeusedtoextendanexistinggenerictype,asdescribedinExtendingaGenericType.Youcanalsoextendagenerictypetoconditionallyaddfunctionality,asdescribedinExtensionswithaGenericWhereClause.

NOTE

Ifyoudefineanextensiontoaddnewfunctionalitytoanexistingtype,thenewfunctionalitywillbeavailableonallexistinginstancesofthattype,eveniftheywerecreatedbeforetheextensionwasdefined.

ComputedProperties

Extensionscanaddcomputedinstancepropertiesandcomputedtypepropertiestoexistingtypes.ThisexampleaddsfivecomputedinstancepropertiestoSwift’sbuilt-inDoubletype,toprovidebasicsupportforworkingwithdistanceunits:

1 extensionDouble{

2 varkm:Double{returnself*1_000.0}

3 varm:Double{returnself}

4 varcm:Double{returnself/100.0}

5 varmm:Double{returnself/1_000.0}

6 varft:Double{returnself/3.28084}

7 }

8 letoneInch=25.4.mm

9 print("Oneinchis\(oneInch)meters")

10 //Prints"Oneinchis0.0254meters"

11 letthreeFeet=3.ft

12 print("Threefeetis\(threeFeet)meters")

13 //Prints"Threefeetis0.914399970739201meters"

ThesecomputedpropertiesexpressthataDoublevalueshouldbeconsideredasacertainunitoflength.Althoughtheyareimplementedascomputedproperties,thenamesofthesepropertiescanbeappendedtoafloating-pointliteralvaluewithdotsyntax,asawaytousethatliteralvaluetoperformdistanceconversions.

Inthisexample,aDoublevalueof1.0isconsideredtorepresent“onemeter”.Thisiswhythemcomputedpropertyreturnsself—theexpression1.misconsideredtocalculateaDoublevalueof1.0.

Otherunitsrequiresomeconversiontobeexpressedasavaluemeasuredinmeters.Onekilometeristhesameas1,000meters,sothekmcomputedpropertymultipliesthevalueby1_000.00toconvertintoanumberexpressedinmeters.Similarly,thereare3.28084feetinameter,andsotheftcomputedpropertydividestheunderlyingDoublevalueby3.28084,toconvertitfromfeettometers.

Thesepropertiesareread-onlycomputedproperties,andsotheyareexpressedwithoutthegetkeyword,forbrevity.TheirreturnvalueisoftypeDouble,andcanbeusedwithinmathematicalcalculationswhereveraDoubleisaccepted:

1 letaMarathon=42.km+195.m

2 print("Amarathonis\(aMarathon)meterslong")

3 //Prints"Amarathonis42195.0meterslong"

NOTE

Extensionscanaddnewcomputedproperties,buttheycannotaddstoredproperties,oraddpropertyobserverstoexistingproperties.

Initializers

Extensionscanaddnewinitializerstoexistingtypes.Thisenablesyoutoextendothertypestoacceptyourowncustomtypesasinitializerparameters,ortoprovideadditionalinitializationoptionsthatwerenotincludedaspartofthetype’soriginalimplementation.

Extensionscanaddnewconvenienceinitializerstoaclass,buttheycannotaddnewdesignatedinitializersordeinitializerstoaclass.Designatedinitializersanddeinitializersmustalwaysbeprovidedbytheoriginalclassimplementation.

Ifyouuseanextensiontoaddaninitializertoavaluetypethatprovidesdefaultvaluesforallofitsstoredpropertiesanddoesnotdefineanycustominitializers,youcancallthedefaultinitializerandmemberwiseinitializerforthatvaluetypefromwithinyourextension’sinitializer.Thiswouldn’tbethecaseifyouhadwrittentheinitializeraspartofthevaluetype’soriginalimplementation,asdescribedinInitializerDelegationforValueTypes.

Ifyouuseanextensiontoaddaninitializertoastructurethatwasdeclaredinanothermodule,thenewinitializercan’taccessselfuntilitcallsaninitializerfromthedefiningmodule.

TheexamplebelowdefinesacustomRectstructuretorepresentageometricrectangle.TheexamplealsodefinestwosupportingstructurescalledSizeandPoint,bothofwhichprovidedefaultvaluesof0.0foralloftheirproperties:

1 structSize{

2 varwidth=0.0,height=0.0

3 }

4 structPoint{

5 varx=0.0,y=0.0

6 }

7 structRect{

8 varorigin=Point()

9 varsize=Size()

10 }

BecausetheRectstructureprovidesdefaultvaluesforallofitsproperties,itreceivesadefaultinitializerandamemberwiseinitializerautomatically,asdescribedinDefaultInitializers.TheseinitializerscanbeusedtocreatenewRectinstances:

1 letdefaultRect=Rect()

2 letmemberwiseRect=Rect(origin:Point(x:2.0,y:2.0),

3 size:Size(width:5.0,height:5.0))

YoucanextendtheRectstructuretoprovideanadditionalinitializerthattakesaspecificcenterpointandsize:

1 extensionRect{

2 init(center:Point,size:Size){

3 letoriginX=center.x-(size.width/2)

4 letoriginY=center.y-(size.height/2)

5 self.init(origin:Point(x:originX,y:originY),size:

size)

6 }

7 }

Thisnewinitializerstartsbycalculatinganappropriateoriginpointbasedontheprovidedcenterpointandsizevalue.Theinitializerthencallsthestructure’sautomaticmemberwiseinitializerinit(origin:size:),whichstorestheneworiginandsizevaluesintheappropriateproperties:

1 letcenterRect=Rect(center:Point(x:4.0,y:4.0),

2 size:Size(width:3.0,height:3.0))

3 //centerRect'soriginis(2.5,2.5)anditssizeis(3.0,3.0)

NOTE

Ifyouprovideanewinitializerwithanextension,youarestillresponsibleformakingsurethateachinstanceisfullyinitializedoncetheinitializercompletes.

Methods

Extensionscanaddnewinstancemethodsandtypemethodstoexistingtypes.ThefollowingexampleaddsanewinstancemethodcalledrepetitionstotheInttype:

1 extensionInt{

2 funcrepetitions(task:()->Void){

3 for_in0..<self{

4 task()

5 }

6 }

7 }

Therepetitions(task:)methodtakesasingleargumentoftype()->Void,whichindicatesafunctionthathasnoparametersanddoesnotreturnavalue.

Afterdefiningthisextension,youcancalltherepetitions(task:)methodonanyintegertoperformataskthatmanynumberoftimes:

1 3.repetitions{

2 print("Hello!")

3 }

4 //Hello!

5 //Hello!

6 //Hello!

MutatingInstanceMethodsInstancemethodsaddedwithanextensioncanalsomodify(ormutate)theinstanceitself.Structureandenumerationmethodsthatmodifyselforitspropertiesmustmarktheinstancemethodasmutating,justlikemutatingmethodsfromanoriginalimplementation.

TheexamplebelowaddsanewmutatingmethodcalledsquaretoSwift’sInttype,whichsquarestheoriginalvalue:

1 extensionInt{

2 mutatingfuncsquare(){

3 self=self*self

4 }

5 }

6 varsomeInt=3

7 someInt.square()

8 //someIntisnow9

Subscripts

Extensionscanaddnewsubscriptstoanexistingtype.ThisexampleaddsanintegersubscripttoSwift’sbuilt-inInttype.Thissubscript[n]returnsthedecimaldigitnplacesinfromtherightofthenumber:

123456789[0]returns9

123456789[1]returns8

…andsoon:

1 extensionInt{

2 subscript(digitIndex:Int)->Int{

3 vardecimalBase=1

4 for_in0..<digitIndex{

5 decimalBase*=10

6 }

7 return(self/decimalBase)%10

8 }

9 }

10 746381295[0]

11 //returns5

12 746381295[1]

13 //returns9

14 746381295[2]

15 //returns2

16 746381295[8]

17 //returns7

IftheIntvaluedoesnothaveenoughdigitsfortherequestedindex,thesubscriptimplementationreturns0,asifthenumberhadbeenpaddedwithzerostotheleft:

1 746381295[9]

2 //returns0,asifyouhadrequested:

3 0746381295[9]

NestedTypes

Extensionscanaddnewnestedtypestoexistingclasses,structures,andenumerations:

1 extensionInt{

2 enumKind{

3 casenegative,zero,positive

4 }

5 varkind:Kind{

6 switchself{

7 case0:

8 return.zero

9 caseletxwherex>0:

10 return.positive

11 default:

12 return.negative

13 }

14 }

15 }

ThisexampleaddsanewnestedenumerationtoInt.Thisenumeration,calledKind,expressesthekindofnumberthataparticularintegerrepresents.Specifically,itexpresseswhetherthenumberisnegative,zero,orpositive.

ThisexamplealsoaddsanewcomputedinstancepropertytoInt,calledkind,whichreturnstheappropriateKindenumerationcaseforthatinteger.

ThenestedenumerationcannowbeusedwithanyIntvalue:

1 funcprintIntegerKinds(_numbers:[Int]){

2 fornumberinnumbers{

3 switchnumber.kind{

4 case.negative:

5 print("-",terminator:"")

6 case.zero:

7 print("0",terminator:"")

8 case.positive:

9 print("+",terminator:"")

10 }

11 }

12 print("")

13 }

14 printIntegerKinds([3,19,-27,0,-6,0,7])

15 //Prints"++-0-0+"

Thisfunction,printIntegerKinds(_:),takesaninputarrayofIntvaluesanditeratesoverthosevaluesinturn.Foreachintegerinthearray,thefunctionconsidersthekindcomputedpropertyforthatinteger,andprintsanappropriatedescription.

NOTE

number.kindisalreadyknowntobeoftypeInt.Kind.Becauseofthis,alloftheInt.Kindcasevaluescanbewritteninshorthandforminsidetheswitchstatement,suchas.negativeratherthanInt.Kind.negative.

Protocols

Aprotocoldefinesablueprintofmethods,properties,andotherrequirementsthatsuitaparticulartaskorpieceoffunctionality.Theprotocolcanthenbeadoptedbyaclass,structure,orenumerationtoprovideanactualimplementationofthoserequirements.Anytypethatsatisfiestherequirementsofaprotocolissaidtoconformtothatprotocol.

Inadditiontospecifyingrequirementsthatconformingtypesmustimplement,youcanextendaprotocoltoimplementsomeoftheserequirementsortoimplementadditionalfunctionalitythatconformingtypescantakeadvantageof.

ProtocolSyntax

Youdefineprotocolsinaverysimilarwaytoclasses,structures,andenumerations:

1 protocolSomeProtocol{

2 //protocoldefinitiongoeshere

3 }

Customtypesstatethattheyadoptaparticularprotocolbyplacingtheprotocol’snameafterthetype’sname,separatedbyacolon,aspartoftheirdefinition.Multipleprotocolscanbelisted,andareseparatedbycommas:

1 structSomeStructure:FirstProtocol,AnotherProtocol{

2 //structuredefinitiongoeshere

3 }

Ifaclasshasasuperclass,listthesuperclassnamebeforeanyprotocolsitadopts,followedbyacomma:

1 classSomeClass:SomeSuperclass,FirstProtocol,AnotherProtocol

{

2 //classdefinitiongoeshere

3 }

PropertyRequirements

Aprotocolcanrequireanyconformingtypetoprovideaninstancepropertyortypepropertywithaparticularnameandtype.Theprotocoldoesn’tspecifywhetherthepropertyshouldbeastoredpropertyoracomputedproperty—itonlyspecifiestherequiredpropertynameandtype.Theprotocolalsospecifieswhethereachpropertymustbegettableorgettableandsettable.

Ifaprotocolrequiresapropertytobegettableandsettable,thatpropertyrequirementcan’tbefulfilledbyaconstantstoredpropertyoraread-onlycomputedproperty.Iftheprotocolonlyrequiresapropertytobegettable,therequirementcanbesatisfiedbyanykindofproperty,andit’svalidforthepropertytobealsosettableifthisisusefulforyourowncode.

Propertyrequirementsarealwaysdeclaredasvariableproperties,prefixedwiththevarkeyword.Gettableandsettablepropertiesareindicatedbywriting{getset}aftertheirtypedeclaration,andgettablepropertiesareindicatedbywriting{get}.

1 protocolSomeProtocol{

2 varmustBeSettable:Int{getset}

3 vardoesNotNeedToBeSettable:Int{get}

4 }

Alwaysprefixtypepropertyrequirementswiththestatickeywordwhenyoudefinetheminaprotocol.Thisrulepertainseventhoughtypepropertyrequirementscanbeprefixedwiththeclassorstatickeywordwhenimplementedbyaclass:

1 protocolAnotherProtocol{

2 staticvarsomeTypeProperty:Int{getset}

3 }

Here’sanexampleofaprotocolwithasingleinstancepropertyrequirement:

1 protocolFullyNamed{

2 varfullName:String{get}

3 }

TheFullyNamedprotocolrequiresaconformingtypetoprovideafullyqualifiedname.Theprotocoldoesn’tspecifyanythingelseaboutthenatureoftheconformingtype—itonlyspecifiesthatthetypemustbeabletoprovideafullnameforitself.TheprotocolstatesthatanyFullyNamedtypemusthaveagettableinstancepropertycalledfullName,whichisoftypeString.

Here’sanexampleofasimplestructurethatadoptsandconformstotheFullyNamedprotocol:

1 structPerson:FullyNamed{

2 varfullName:String

3 }

4 letjohn=Person(fullName:"JohnAppleseed")

5 //john.fullNameis"JohnAppleseed"

ThisexampledefinesastructurecalledPerson,whichrepresentsaspecificnamedperson.ItstatesthatitadoptstheFullyNamedprotocolaspartofthefirstlineofitsdefinition.

EachinstanceofPersonhasasinglestoredpropertycalledfullName,whichisoftypeString.ThismatchesthesinglerequirementoftheFullyNamedprotocol,andmeansthatPersonhascorrectlyconformedtotheprotocol.(Swiftreportsanerroratcompile-timeifaprotocolrequirementisnotfulfilled.)

Here’samorecomplexclass,whichalsoadoptsandconformstotheFullyNamed

protocol:

1 classStarship:FullyNamed{

2 varprefix:String?

3 varname:String

4 init(name:String,prefix:String?=nil){

5 self.name=name

6 self.prefix=prefix

7 }

8 varfullName:String{

9 return(prefix!=nil?prefix!+"":"")+name

10 }

11 }

12 varncc1701=Starship(name:"Enterprise",prefix:"USS")

13 //ncc1701.fullNameis"USSEnterprise"

ThisclassimplementsthefullNamepropertyrequirementasacomputedread-onlypropertyforastarship.EachStarshipclassinstancestoresamandatorynameandanoptionalprefix.ThefullNamepropertyusestheprefixvalueifitexists,andprependsittothebeginningofnametocreateafullnameforthestarship.

MethodRequirements

Protocolscanrequirespecificinstancemethodsandtypemethodstobeimplementedbyconformingtypes.Thesemethodsarewrittenaspartoftheprotocol’sdefinitioninexactlythesamewayasfornormalinstanceandtypemethods,butwithoutcurlybracesoramethodbody.Variadicparametersareallowed,subjecttothesamerulesasfornormalmethods.Defaultvalues,however,can’tbespecifiedformethodparameterswithinaprotocol’sdefinition.

Aswithtypepropertyrequirements,youalwaysprefixtypemethod

requirementswiththestatickeywordwhenthey’redefinedinaprotocol.Thisistrueeventhoughtypemethodrequirementsareprefixedwiththeclassorstatickeywordwhenimplementedbyaclass:

1 protocolSomeProtocol{

2 staticfuncsomeTypeMethod()

3 }

Thefollowingexampledefinesaprotocolwithasingleinstancemethodrequirement:

1 protocolRandomNumberGenerator{

2 funcrandom()->Double

3 }

Thisprotocol,RandomNumberGenerator,requiresanyconformingtypetohaveaninstancemethodcalledrandom,whichreturnsaDoublevaluewheneverit’scalled.Althoughit’snotspecifiedaspartoftheprotocol,it’sassumedthatthisvaluewillbeanumberfrom0.0upto(butnotincluding)1.0.

TheRandomNumberGeneratorprotocoldoesn’tmakeanyassumptionsabouthoweachrandomnumberwillbegenerated—itsimplyrequiresthegeneratortoprovideastandardwaytogenerateanewrandomnumber.

Here’sanimplementationofaclassthatadoptsandconformstotheRandomNumberGeneratorprotocol.Thisclassimplementsapseudorandomnumbergeneratoralgorithmknownasalinearcongruentialgenerator:

1 classLinearCongruentialGenerator:RandomNumberGenerator{

2 varlastRandom=42.0

3 letm=139968.0

4 leta=3877.0

5 letc=29573.0

6 funcrandom()->Double{

7 lastRandom=((lastRandom*a+c)

8 .truncatingRemainder(dividingBy:m))

9 returnlastRandom/m

10 }

11 }

12 letgenerator=LinearCongruentialGenerator()

13 print("Here'sarandomnumber:\(generator.random())")

14 //Prints"Here'sarandomnumber:0.3746499199817101"

15 print("Andanotherone:\(generator.random())")

16 //Prints"Andanotherone:0.729023776863283"

MutatingMethodRequirements

It’ssometimesnecessaryforamethodtomodify(ormutate)theinstanceitbelongsto.Forinstancemethodsonvaluetypes(thatis,structuresandenumerations)youplacethemutatingkeywordbeforeamethod’sfunckeywordtoindicatethatthemethodisallowedtomodifytheinstanceitbelongstoandanypropertiesofthatinstance.ThisprocessisdescribedinModifyingValueTypesfromWithinInstanceMethods.

Ifyoudefineaprotocolinstancemethodrequirementthatisintendedtomutateinstancesofanytypethatadoptstheprotocol,markthemethodwiththemutatingkeywordaspartoftheprotocol’sdefinition.Thisenablesstructuresandenumerationstoadopttheprotocolandsatisfythatmethodrequirement.

NOTE

Ifyoumarkaprotocolinstancemethodrequirementasmutating,youdon’tneedtowritethemutatingkeywordwhenwritinganimplementationofthatmethodforaclass.Themutatingkeywordisonlyusedbystructuresandenumerations.

TheexamplebelowdefinesaprotocolcalledTogglable,whichdefinesasingleinstancemethodrequirementcalledtoggle.Asitsnamesuggests,thetoggle()

methodisintendedtotoggleorinvertthestateofanyconformingtype,typicallybymodifyingapropertyofthattype.

Thetoggle()methodismarkedwiththemutatingkeywordaspartoftheTogglableprotocoldefinition,toindicatethatthemethodisexpectedtomutatethestateofaconforminginstancewhenit’scalled:

1 protocolTogglable{

2 mutatingfunctoggle()

3 }

IfyouimplementtheTogglableprotocolforastructureorenumeration,thatstructureorenumerationcanconformtotheprotocolbyprovidinganimplementationofthetoggle()methodthatisalsomarkedasmutating.

TheexamplebelowdefinesanenumerationcalledOnOffSwitch.Thisenumerationtogglesbetweentwostates,indicatedbytheenumerationcasesonandoff.Theenumeration’stoggleimplementationismarkedasmutating,tomatchtheTogglableprotocol’srequirements:

1 enumOnOffSwitch:Togglable{

2 caseoff,on

3 mutatingfunctoggle(){

4 switchself{

5 case.off:

6 self=.on

7 case.on:

8 self=.off

9 }

10 }

11 }

12 varlightSwitch=OnOffSwitch.off

13 lightSwitch.toggle()

14 //lightSwitchisnowequalto.on

InitializerRequirements

Protocolscanrequirespecificinitializerstobeimplementedbyconformingtypes.Youwritetheseinitializersaspartoftheprotocol’sdefinitioninexactlythesamewayasfornormalinitializers,butwithoutcurlybracesoraninitializerbody:

1 protocolSomeProtocol{

2 init(someParameter:Int)

3 }

ClassImplementationsofProtocolInitializerRequirementsYoucanimplementaprotocolinitializerrequirementonaconformingclassaseitheradesignatedinitializeroraconvenienceinitializer.Inbothcases,youmustmarktheinitializerimplementationwiththerequiredmodifier:

1 classSomeClass:SomeProtocol{

2 requiredinit(someParameter:Int){

3 //initializerimplementationgoeshere

4 }

5 }

Theuseoftherequiredmodifierensuresthatyouprovideanexplicitorinheritedimplementationoftheinitializerrequirementonallsubclassesoftheconformingclass,suchthattheyalsoconformtotheprotocol.

Formoreinformationonrequiredinitializers,seeRequiredInitializers.

NOTE

Youdon’tneedtomarkprotocolinitializerimplementationswiththerequiredmodifieronclassesthataremarkedwiththefinalmodifier,becausefinalclassescan’tsubclassed.Formoreaboutthefinalmodifier,seePreventingOverrides.

Ifasubclassoverridesadesignatedinitializerfromasuperclass,andalsoimplementsamatchinginitializerrequirementfromaprotocol,marktheinitializerimplementationwithboththerequiredandoverridemodifiers:

1 protocolSomeProtocol{

2 init()

3 }

4

5 classSomeSuperClass{

6 init(){

7 //initializerimplementationgoeshere

8 }

9 }

10

11 classSomeSubClass:SomeSuperClass,SomeProtocol{

12 //"required"fromSomeProtocolconformance;"override"

fromSomeSuperClass

13 requiredoverrideinit(){

14 //initializerimplementationgoeshere

15 }

16 }

FailableInitializerRequirementsProtocolscandefinefailableinitializerrequirementsforconformingtypes,asdefinedinFailableInitializers.

Afailableinitializerrequirementcanbesatisfiedbyafailableornonfailable

initializeronaconformingtype.Anonfailableinitializerrequirementcanbesatisfiedbyanonfailableinitializeroranimplicitlyunwrappedfailableinitializer.

ProtocolsasTypes

Protocolsdon’tactuallyimplementanyfunctionalitythemselves.Nonetheless,youcanuseprotocolsasafullyfledgedtypesinyourcode.Usingaprotocolasatypeissometimescalledanexistentialtype,whichcomesfromthephrase“thereexistsatypeTsuchthatTconformstotheprotocol”.

Youcanuseaprotocolinmanyplaceswhereothertypesareallowed,including:

Asaparametertypeorreturntypeinafunction,method,orinitializer

Asthetypeofaconstant,variable,orproperty

Asthetypeofitemsinanarray,dictionary,orothercontainer

NOTE

Becauseprotocolsaretypes,begintheirnameswithacapitalletter(suchasFullyNamedandRandomNumberGenerator)tomatchthenamesofothertypesinSwift(suchasInt,String,andDouble).

Here’sanexampleofaprotocolusedasatype:

1 classDice{

2 letsides:Int

3 letgenerator:RandomNumberGenerator

4 init(sides:Int,generator:RandomNumberGenerator){

5 self.sides=sides

6 self.generator=generator

7 }

8 funcroll()->Int{

9 returnInt(generator.random()*Double(sides))+1

10 }

11 }

ThisexampledefinesanewclasscalledDice,whichrepresentsann-sideddiceforuseinaboardgame.Diceinstanceshaveanintegerpropertycalledsides,whichrepresentshowmanysidestheyhave,andapropertycalledgenerator,whichprovidesarandomnumbergeneratorfromwhichtocreatedicerollvalues.

ThegeneratorpropertyisoftypeRandomNumberGenerator.Therefore,youcansetittoaninstanceofanytypethatadoptstheRandomNumberGeneratorprotocol.Nothingelseisrequiredoftheinstanceyouassigntothisproperty,exceptthattheinstancemustadopttheRandomNumberGeneratorprotocol.BecauseitstypeisRandomNumberGenerator,codeinsidetheDiceclasscanonlyinteractwithgeneratorinwaysthatapplytoallgeneratorsthatconformtothisprotocol.Thatmeansitcan’tuseanymethodsorpropertiesthataredefinedbytheunderlyingtypeofthegenerator.However,youcandowncastfromaprotocoltypetoanunderlyingtypeinthesamewayyoucandowncastfromasuperclasstoasubclass,asdiscussedinDowncasting.

Dicealsohasaninitializer,tosetupitsinitialstate.Thisinitializerhasaparametercalledgenerator,whichisalsooftypeRandomNumberGenerator.YoucanpassavalueofanyconformingtypeintothisparameterwheninitializinganewDiceinstance.

Diceprovidesoneinstancemethod,roll,whichreturnsanintegervaluebetween1andthenumberofsidesonthedice.Thismethodcallsthegenerator’srandom()methodtocreateanewrandomnumberbetween0.0and1.0,andusesthisrandomnumbertocreateadicerollvaluewithinthecorrectrange.BecausegeneratorisknowntoadoptRandomNumberGenerator,it’sguaranteedtohavearandom()methodtocall.

Here’showtheDiceclasscanbeusedtocreateasix-sideddicewithaLinearCongruentialGeneratorinstanceasitsrandomnumbergenerator:

1 vard6=Dice(sides:6,generator:

LinearCongruentialGenerator())

2 for_in1...5{

3 print("Randomdicerollis\(d6.roll())")

4 }

5 //Randomdicerollis3

6 //Randomdicerollis5

7 //Randomdicerollis4

8 //Randomdicerollis5

9 //Randomdicerollis4

Delegation

Delegationisadesignpatternthatenablesaclassorstructuretohandoff(ordelegate)someofitsresponsibilitiestoaninstanceofanothertype.Thisdesignpatternisimplementedbydefiningaprotocolthatencapsulatesthedelegatedresponsibilities,suchthataconformingtype(knownasadelegate)isguaranteedtoprovidethefunctionalitythathasbeendelegated.Delegationcanbeusedtorespondtoaparticularaction,ortoretrievedatafromanexternalsourcewithoutneedingtoknowtheunderlyingtypeofthatsource.

Theexamplebelowdefinestwoprotocolsforusewithdice-basedboardgames:

1 protocolDiceGame{

2 vardice:Dice{get}

3 funcplay()

4 }

5 protocolDiceGameDelegate:AnyObject{

6 funcgameDidStart(_game:DiceGame)

7 funcgame(_game:DiceGame,didStartNewTurnWithDiceRoll

diceRoll:Int)

8 funcgameDidEnd(_game:DiceGame)

9 }

TheDiceGameprotocolisaprotocolthatcanbeadoptedbyanygamethatinvolvesdice.

TheDiceGameDelegateprotocolcanbeadoptedtotracktheprogressofaDiceGame.Topreventstrongreferencecycles,delegatesaredeclaredasweakreferences.Forinformationaboutweakreferences,seeStrongReferenceCyclesBetweenClassInstances.Markingtheprotocolasclass-onlyletstheSnakesAndLaddersclasslaterinthischapterdeclarethatitsdelegatemustuseaweakreference.Aclass-onlyprotocolismarkedbyitsinheritancefromAnyObject,asdiscussedinClass-OnlyProtocols.

Here’saversionoftheSnakesandLaddersgameoriginallyintroducedinControlFlow.ThisversionisadaptedtouseaDiceinstanceforitsdice-rolls;toadopttheDiceGameprotocol;andtonotifyaDiceGameDelegateaboutitsprogress:

1 classSnakesAndLadders:DiceGame{

2 letfinalSquare=25

3 letdice=Dice(sides:6,generator:

LinearCongruentialGenerator())

4 varsquare=0

5 varboard:[Int]

6 init(){

7 board=Array(repeating:0,count:finalSquare+1)

8 board[03]=+08;board[06]=+11;board[09]=+09;

board[10]=+02

9 board[14]=-10;board[19]=-11;board[22]=-02;

board[24]=-08

10 }

11 weakvardelegate:DiceGameDelegate?

12 funcplay(){

13 square=0

14 delegate?.gameDidStart(self)

15 gameLoop:whilesquare!=finalSquare{

16 letdiceRoll=dice.roll()

17 delegate?.game(self,didStartNewTurnWithDiceRoll:

diceRoll)

18 switchsquare+diceRoll{

19 casefinalSquare:

20 breakgameLoop

21 caseletnewSquarewherenewSquare>finalSquare:

22 continuegameLoop

23 default:

24 square+=diceRoll

25 square+=board[square]

26 }

27 }

28 delegate?.gameDidEnd(self)

29 }

30 }

ForadescriptionoftheSnakesandLaddersgameplay,seeBreak.

ThisversionofthegameiswrappedupasaclasscalledSnakesAndLadders,whichadoptstheDiceGameprotocol.Itprovidesagettabledicepropertyandaplay()methodinordertoconformtotheprotocol.(Thedicepropertyisdeclaredasaconstantpropertybecauseitdoesn’tneedtochangeafterinitialization,andtheprotocolonlyrequiresthatitmustbegettable.)

TheSnakesandLaddersgameboardsetuptakesplacewithintheclass’sinit()initializer.Allgamelogicismovedintotheprotocol’splaymethod,whichusestheprotocol’srequireddicepropertytoprovideitsdicerollvalues.

NotethatthedelegatepropertyisdefinedasanoptionalDiceGameDelegate,becauseadelegateisn’trequiredinordertoplaythegame.Becauseit’sofanoptionaltype,thedelegatepropertyisautomaticallysettoaninitialvalueofnil.Thereafter,thegameinstantiatorhastheoptiontosetthepropertytoasuitabledelegate.BecausetheDiceGameDelegateprotocolisclass-only,youcandeclarethedelegatetobeweaktopreventreferencecycles.

DiceGameDelegateprovidesthreemethodsfortrackingtheprogressofagame.Thesethreemethodshavebeenincorporatedintothegamelogicwithintheplay()methodabove,andarecalledwhenanewgamestarts,anewturnbegins,orthegameends.

BecausethedelegatepropertyisanoptionalDiceGameDelegate,theplay()methodusesoptionalchainingeachtimeitcallsamethodonthedelegate.Ifthedelegatepropertyisnil,thesedelegatecallsfailgracefullyandwithouterror.Ifthedelegatepropertyisnon-nil,thedelegatemethodsarecalled,andarepassedtheSnakesAndLaddersinstanceasaparameter.

ThisnextexampleshowsaclasscalledDiceGameTracker,whichadoptstheDiceGameDelegateprotocol:

1 classDiceGameTracker:DiceGameDelegate{

2 varnumberOfTurns=0

3 funcgameDidStart(_game:DiceGame){

4 numberOfTurns=0

5 ifgameisSnakesAndLadders{

6 print("StartedanewgameofSnakesandLadders")

7 }

8 print("Thegameisusinga\(game.dice.sides)-sided

dice")

9 }

10 funcgame(_game:DiceGame,didStartNewTurnWithDiceRoll

diceRoll:Int){

11 numberOfTurns+=1

12 print("Rolleda\(diceRoll)")

13 }

14 funcgameDidEnd(_game:DiceGame){

15 print("Thegamelastedfor\(numberOfTurns)turns")

16 }

17 }

DiceGameTrackerimplementsallthreemethodsrequiredbyDiceGameDelegate.Itusesthesemethodstokeeptrackofthenumberofturnsagamehastaken.ItresetsanumberOfTurnspropertytozerowhenthegamestarts,incrementsiteachtimeanewturnbegins,andprintsoutthetotalnumberofturnsoncethegamehasended.

TheimplementationofgameDidStart(_:)shownaboveusesthegameparametertoprintsomeintroductoryinformationaboutthegamethatisabouttobeplayed.ThegameparameterhasatypeofDiceGame,notSnakesAndLadders,andsogameDidStart(_:)canaccessanduseonlymethodsandpropertiesthatareimplementedaspartoftheDiceGameprotocol.However,themethodisstillabletousetypecastingtoquerythetypeoftheunderlyinginstance.Inthisexample,itcheckswhethergameisactuallyaninstanceofSnakesAndLaddersbehindthescenes,andprintsanappropriatemessageifso.

ThegameDidStart(_:)methodalsoaccessesthedicepropertyofthepassedgameparameter.BecausegameisknowntoconformtotheDiceGameprotocol,it’sguaranteedtohaveadiceproperty,andsothegameDidStart(_:)methodisabletoaccessandprintthedice’ssidesproperty,regardlessofwhatkindofgameisbeingplayed.

Here’showDiceGameTrackerlooksinaction:

1 lettracker=DiceGameTracker()

2 letgame=SnakesAndLadders()

3 game.delegate=tracker

4 game.play()

5 //StartedanewgameofSnakesandLadders

6 //Thegameisusinga6-sideddice

7 //Rolleda3

8 //Rolleda5

9 //Rolleda4

10 //Rolleda5

11 //Thegamelastedfor4turns

AddingProtocolConformancewithanExtension

Youcanextendanexistingtypetoadoptandconformtoanewprotocol,evenifyoudon’thaveaccesstothesourcecodefortheexistingtype.Extensionscanaddnewproperties,methods,andsubscriptstoanexistingtype,andarethereforeabletoaddanyrequirementsthataprotocolmaydemand.Formoreaboutextensions,seeExtensions.

NOTE

Existinginstancesofatypeautomaticallyadoptandconformtoaprotocolwhenthatconformanceisaddedtotheinstance’stypeinanextension.

Forexample,thisprotocol,calledTextRepresentable,canbeimplementedbyanytypethathasawaytoberepresentedastext.Thismightbeadescriptionofitself,oratextversionofitscurrentstate:

1 protocolTextRepresentable{

2 vartextualDescription:String{get}

3 }

TheDiceclassfromabovecanbeextendedtoadoptandconformtoTextRepresentable:

1 extensionDice:TextRepresentable{

2 vartextualDescription:String{

3 return"A\(sides)-sideddice"

4 }

5 }

ThisextensionadoptsthenewprotocolinexactlythesamewayasifDicehadprovideditinitsoriginalimplementation.Theprotocolnameisprovidedafterthetypename,separatedbyacolon,andanimplementationofallrequirementsoftheprotocolisprovidedwithintheextension’scurlybraces.

AnyDiceinstancecannowbetreatedasTextRepresentable:

1 letd12=Dice(sides:12,generator:

LinearCongruentialGenerator())

2 print(d12.textualDescription)

3 //Prints"A12-sideddice"

Similarly,theSnakesAndLaddersgameclasscanbeextendedtoadoptandconformtotheTextRepresentableprotocol:

1 extensionSnakesAndLadders:TextRepresentable{

2 vartextualDescription:String{

3 return"AgameofSnakesandLadderswith\

(finalSquare)squares"

4 }

5 }

6 print(game.textualDescription)

7 //Prints"AgameofSnakesandLadderswith25squares"

ConditionallyConformingtoaProtocolAgenerictypemaybeabletosatisfytherequirementsofaprotocolonlyundercertainconditions,suchaswhenthetype’sgenericparameterconformstotheprotocol.Youcanmakeagenerictypeconditionallyconformtoaprotocolby

listingconstraintswhenextendingthetype.Writetheseconstraintsafterthenameoftheprotocolyou’readoptingbywritingagenericwhereclause.Formoreaboutgenericwhereclauses,seeGenericWhereClauses.

ThefollowingextensionmakesArrayinstancesconformtotheTextRepresentableprotocolwhenevertheystoreelementsofatypethatconformstoTextRepresentable.

1 extensionArray:TextRepresentablewhereElement:

TextRepresentable{

2 vartextualDescription:String{

3 letitemsAsText=self.map{$0.textualDescription}

4 return"["+itemsAsText.joined(separator:",")+"]"

5 }

6 }

7 letmyDice=[d6,d12]

8 print(myDice.textualDescription)

9 //Prints"[A6-sideddice,A12-sideddice]"

DeclaringProtocolAdoptionwithanExtensionIfatypealreadyconformstoalloftherequirementsofaprotocol,buthasnotyetstatedthatitadoptsthatprotocol,youcanmakeitadopttheprotocolwithanemptyextension:

1 structHamster{

2 varname:String

3 vartextualDescription:String{

4 return"Ahamsternamed\(name)"

5 }

6 }

7 extensionHamster:TextRepresentable{}

InstancesofHamstercannowbeusedwhereverTextRepresentableistherequiredtype:

1 letsimonTheHamster=Hamster(name:"Simon")

2 letsomethingTextRepresentable:TextRepresentable=

simonTheHamster

3 print(somethingTextRepresentable.textualDescription)

4 //Prints"AhamsternamedSimon"

NOTE

Typesdon’tautomaticallyadoptaprotocoljustbysatisfyingitsrequirements.Theymustalwaysexplicitlydeclaretheiradoptionoftheprotocol.

CollectionsofProtocolTypes

Aprotocolcanbeusedasthetypetobestoredinacollectionsuchasanarrayoradictionary,asmentionedinProtocolsasTypes.ThisexamplecreatesanarrayofTextRepresentablethings:

letthings:[TextRepresentable]=[game,d12,simonTheHamster]

It’snowpossibletoiterateovertheitemsinthearray,andprinteachitem’stextualdescription:

1 forthinginthings{

2 print(thing.textualDescription)

3 }

4 //AgameofSnakesandLadderswith25squares

5 //A12-sideddice

6 //AhamsternamedSimon

NotethatthethingconstantisoftypeTextRepresentable.It’snotoftypeDice,orDiceGame,orHamster,eveniftheactualinstancebehindthescenesisofoneofthosetypes.Nonetheless,becauseit’softypeTextRepresentable,andanythingthatisTextRepresentableisknowntohaveatextualDescriptionproperty,it’ssafetoaccessthing.textualDescriptioneachtimethroughtheloop.

ProtocolInheritance

Aprotocolcaninheritoneormoreotherprotocolsandcanaddfurtherrequirementsontopoftherequirementsitinherits.Thesyntaxforprotocolinheritanceissimilartothesyntaxforclassinheritance,butwiththeoptiontolistmultipleinheritedprotocols,separatedbycommas:

1 protocolInheritingProtocol:SomeProtocol,AnotherProtocol{

2 //protocoldefinitiongoeshere

3 }

Here’sanexampleofaprotocolthatinheritstheTextRepresentableprotocolfromabove:

1 protocolPrettyTextRepresentable:TextRepresentable{

2 varprettyTextualDescription:String{get}

3 }

Thisexampledefinesanewprotocol,PrettyTextRepresentable,whichinheritsfromTextRepresentable.AnythingthatadoptsPrettyTextRepresentablemustsatisfyalloftherequirementsenforcedbyTextRepresentable,plustheadditionalrequirementsenforcedbyPrettyTextRepresentable.Inthisexample,PrettyTextRepresentableaddsasinglerequirementtoprovideagettablepropertycalledprettyTextualDescriptionthatreturnsaString.

TheSnakesAndLaddersclasscanbeextendedtoadoptandconformtoPrettyTextRepresentable:

1 extensionSnakesAndLadders:PrettyTextRepresentable{

2 varprettyTextualDescription:String{

3 varoutput=textualDescription+":\n"

4 forindexin1...finalSquare{

5 switchboard[index]{

6 caseletladderwhereladder>0:

7 output+="▲"

8 caseletsnakewheresnake<0:

9 output+="▼"

10 default:

11 output+="○"

12 }

13 }

14 returnoutput

15 }

16 }

ThisextensionstatesthatitadoptsthePrettyTextRepresentableprotocolandprovidesanimplementationoftheprettyTextualDescriptionpropertyfortheSnakesAndLadderstype.AnythingthatisPrettyTextRepresentablemustalsobeTextRepresentable,andsotheimplementationofprettyTextualDescriptionstartsbyaccessingthetextualDescriptionpropertyfromtheTextRepresentableprotocoltobeginanoutputstring.Itappendsacolonandalinebreak,andusesthisasthestartofitsprettytextrepresentation.Ittheniteratesthroughthearrayofboardsquares,andappendsageometricshapetorepresentthecontentsofeachsquare:

Ifthesquare’svalueisgreaterthan0,it’sthebaseofaladder,andisrepresentedby▲.

Ifthesquare’svalueislessthan0,it’stheheadofasnake,andisrepresentedby▼.

Otherwise,thesquare’svalueis0,andit’sa“free”square,representedby○.

TheprettyTextualDescriptionpropertycannowbeusedtoprintaprettytextdescriptionofanySnakesAndLaddersinstance:

1 print(game.prettyTextualDescription)

2 //AgameofSnakesandLadderswith25squares:

3 //○○▲○○▲○○▲▲○○○▼○○○○▼○○▼○▼○

Class-OnlyProtocols

Youcanlimitprotocoladoptiontoclasstypes(andnotstructuresorenumerations)byaddingtheAnyObjectprotocoltoaprotocol’sinheritancelist.

1 protocolSomeClassOnlyProtocol:AnyObject,

SomeInheritedProtocol{

2 //class-onlyprotocoldefinitiongoeshere

3 }

Intheexampleabove,SomeClassOnlyProtocolcanonlybeadoptedbyclasstypes.It’sacompile-timeerrortowriteastructureorenumerationdefinitionthattriestoadoptSomeClassOnlyProtocol.

NOTE

Useaclass-onlyprotocolwhenthebehaviordefinedbythatprotocol’srequirementsassumesorrequiresthataconformingtypehasreferencesemanticsratherthanvaluesemantics.Formoreaboutreferenceandvaluesemantics,seeStructuresandEnumerationsAreValueTypesandClassesAreReferenceTypes.

ProtocolComposition

Itcanbeusefultorequireatypetoconformtomultipleprotocolsatthesametime.Youcancombinemultipleprotocolsintoasinglerequirementwithaprotocolcomposition.Protocolcompositionsbehaveasifyoudefinedatemporarylocalprotocolthathasthecombinedrequirementsofallprotocolsinthecomposition.Protocolcompositionsdon’tdefineanynewprotocoltypes.

ProtocolcompositionshavetheformSomeProtocol&AnotherProtocol.Youcanlistasmanyprotocolsasyouneed,separatingthemwithampersands(&).Inadditiontoitslistofprotocols,aprotocolcompositioncanalsocontainoneclasstype,whichyoucanusetospecifyarequiredsuperclass.

Here’sanexamplethatcombinestwoprotocolscalledNamedandAgedintoasingleprotocolcompositionrequirementonafunctionparameter:

1 protocolNamed{

2 varname:String{get}

3 }

4 protocolAged{

5 varage:Int{get}

6 }

7 structPerson:Named,Aged{

8 varname:String

9 varage:Int

10 }

11 funcwishHappyBirthday(tocelebrator:Named&Aged){

12 print("Happybirthday,\(celebrator.name),you're\

(celebrator.age)!")

13 }

14 letbirthdayPerson=Person(name:"Malcolm",age:21)

15 wishHappyBirthday(to:birthdayPerson)

16 //Prints"Happybirthday,Malcolm,you're21!"

Inthisexample,theNamedprotocolhasasinglerequirementforagettableString

propertycalledname.TheAgedprotocolhasasinglerequirementforagettableIntpropertycalledage.BothprotocolsareadoptedbyastructurecalledPerson.

TheexamplealsodefinesawishHappyBirthday(to:)function.ThetypeofthecelebratorparameterisNamed&Aged,whichmeans“anytypethatconformstoboththeNamedandAgedprotocols.”Itdoesn’tmatterwhichspecifictypeispassedtothefunction,aslongasitconformstobothoftherequiredprotocols.

TheexamplethencreatesanewPersoninstancecalledbirthdayPersonandpassesthisnewinstancetothewishHappyBirthday(to:)function.BecausePersonconformstobothprotocols,thiscallisvalid,andthewishHappyBirthday(to:)functioncanprintitsbirthdaygreeting.

Here’sanexamplethatcombinestheNamedprotocolfromthepreviousexamplewithaLocationclass:

1 classLocation{

2 varlatitude:Double

3 varlongitude:Double

4 init(latitude:Double,longitude:Double){

5 self.latitude=latitude

6 self.longitude=longitude

7 }

8 }

9 classCity:Location,Named{

10 varname:String

11 init(name:String,latitude:Double,longitude:Double){

12 self.name=name

13 super.init(latitude:latitude,longitude:longitude)

14 }

15 }

16 funcbeginConcert(inlocation:Location&Named){

17 print("Hello,\(location.name)!")

18 }

19

20 letseattle=City(name:"Seattle",latitude:47.6,longitude:

-122.3)

21 beginConcert(in:seattle)

22 //Prints"Hello,Seattle!"

ThebeginConcert(in:)functiontakesaparameteroftypeLocation&Named,whichmeans“anytypethat’sasubclassofLocationandthatconformstotheNamedprotocol.”Inthiscase,Citysatisfiesbothrequirements.

PassingbirthdayPersontothebeginConcert(in:)functionisinvalidbecausePersonisn’tasubclassofLocation.Likewise,ifyoumadeasubclassofLocationthatdidn’tconformtotheNamedprotocol,callingbeginConcert(in:)withaninstanceofthattypeisalsoinvalid.

CheckingforProtocolConformance

YoucanusetheisandasoperatorsdescribedinTypeCastingtocheckforprotocolconformance,andtocasttoaspecificprotocol.Checkingforandcastingtoaprotocolfollowsexactlythesamesyntaxascheckingforandcastingtoatype:

Theisoperatorreturnstrueifaninstanceconformstoaprotocolandreturnsfalseifitdoesn’t.

Theas?versionofthedowncastoperatorreturnsanoptionalvalueoftheprotocol’stype,andthisvalueisniliftheinstancedoesn’tconformtothatprotocol.

Theas!versionofthedowncastoperatorforcesthedowncasttotheprotocoltypeandtriggersaruntimeerrorifthedowncastdoesn’tsucceed.

ThisexampledefinesaprotocolcalledHasArea,withasingleproperty

requirementofagettableDoublepropertycalledarea:

1 protocolHasArea{

2 vararea:Double{get}

3 }

Herearetwoclasses,CircleandCountry,bothofwhichconformtotheHasAreaprotocol:

1 classCircle:HasArea{

2 letpi=3.1415927

3 varradius:Double

4 vararea:Double{returnpi*radius*radius}

5 init(radius:Double){self.radius=radius}

6 }

7 classCountry:HasArea{

8 vararea:Double

9 init(area:Double){self.area=area}

10 }

TheCircleclassimplementstheareapropertyrequirementasacomputedproperty,basedonastoredradiusproperty.TheCountryclassimplementsthearearequirementdirectlyasastoredproperty.BothclassescorrectlyconformtotheHasAreaprotocol.

Here’saclasscalledAnimal,whichdoesn’tconformtotheHasAreaprotocol:

1 classAnimal{

2 varlegs:Int

3 init(legs:Int){self.legs=legs}

4 }

TheCircle,CountryandAnimalclassesdon’thaveasharedbaseclass.

Nonetheless,they’reallclasses,andsoinstancesofallthreetypescanbeusedtoinitializeanarraythatstoresvaluesoftypeAnyObject:

1 letobjects:[AnyObject]=[

2 Circle(radius:2.0),

3 Country(area:243_610),

4 Animal(legs:4)

5 ]

TheobjectsarrayisinitializedwithanarrayliteralcontainingaCircleinstancewitharadiusof2units;aCountryinstanceinitializedwiththesurfaceareaoftheUnitedKingdominsquarekilometers;andanAnimalinstancewithfourlegs.

Theobjectsarraycannowbeiterated,andeachobjectinthearraycanbecheckedtoseeifitconformstotheHasAreaprotocol:

1 forobjectinobjects{

2 ifletobjectWithArea=objectas?HasArea{

3 print("Areais\(objectWithArea.area)")

4 }else{

5 print("Somethingthatdoesn'thaveanarea")

6 }

7 }

8 //Areais12.5663708

9 //Areais243610.0

10 //Somethingthatdoesn'thaveanarea

WheneveranobjectinthearrayconformstotheHasAreaprotocol,theoptionalvaluereturnedbytheas?operatorisunwrappedwithoptionalbindingintoaconstantcalledobjectWithArea.TheobjectWithAreaconstantisknowntobeoftypeHasArea,andsoitsareapropertycanbeaccessedandprintedinatype-safeway.

Notethattheunderlyingobjectsaren’tchangedbythecastingprocess.They

continuetobeaCircle,aCountryandanAnimal.However,atthepointthatthey’restoredintheobjectWithAreaconstant,they’reonlyknowntobeoftypeHasArea,andsoonlytheirareapropertycanbeaccessed.

OptionalProtocolRequirements

Youcandefineoptionalrequirementsforprotocols.Theserequirementsdon’thavetobeimplementedbytypesthatconformtotheprotocol.Optionalrequirementsareprefixedbytheoptionalmodifieraspartoftheprotocol’sdefinition.OptionalrequirementsareavailablesothatyoucanwritecodethatinteroperateswithObjective-C.Boththeprotocolandtheoptionalrequirementmustbemarkedwiththe@objcattribute.Notethat@objcprotocolscanbeadoptedonlybyclassesthatinheritfromObjective-Cclassesorother@objcclasses.Theycan’tbeadoptedbystructuresorenumerations.

Whenyouuseamethodorpropertyinanoptionalrequirement,itstypeautomaticallybecomesanoptional.Forexample,amethodoftype(Int)->Stringbecomes((Int)->String)?.Notethattheentirefunctiontypeiswrappedintheoptional,notthemethod’sreturnvalue.

Anoptionalprotocolrequirementcanbecalledwithoptionalchaining,toaccountforthepossibilitythattherequirementwasnotimplementedbyatypethatconformstotheprotocol.Youcheckforanimplementationofanoptionalmethodbywritingaquestionmarkafterthenameofthemethodwhenit’scalled,suchassomeOptionalMethod?(someArgument).Forinformationonoptionalchaining,seeOptionalChaining.

Thefollowingexampledefinesaninteger-countingclasscalledCounter,whichusesanexternaldatasourcetoprovideitsincrementamount.ThisdatasourceisdefinedbytheCounterDataSourceprotocol,whichhastwooptionalrequirements:

1 @objcprotocolCounterDataSource{

2 @objcoptionalfuncincrement(forCountcount:Int)->Int

3 @objcoptionalvarfixedIncrement:Int{get}

4 }

TheCounterDataSourceprotocoldefinesanoptionalmethodrequirementcalledincrement(forCount:)andanoptionalpropertyrequirementcalledfixedIncrement.TheserequirementsdefinetwodifferentwaysfordatasourcestoprovideanappropriateincrementamountforaCounterinstance.

NOTE

Strictlyspeaking,youcanwriteacustomclassthatconformstoCounterDataSourcewithoutimplementingeitherprotocolrequirement.They’rebothoptional,afterall.Althoughtechnicallyallowed,thiswouldn’tmakeforaverygooddatasource.

TheCounterclass,definedbelow,hasanoptionaldataSourcepropertyoftypeCounterDataSource?:

1 classCounter{

2 varcount=0

3 vardataSource:CounterDataSource?

4 funcincrement(){

5 ifletamount=dataSource?.increment?(forCount:count)

{

6 count+=amount

7 }elseifletamount=dataSource?.fixedIncrement{

8 count+=amount

9 }

10 }

11 }

TheCounterclassstoresitscurrentvalueinavariablepropertycalledcount.TheCounterclassalsodefinesamethodcalledincrement,whichincrementsthecountpropertyeverytimethemethodiscalled.

Theincrement()methodfirsttriestoretrieveanincrementamountbylookingforanimplementationoftheincrement(forCount:)methodonitsdatasource.

Theincrement()methodusesoptionalchainingtotrytocallincrement(forCount:),andpassesthecurrentcountvalueasthemethod’ssingleargument.

Notethattwolevelsofoptionalchainingareatplayhere.First,it’spossiblethatdataSourcemaybenil,andsodataSourcehasaquestionmarkafteritsnametoindicatethatincrement(forCount:)shouldbecalledonlyifdataSourceisn’tnil.Second,evenifdataSourcedoesexist,there’snoguaranteethatitimplementsincrement(forCount:),becauseit’sanoptionalrequirement.Here,thepossibilitythatincrement(forCount:)mightnotbeimplementedisalsohandledbyoptionalchaining.Thecalltoincrement(forCount:)happensonlyifincrement(forCount:)exists—thatis,ifitisn’tnil.Thisiswhyincrement(forCount:)isalsowrittenwithaquestionmarkafteritsname.

Becausethecalltoincrement(forCount:)canfailforeitherofthesetworeasons,thecallreturnsanoptionalIntvalue.Thisistrueeventhoughincrement(forCount:)isdefinedasreturninganon-optionalIntvalueinthedefinitionofCounterDataSource.Eventhoughtherearetwooptionalchainingoperations,oneafteranother,theresultisstillwrappedinasingleoptional.Formoreinformationaboutusingmultipleoptionalchainingoperations,seeLinkingMultipleLevelsofChaining.

Aftercallingincrement(forCount:),theoptionalIntthatitreturnsisunwrappedintoaconstantcalledamount,usingoptionalbinding.IftheoptionalIntdoescontainavalue—thatis,ifthedelegateandmethodbothexist,andthemethodreturnedavalue—theunwrappedamountisaddedontothestoredcountproperty,andincrementationiscomplete.

Ifit’snotpossibletoretrieveavaluefromtheincrement(forCount:)method—eitherbecausedataSourceisnil,orbecausethedatasourcedoesn’timplementincrement(forCount:)—thentheincrement()methodtriestoretrieveavaluefromthedatasource’sfixedIncrementpropertyinstead.ThefixedIncrementpropertyisalsoanoptionalrequirement,soitsvalueisanoptionalIntvalue,eventhoughfixedIncrementisdefinedasanon-optionalIntpropertyaspartoftheCounterDataSourceprotocoldefinition.

Here’sasimpleCounterDataSourceimplementationwherethedatasourcereturnsaconstantvalueof3everytimeit’squeried.Itdoesthisbyimplementing

theoptionalfixedIncrementpropertyrequirement:

1 classThreeSource:NSObject,CounterDataSource{

2 letfixedIncrement=3

3 }

YoucanuseaninstanceofThreeSourceasthedatasourceforanewCounterinstance:

1 varcounter=Counter()

2 counter.dataSource=ThreeSource()

3 for_in1...4{

4 counter.increment()

5 print(counter.count)

6 }

7 //3

8 //6

9 //9

10 //12

ThecodeabovecreatesanewCounterinstance;setsitsdatasourcetobeanewThreeSourceinstance;andcallsthecounter’sincrement()methodfourtimes.Asexpected,thecounter’scountpropertyincreasesbythreeeachtimeincrement()iscalled.

Here’samorecomplexdatasourcecalledTowardsZeroSource,whichmakesaCounterinstancecountupordowntowardszerofromitscurrentcountvalue:

1 classTowardsZeroSource:NSObject,CounterDataSource{

2 funcincrement(forCountcount:Int)->Int{

3 ifcount==0{

4 return0

5 }elseifcount<0{

6 return1

7 }else{

8 return-1

9 }

10 }

11 }

TheTowardsZeroSourceclassimplementstheoptionalincrement(forCount:)methodfromtheCounterDataSourceprotocolandusesthecountargumentvaluetoworkoutwhichdirectiontocountin.Ifcountisalreadyzero,themethodreturns0toindicatethatnofurthercountingshouldtakeplace.

YoucanuseaninstanceofTowardsZeroSourcewiththeexistingCounterinstancetocountfrom-4tozero.Oncethecounterreacheszero,nomorecountingtakesplace:

1 counter.count=-4

2 counter.dataSource=TowardsZeroSource()

3 for_in1...5{

4 counter.increment()

5 print(counter.count)

6 }

7 //-3

8 //-2

9 //-1

10 //0

11 //0

ProtocolExtensions

Protocolscanbeextendedtoprovidemethod,initializer,subscript,and

computedpropertyimplementationstoconformingtypes.Thisallowsyoutodefinebehavioronprotocolsthemselves,ratherthanineachtype’sindividualconformanceorinaglobalfunction.

Forexample,theRandomNumberGeneratorprotocolcanbeextendedtoprovidearandomBool()method,whichusestheresultoftherequiredrandom()methodtoreturnarandomBoolvalue:

1 extensionRandomNumberGenerator{

2 funcrandomBool()->Bool{

3 returnrandom()>0.5

4 }

5 }

Bycreatinganextensionontheprotocol,allconformingtypesautomaticallygainthismethodimplementationwithoutanyadditionalmodification.

1 letgenerator=LinearCongruentialGenerator()

2 print("Here'sarandomnumber:\(generator.random())")

3 //Prints"Here'sarandomnumber:0.3746499199817101"

4 print("Andhere'sarandomBoolean:\(generator.randomBool())")

5 //Prints"Andhere'sarandomBoolean:true"

Protocolextensionscanaddimplementationstoconformingtypesbutcan’tmakeaprotocolextendorinheritfromanotherprotocol.Protocolinheritanceisalwaysspecifiedintheprotocoldeclarationitself.

ProvidingDefaultImplementationsYoucanuseprotocolextensionstoprovideadefaultimplementationtoanymethodorcomputedpropertyrequirementofthatprotocol.Ifaconformingtypeprovidesitsownimplementationofarequiredmethodorproperty,thatimplementationwillbeusedinsteadoftheoneprovidedbytheextension.

NOTE

Protocolrequirementswithdefaultimplementationsprovidedbyextensionsaredistinctfromoptionalprotocolrequirements.Althoughconformingtypesdon’thavetoprovidetheirownimplementationofeither,requirementswithdefaultimplementationscanbecalledwithoutoptionalchaining.

Forexample,thePrettyTextRepresentableprotocol,whichinheritstheTextRepresentableprotocolcanprovideadefaultimplementationofitsrequiredprettyTextualDescriptionpropertytosimplyreturntheresultofaccessingthetextualDescriptionproperty:

1 extensionPrettyTextRepresentable{

2 varprettyTextualDescription:String{

3 returntextualDescription

4 }

5 }

AddingConstraintstoProtocolExtensionsWhenyoudefineaprotocolextension,youcanspecifyconstraintsthatconformingtypesmustsatisfybeforethemethodsandpropertiesoftheextensionareavailable.Youwritetheseconstraintsafterthenameoftheprotocolyou’reextendingbywritingagenericwhereclause.Formoreaboutgenericwhereclauses,seeGenericWhereClauses.

Forexample,youcandefineanextensiontotheCollectionprotocolthatappliestoanycollectionwhoseelementsconformtotheEquatableprotocol.Byconstrainingacollection’selementstotheEquatableprotocol,apartofthestandardlibrary,youcanusethe==and!=operatorstocheckforequalityandinequalitybetweentwoelements.

1 extensionCollectionwhereElement:Equatable{

2 funcallEqual()->Bool{

3 forelementinself{

4 ifelement!=self.first{

5 returnfalse

6 }

7 }

8 returntrue

9 }

10 }

TheallEqual()methodreturnstrueonlyifalltheelementsinthecollectionareequal.

Considertwoarraysofintegers,onewherealltheelementsarethesame,andonewheretheyaren’t:

1 letequalNumbers=[100,100,100,100,100]

2 letdifferentNumbers=[100,100,200,100,200]

BecausearraysconformtoCollectionandintegersconformtoEquatable,equalNumbersanddifferentNumberscanusetheallEqual()method:

1 print(equalNumbers.allEqual())

2 //Prints"true"

3 print(differentNumbers.allEqual())

4 //Prints"false"

NOTE

Ifaconformingtypesatisfiestherequirementsformultipleconstrainedextensionsthatprovideimplementationsforthesamemethodorproperty,Swiftusestheimplementationcorrespondingtothemostspecializedconstraints.

Generics

Genericcodeenablesyoutowriteflexible,reusablefunctionsandtypesthatcanworkwithanytype,subjecttorequirementsthatyoudefine.Youcanwritecodethatavoidsduplicationandexpressesitsintentinaclear,abstractedmanner.

GenericsareoneofthemostpowerfulfeaturesofSwift,andmuchoftheSwiftstandardlibraryisbuiltwithgenericcode.Infact,you’vebeenusinggenericsthroughouttheLanguageGuide,evenifyoudidn’trealizeit.Forexample,Swift’sArrayandDictionarytypesarebothgenericcollections.YoucancreateanarraythatholdsIntvalues,oranarraythatholdsStringvalues,orindeedanarrayforanyothertypethatcanbecreatedinSwift.Similarly,youcancreateadictionarytostorevaluesofanyspecifiedtype,andtherearenolimitationsonwhatthattypecanbe.

TheProblemThatGenericsSolve

Here’sastandard,nongenericfunctioncalledswapTwoInts(_:_:),whichswapstwoIntvalues:

1 funcswapTwoInts(_a:inoutInt,_b:inoutInt){

2 lettemporaryA=a

3 a=b

4 b=temporaryA

5 }

Thisfunctionmakesuseofin-outparameterstoswapthevaluesofaandb,asdescribedinIn-OutParameters.

TheswapTwoInts(_:_:)functionswapstheoriginalvalueofbintoa,andtheoriginalvalueofaintob.YoucancallthisfunctiontoswapthevaluesintwoIntvariables:

1 varsomeInt=3

2 varanotherInt=107

3 swapTwoInts(&someInt,&anotherInt)

4 print("someIntisnow\(someInt),andanotherIntisnow\

(anotherInt)")

5 //Prints"someIntisnow107,andanotherIntisnow3"

TheswapTwoInts(_:_:)functionisuseful,butitcanonlybeusedwithIntvalues.IfyouwanttoswaptwoStringvalues,ortwoDoublevalues,youhavetowritemorefunctions,suchastheswapTwoStrings(_:_:)andswapTwoDoubles(_:_:)functionsshownbelow:

1 funcswapTwoStrings(_a:inoutString,_b:inoutString){

2 lettemporaryA=a

3 a=b

4 b=temporaryA

5 }

6

7 funcswapTwoDoubles(_a:inoutDouble,_b:inoutDouble){

8 lettemporaryA=a

9 a=b

10 b=temporaryA

11 }

YoumayhavenoticedthatthebodiesoftheswapTwoInts(_:_:),swapTwoStrings(_:_:),andswapTwoDoubles(_:_:)functionsareidentical.Theonlydifferenceisthetypeofthevaluesthattheyaccept(Int,String,andDouble).

It’smoreuseful,andconsiderablymoreflexible,towriteasinglefunctionthatswapstwovaluesofanytype.Genericcodeenablesyoutowritesuchafunction.(Agenericversionofthesefunctionsisdefinedbelow.)

NOTE

Inallthreefunctions,thetypesofaandbmustbethesame.Ifaandbaren’tofthesametype,itisn’tpossibletoswaptheirvalues.Swiftisatype-safelanguage,anddoesn’tallow(forexample)avariableoftypeStringandavariableoftypeDoubletoswapvalueswitheachother.Attemptingtodosoresultsinacompile-timeerror.

GenericFunctions

Genericfunctionscanworkwithanytype.Here’sagenericversionoftheswapTwoInts(_:_:)functionfromabove,calledswapTwoValues(_:_:):

1 funcswapTwoValues<T>(_a:inoutT,_b:inoutT){

2 lettemporaryA=a

3 a=b

4 b=temporaryA

5 }

ThebodyoftheswapTwoValues(_:_:)functionisidenticaltothebodyoftheswapTwoInts(_:_:)function.However,thefirstlineofswapTwoValues(_:_:)isslightlydifferentfromswapTwoInts(_:_:).Here’showthefirstlinescompare:

1 funcswapTwoInts(_a:inoutInt,_b:inoutInt)

2 funcswapTwoValues<T>(_a:inoutT,_b:inoutT)

Thegenericversionofthefunctionusesaplaceholdertypename(calledT,inthiscase)insteadofanactualtypename(suchasInt,String,orDouble).Theplaceholdertypenamedoesn’tsayanythingaboutwhatTmustbe,butitdoessaythatbothaandbmustbeofthesametypeT,whateverTrepresents.TheactualtypetouseinplaceofTisdeterminedeachtimetheswapTwoValues(_:_:)functioniscalled.

Theotherdifferencebetweenagenericfunctionandanongenericfunctionisthatthegenericfunction’sname(swapTwoValues(_:_:))isfollowedbythe

placeholdertypename(T)insideanglebrackets(<T>).ThebracketstellSwiftthatTisaplaceholdertypenamewithintheswapTwoValues(_:_:)functiondefinition.BecauseTisaplaceholder,Swiftdoesn’tlookforanactualtypecalledT.

TheswapTwoValues(_:_:)functioncannowbecalledinthesamewayasswapTwoInts,exceptthatitcanbepassedtwovaluesofanytype,aslongasbothofthosevaluesareofthesametypeaseachother.EachtimeswapTwoValues(_:_:)iscalled,thetypetouseforTisinferredfromthetypesofvaluespassedtothefunction.

Inthetwoexamplesbelow,TisinferredtobeIntandStringrespectively:

1 varsomeInt=3

2 varanotherInt=107

3 swapTwoValues(&someInt,&anotherInt)

4 //someIntisnow107,andanotherIntisnow3

5

6 varsomeString="hello"

7 varanotherString="world"

8 swapTwoValues(&someString,&anotherString)

9 //someStringisnow"world",andanotherStringisnow"hello"

NOTE

TheswapTwoValues(_:_:)functiondefinedaboveisinspiredbyagenericfunctioncalledswap,whichispartoftheSwiftstandardlibrary,andisautomaticallymadeavailableforyoutouseinyourapps.IfyouneedthebehavioroftheswapTwoValues(_:_:)functioninyourowncode,youcanuseSwift’sexistingswap(_:_:)functionratherthanprovidingyourownimplementation.

TypeParameters

IntheswapTwoValues(_:_:)exampleabove,theplaceholdertypeTisanexampleofatypeparameter.Typeparametersspecifyandnameaplaceholdertype,and

arewrittenimmediatelyafterthefunction’sname,betweenapairofmatchinganglebrackets(suchas<T>).

Onceyouspecifyatypeparameter,youcanuseittodefinethetypeofafunction’sparameters(suchastheaandbparametersoftheswapTwoValues(_:_:)function),orasthefunction’sreturntype,orasatypeannotationwithinthebodyofthefunction.Ineachcase,thetypeparameterisreplacedwithanactualtypewheneverthefunctioniscalled.(IntheswapTwoValues(_:_:)exampleabove,TwasreplacedwithIntthefirsttimethefunctionwascalled,andwasreplacedwithStringthesecondtimeitwascalled.)

Youcanprovidemorethanonetypeparameterbywritingmultipletypeparameternameswithintheanglebrackets,separatedbycommas.

NamingTypeParameters

Inmostcases,typeparametershavedescriptivenames,suchasKeyandValueinDictionary<Key,Value>andElementinArray<Element>,whichtellsthereaderabouttherelationshipbetweenthetypeparameterandthegenerictypeorfunctionit’susedin.However,whenthereisn’tameaningfulrelationshipbetweenthem,it’straditionaltonamethemusingsingleletterssuchasT,U,andV,suchasTintheswapTwoValues(_:_:)functionabove.

NOTE

Alwaysgivetypeparametersuppercamelcasenames(suchasTandMyTypeParameter)toindicatethatthey’reaplaceholderforatype,notavalue.

GenericTypes

Inadditiontogenericfunctions,Swiftenablesyoutodefineyourowngenerictypes.Thesearecustomclasses,structures,andenumerationsthatcanworkwithanytype,inasimilarwaytoArrayandDictionary.

ThissectionshowsyouhowtowriteagenericcollectiontypecalledStack.Astackisanorderedsetofvalues,similartoanarray,butwithamorerestrictedsetofoperationsthanSwift’sArraytype.Anarrayallowsnewitemstobeinsertedandremovedatanylocationinthearray.Astack,however,allowsnewitemstobeappendedonlytotheendofthecollection(knownaspushinganewvalueontothestack).Similarly,astackallowsitemstoberemovedonlyfromtheendofthecollection(knownaspoppingavalueoffthestack).

NOTE

TheconceptofastackisusedbytheUINavigationControllerclasstomodeltheviewcontrollersinitsnavigationhierarchy.YoucalltheUINavigationControllerclasspushViewController(_:animated:)methodtoadd(orpush)aviewcontrollerontothenavigationstack,anditspopViewControllerAnimated(_:)methodtoremove(orpop)aviewcontrollerfromthenavigationstack.Astackisausefulcollectionmodelwheneveryouneedastrict“lastin,firstout”approachtomanagingacollection.

Theillustrationbelowshowsthepushandpopbehaviorforastack:

1. Therearecurrentlythreevaluesonthestack.

2. Afourthvalueispushedontothetopofthestack.

3. Thestacknowholdsfourvalues,withthemostrecentoneatthetop.

4. Thetopiteminthestackispopped.

5. Afterpoppingavalue,thestackonceagainholdsthreevalues.

Here’showtowriteanongenericversionofastack,inthiscaseforastackofIntvalues:

1 structIntStack{

2 varitems=[Int]()

3 mutatingfuncpush(_item:Int){

4 items.append(item)

5 }

6 mutatingfuncpop()->Int{

7 returnitems.removeLast()

8 }

9 }

ThisstructureusesanArraypropertycalleditemstostorethevaluesinthestack.Stackprovidestwomethods,pushandpop,topushandpopvaluesonandoffthestack.Thesemethodsaremarkedasmutating,becausetheyneedtomodify(ormutate)thestructure’sitemsarray.

TheIntStacktypeshownabovecanonlybeusedwithIntvalues,however.ItwouldbemuchmoreusefultodefineagenericStackclass,thatcanmanageastackofanytypeofvalue.

Here’sagenericversionofthesamecode:

1 structStack<Element>{

2 varitems=[Element]()

3 mutatingfuncpush(_item:Element){

4 items.append(item)

5 }

6 mutatingfuncpop()->Element{

7 returnitems.removeLast()

8 }

9 }

NotehowthegenericversionofStackisessentiallythesameasthenongenericversion,butwithatypeparametercalledElementinsteadofanactualtypeofInt.Thistypeparameteriswrittenwithinapairofanglebrackets(<Element>)immediatelyafterthestructure’sname.

Elementdefinesaplaceholdernameforatypetobeprovidedlater.ThisfuturetypecanbereferredtoasElementanywherewithinthestructure’sdefinition.Inthiscase,Elementisusedasaplaceholderinthreeplaces:

Tocreateapropertycalleditems,whichisinitializedwithanemptyarrayofvaluesoftypeElement

Tospecifythatthepush(_:)methodhasasingleparametercalleditem,whichmustbeoftypeElement

Tospecifythatthevaluereturnedbythepop()methodwillbeavalueoftypeElement

Becauseit’sagenerictype,StackcanbeusedtocreateastackofanyvalidtypeinSwift,inasimilarmannertoArrayandDictionary.

YoucreateanewStackinstancebywritingthetypetobestoredinthestackwithinanglebrackets.Forexample,tocreateanewstackofstrings,youwriteStack<String>():

1 varstackOfStrings=Stack<String>()

2 stackOfStrings.push("uno")

3 stackOfStrings.push("dos")

4 stackOfStrings.push("tres")

5 stackOfStrings.push("cuatro")

6 //thestacknowcontains4strings

Here’showstackOfStringslooksafterpushingthesefourvaluesontothestack:

Poppingavaluefromthestackremovesandreturnsthetopvalue,"cuatro":

1 letfromTheTop=stackOfStrings.pop()

2 //fromTheTopisequalto"cuatro",andthestacknowcontains

3strings

Here’showthestacklooksafterpoppingitstopvalue:

ExtendingaGenericType

Whenyouextendagenerictype,youdon’tprovideatypeparameterlistaspartoftheextension’sdefinition.Instead,thetypeparameterlistfromtheoriginaltypedefinitionisavailablewithinthebodyoftheextension,andtheoriginaltypeparameternamesareusedtorefertothetypeparametersfromtheoriginaldefinition.

ThefollowingexampleextendsthegenericStacktypetoaddaread-onlycomputedpropertycalledtopItem,whichreturnsthetopitemonthestackwithoutpoppingitfromthestack:

1 extensionStack{

2 vartopItem:Element?{

3 returnitems.isEmpty?nil:items[items.count-1]

4 }

5 }

ThetopItempropertyreturnsanoptionalvalueoftypeElement.Ifthestackisempty,topItemreturnsnil;ifthestackisn’tempty,topItemreturnsthefinalitemintheitemsarray.

Notethatthisextensiondoesn’tdefineatypeparameterlist.Instead,theStacktype’sexistingtypeparametername,Element,isusedwithintheextensiontoindicatetheoptionaltypeofthetopItemcomputedproperty.

ThetopItemcomputedpropertycannowbeusedwithanyStackinstancetoaccessandqueryitstopitemwithoutremovingit.

1 iflettopItem=stackOfStrings.topItem{

2 print("Thetopitemonthestackis\(topItem).")

3 }

4 //Prints"Thetopitemonthestackistres."

Extensionsofagenerictypecanalsoincluderequirementsthatinstancesoftheextendedtypemustsatisfyinordertogainthenewfunctionality,asdiscussedinExtensionswithaGenericWhereClausebelow.

TypeConstraints

TheswapTwoValues(_:_:)functionandtheStacktypecanworkwithanytype.

However,it’ssometimesusefultoenforcecertaintypeconstraintsonthetypesthatcanbeusedwithgenericfunctionsandgenerictypes.Typeconstraintsspecifythatatypeparametermustinheritfromaspecificclass,orconformtoaparticularprotocolorprotocolcomposition.

Forexample,Swift’sDictionarytypeplacesalimitationonthetypesthatcanbeusedaskeysforadictionary.AsdescribedinDictionaries,thetypeofadictionary’skeysmustbehashable.Thatis,itmustprovideawaytomakeitselfuniquelyrepresentable.Dictionaryneedsitskeystobehashablesothatitcancheckwhetheritalreadycontainsavalueforaparticularkey.Withoutthisrequirement,Dictionarycouldnottellwhetheritshouldinsertorreplaceavalueforaparticularkey,norwoulditbeabletofindavalueforagivenkeythatisalreadyinthedictionary.

ThisrequirementisenforcedbyatypeconstraintonthekeytypeforDictionary,whichspecifiesthatthekeytypemustconformtotheHashableprotocol,aspecialprotocoldefinedintheSwiftstandardlibrary.AllofSwift’sbasictypes(suchasString,Int,Double,andBool)arehashablebydefault.

Youcandefineyourowntypeconstraintswhencreatingcustomgenerictypes,andtheseconstraintsprovidemuchofthepowerofgenericprogramming.AbstractconceptslikeHashablecharacterizetypesintermsoftheirconceptualcharacteristics,ratherthantheirconcretetype.

TypeConstraintSyntaxYouwritetypeconstraintsbyplacingasingleclassorprotocolconstraintafteratypeparameter’sname,separatedbyacolon,aspartofthetypeparameterlist.Thebasicsyntaxfortypeconstraintsonagenericfunctionisshownbelow(althoughthesyntaxisthesameforgenerictypes):

1 funcsomeFunction<T:SomeClass,U:SomeProtocol>(someT:T,

someU:U){

2 //functionbodygoeshere

3 }

Thehypotheticalfunctionabovehastwotypeparameters.Thefirsttypeparameter,T,hasatypeconstraintthatrequiresTtobeasubclassofSomeClass.Thesecondtypeparameter,U,hasatypeconstraintthatrequiresUtoconformtotheprotocolSomeProtocol.

TypeConstraintsinActionHere’sanongenericfunctioncalledfindIndex(ofString:in:),whichisgivenaStringvaluetofindandanarrayofStringvalueswithinwhichtofindit.ThefindIndex(ofString:in:)functionreturnsanoptionalIntvalue,whichwillbetheindexofthefirstmatchingstringinthearrayifit’sfound,ornilifthestringcan’tbefound:

1 funcfindIndex(ofStringvalueToFind:String,inarray:

[String])->Int?{

2 for(index,value)inarray.enumerated(){

3 ifvalue==valueToFind{

4 returnindex

5 }

6 }

7 returnnil

8 }

ThefindIndex(ofString:in:)functioncanbeusedtofindastringvalueinanarrayofstrings:

1 letstrings=["cat","dog","llama","parakeet","terrapin"]

2 ifletfoundIndex=findIndex(ofString:"llama",in:strings){

3 print("Theindexofllamais\(foundIndex)")

4 }

5 //Prints"Theindexofllamais2"

Theprincipleoffindingtheindexofavalueinanarrayisn’tusefulonlyfor

strings,however.YoucanwritethesamefunctionalityasagenericfunctionbyreplacinganymentionofstringswithvaluesofsometypeTinstead.

Here’showyoumightexpectagenericversionoffindIndex(ofString:in:),calledfindIndex(of:in:),tobewritten.NotethatthereturntypeofthisfunctionisstillInt?,becausethefunctionreturnsanoptionalindexnumber,notanoptionalvaluefromthearray.Bewarned,though—thisfunctiondoesn’tcompile,forreasonsexplainedaftertheexample:

1 funcfindIndex<T>(ofvalueToFind:T,inarray:[T])->Int?{

2 for(index,value)inarray.enumerated(){

3 ifvalue==valueToFind{

4 returnindex

5 }

6 }

7 returnnil

8 }

Thisfunctiondoesn’tcompileaswrittenabove.Theproblemlieswiththeequalitycheck,“ifvalue==valueToFind”.NoteverytypeinSwiftcanbecomparedwiththeequaltooperator(==).Ifyoucreateyourownclassorstructuretorepresentacomplexdatamodel,forexample,thenthemeaningof“equalto”forthatclassorstructureisn’tsomethingthatSwiftcanguessforyou.Becauseofthis,itisn’tpossibletoguaranteethatthiscodewillworkforeverypossibletypeT,andanappropriateerrorisreportedwhenyoutrytocompilethecode.

Allisnotlost,however.TheSwiftstandardlibrarydefinesaprotocolcalledEquatable,whichrequiresanyconformingtypetoimplementtheequaltooperator(==)andthenotequaltooperator(!=)tocompareanytwovaluesofthattype.AllofSwift’sstandardtypesautomaticallysupporttheEquatableprotocol.

AnytypethatisEquatablecanbeusedsafelywiththefindIndex(of:in:)function,becauseit’sguaranteedtosupporttheequaltooperator.Toexpressthisfact,youwriteatypeconstraintofEquatableaspartofthetypeparameter’sdefinitionwhenyoudefinethefunction:

1 funcfindIndex<T:Equatable>(ofvalueToFind:T,inarray:[T])-

>Int?{

2 for(index,value)inarray.enumerated(){

3 ifvalue==valueToFind{

4 returnindex

5 }

6 }

7 returnnil

8 }

ThesingletypeparameterforfindIndex(of:in:)iswrittenasT:Equatable,whichmeans“anytypeTthatconformstotheEquatableprotocol.”

ThefindIndex(of:in:)functionnowcompilessuccessfullyandcanbeusedwithanytypethatisEquatable,suchasDoubleorString:

1 letdoubleIndex=findIndex(of:9.3,in:[3.14159,0.1,0.25])

2 //doubleIndexisanoptionalIntwithnovalue,because9.3

isn'tinthearray

3 letstringIndex=findIndex(of:"Andrea",in:["Mike",

"Malcolm","Andrea"])

4 //stringIndexisanoptionalIntcontainingavalueof2

AssociatedTypes

Whendefiningaprotocol,it’ssometimesusefultodeclareoneormoreassociatedtypesaspartoftheprotocol’sdefinition.Anassociatedtypegivesaplaceholdernametoatypethatisusedaspartoftheprotocol.Theactualtypetouseforthatassociatedtypeisn’tspecifieduntiltheprotocolisadopted.Associatedtypesarespecifiedwiththeassociatedtypekeyword.

AssociatedTypesinActionHere’sanexampleofaprotocolcalledContainer,whichdeclaresanassociatedtypecalledItem:

1 protocolContainer{

2 associatedtypeItem

3 mutatingfuncappend(_item:Item)

4 varcount:Int{get}

5 subscript(i:Int)->Item{get}

6 }

TheContainerprotocoldefinesthreerequiredcapabilitiesthatanycontainermustprovide:

Itmustbepossibletoaddanewitemtothecontainerwithanappend(_:)method.

ItmustbepossibletoaccessacountoftheitemsinthecontainerthroughacountpropertythatreturnsanIntvalue.

ItmustbepossibletoretrieveeachiteminthecontainerwithasubscriptthattakesanIntindexvalue.

Thisprotocoldoesn’tspecifyhowtheitemsinthecontainershouldbestoredorwhattypethey’reallowedtobe.TheprotocolonlyspecifiesthethreebitsoffunctionalitythatanytypemustprovideinordertobeconsideredaContainer.Aconformingtypecanprovideadditionalfunctionality,aslongasitsatisfiesthesethreerequirements.

AnytypethatconformstotheContainerprotocolmustbeabletospecifythetypeofvaluesitstores.Specifically,itmustensurethatonlyitemsoftherighttypeareaddedtothecontainer,anditmustbeclearaboutthetypeoftheitemsreturnedbyitssubscript.

Todefinetheserequirements,theContainerprotocolneedsawaytorefertothetypeoftheelementsthatacontainerwillhold,withoutknowingwhatthattypeis

foraspecificcontainer.TheContainerprotocolneedstospecifythatanyvaluepassedtotheappend(_:)methodmusthavethesametypeasthecontainer’selementtype,andthatthevaluereturnedbythecontainer’ssubscriptwillbeofthesametypeasthecontainer’selementtype.

Toachievethis,theContainerprotocoldeclaresanassociatedtypecalledItem,writtenasassociatedtypeItem.Theprotocoldoesn’tdefinewhatItemis—thatinformationisleftforanyconformingtypetoprovide.Nonetheless,theItemaliasprovidesawaytorefertothetypeoftheitemsinaContainer,andtodefineatypeforusewiththeappend(_:)methodandsubscript,toensurethattheexpectedbehaviorofanyContainerisenforced.

Here’saversionofthenongenericIntStacktypefromGenericTypesabove,adaptedtoconformtotheContainerprotocol:

1 structIntStack:Container{

2 //originalIntStackimplementation

3 varitems=[Int]()

4 mutatingfuncpush(_item:Int){

5 items.append(item)

6 }

7 mutatingfuncpop()->Int{

8 returnitems.removeLast()

9 }

10 //conformancetotheContainerprotocol

11 typealiasItem=Int

12 mutatingfuncappend(_item:Int){

13 self.push(item)

14 }

15 varcount:Int{

16 returnitems.count

17 }

18 subscript(i:Int)->Int{

19 returnitems[i]

20 }

21 }

TheIntStacktypeimplementsallthreeoftheContainerprotocol’srequirements,andineachcasewrapspartoftheIntStacktype’sexistingfunctionalitytosatisfytheserequirements.

Moreover,IntStackspecifiesthatforthisimplementationofContainer,theappropriateItemtouseisatypeofInt.ThedefinitionoftypealiasItem=IntturnstheabstracttypeofItemintoaconcretetypeofIntforthisimplementationoftheContainerprotocol.

ThankstoSwift’stypeinference,youdon’tactuallyneedtodeclareaconcreteItemofIntaspartofthedefinitionofIntStack.BecauseIntStackconformstoalloftherequirementsoftheContainerprotocol,SwiftcaninfertheappropriateItemtouse,simplybylookingatthetypeoftheappend(_:)method’sitemparameterandthereturntypeofthesubscript.Indeed,ifyoudeletethetypealiasItem=Intlinefromthecodeabove,everythingstillworks,becauseit’sclearwhattypeshouldbeusedforItem.

YoucanalsomakethegenericStacktypeconformtotheContainerprotocol:

1 structStack<Element>:Container{

2 //originalStack<Element>implementation

3 varitems=[Element]()

4 mutatingfuncpush(_item:Element){

5 items.append(item)

6 }

7 mutatingfuncpop()->Element{

8 returnitems.removeLast()

9 }

10 //conformancetotheContainerprotocol

11 mutatingfuncappend(_item:Element){

12 self.push(item)

13 }

14 varcount:Int{

15 returnitems.count

16 }

17 subscript(i:Int)->Element{

18 returnitems[i]

19 }

20 }

Thistime,thetypeparameterElementisusedasthetypeoftheappend(_:)method’sitemparameterandthereturntypeofthesubscript.SwiftcanthereforeinferthatElementistheappropriatetypetouseastheItemforthisparticularcontainer.

ExtendinganExistingTypetoSpecifyanAssociatedTypeYoucanextendanexistingtypetoaddconformancetoaprotocol,asdescribedinAddingProtocolConformancewithanExtension.Thisincludesaprotocolwithanassociatedtype.

Swift’sArraytypealreadyprovidesanappend(_:)method,acountproperty,andasubscriptwithanIntindextoretrieveitselements.ThesethreecapabilitiesmatchtherequirementsoftheContainerprotocol.ThismeansthatyoucanextendArraytoconformtotheContainerprotocolsimplybydeclaringthatArrayadoptstheprotocol.Youdothiswithanemptyextension,asdescribedinDeclaringProtocolAdoptionwithanExtension:

extensionArray:Container{}

Array’sexistingappend(_:)methodandsubscriptenableSwifttoinfertheappropriatetypetouseforItem,justasforthegenericStacktypeabove.Afterdefiningthisextension,youcanuseanyArrayasaContainer.

AddingConstraintstoanAssociatedTypeYoucanaddtypeconstraintstoanassociatedtypeinaprotocoltorequirethatconformingtypessatisfythoseconstraints.Forexample,thefollowingcodedefinesaversionofContainerthatrequirestheitemsinthecontainertobeequatable.

1 protocolContainer{

2 associatedtypeItem:Equatable

3 mutatingfuncappend(_item:Item)

4 varcount:Int{get}

5 subscript(i:Int)->Item{get}

6 }

ToconformtothisversionofContainer,thecontainer’sItemtypehastoconformtotheEquatableprotocol.

UsingaProtocolinItsAssociatedType’sConstraintsAprotocolcanappearaspartofitsownrequirements.Forexample,here’saprotocolthatrefinestheContainerprotocol,addingtherequirementofasuffix(_:)method.Thesuffix(_:)methodreturnsagivennumberofelementsfromtheendofthecontainer,storingtheminaninstanceoftheSuffixtype.

1 protocolSuffixableContainer:Container{

2 associatedtypeSuffix:SuffixableContainerwhere

Suffix.Item==Item

3 funcsuffix(_size:Int)->Suffix

4 }

Inthisprotocol,Suffixisanassociatedtype,liketheItemtypeintheContainerexampleabove.Suffixhastwoconstraints:ItmustconformtotheSuffixableContainerprotocol(theprotocolcurrentlybeingdefined),anditsItemtypemustbethesameasthecontainer’sItemtype.TheconstraintonItem

isagenericwhereclause,whichisdiscussedinAssociatedTypeswithaGenericWhereClausebelow.

Here’sanextensionoftheStacktypefromGenericTypesabovethataddsconformancetotheSuffixableContainerprotocol:

1 extensionStack:SuffixableContainer{

2 funcsuffix(_size:Int)->Stack{

3 varresult=Stack()

4 forindexin(count-size)..<count{

5 result.append(self[index])

6 }

7 returnresult

8 }

9 //InferredthatSuffixisStack.

10 }

11 varstackOfInts=Stack<Int>()

12 stackOfInts.append(10)

13 stackOfInts.append(20)

14 stackOfInts.append(30)

15 letsuffix=stackOfInts.suffix(2)

16 //suffixcontains20and30

Intheexampleabove,theSuffixassociatedtypeforStackisalsoStack,sothesuffixoperationonStackreturnsanotherStack.Alternatively,atypethatconformstoSuffixableContainercanhaveaSuffixtypethat’sdifferentfromitself—meaningthesuffixoperationcanreturnadifferenttype.Forexample,here’sanextensiontothenongenericIntStacktypethataddsSuffixableContainerconformance,usingStack<Int>asitssuffixtypeinsteadofIntStack:

1 extensionIntStack:SuffixableContainer{

2 funcsuffix(_size:Int)->Stack<Int>{

3 varresult=Stack<Int>()

4 forindexin(count-size)..<count{

5 result.append(self[index])

6 }

7 returnresult

8 }

9 //InferredthatSuffixisStack<Int>.

10 }

GenericWhereClauses

Typeconstraints,asdescribedinTypeConstraints,enableyoutodefinerequirementsonthetypeparametersassociatedwithagenericfunction,subscript,ortype.

Itcanalsobeusefultodefinerequirementsforassociatedtypes.Youdothisbydefiningagenericwhereclause.Agenericwhereclauseenablesyoutorequirethatanassociatedtypemustconformtoacertainprotocol,orthatcertaintypeparametersandassociatedtypesmustbethesame.Agenericwhereclausestartswiththewherekeyword,followedbyconstraintsforassociatedtypesorequalityrelationshipsbetweentypesandassociatedtypes.Youwriteagenericwhereclauserightbeforetheopeningcurlybraceofatypeorfunction’sbody.

TheexamplebelowdefinesagenericfunctioncalledallItemsMatch,whichcheckstoseeiftwoContainerinstancescontainthesameitemsinthesameorder.ThefunctionreturnsaBooleanvalueoftrueifallitemsmatchandavalueoffalseiftheydon’t.

Thetwocontainerstobecheckeddon’thavetobethesametypeofcontainer(althoughtheycanbe),buttheydohavetoholdthesametypeofitems.Thisrequirementisexpressedthroughacombinationoftypeconstraintsandagenericwhereclause:

1 funcallItemsMatch<C1:Container,C2:Container>

2 (_someContainer:C1,_anotherContainer:C2)->Bool

3 whereC1.Item==C2.Item,C1.Item:Equatable{

4

5 //Checkthatbothcontainerscontainthesamenumber

ofitems.

6 ifsomeContainer.count!=anotherContainer.count{

7 returnfalse

8 }

9

10 //Checkeachpairofitemstoseeifthey're

equivalent.

11 foriin0..<someContainer.count{

12 ifsomeContainer[i]!=anotherContainer[i]{

13 returnfalse

14 }

15 }

16

17 //Allitemsmatch,soreturntrue.

18 returntrue

19 }

ThisfunctiontakestwoargumentscalledsomeContainerandanotherContainer.ThesomeContainerargumentisoftypeC1,andtheanotherContainerargumentisoftypeC2.BothC1andC2aretypeparametersfortwocontainertypestobedeterminedwhenthefunctioniscalled.

Thefollowingrequirementsareplacedonthefunction’stwotypeparameters:

C1mustconformtotheContainerprotocol(writtenasC1:Container).

C2mustalsoconformtotheContainerprotocol(writtenasC2:Container).

TheItemforC1mustbethesameastheItemforC2(writtenasC1.Item==C2.Item).

TheItemforC1mustconformtotheEquatableprotocol(writtenasC1.Item:Equatable).

Thefirstandsecondrequirementsaredefinedinthefunction’stypeparameterlist,andthethirdandfourthrequirementsaredefinedinthefunction’sgenericwhereclause.

Theserequirementsmean:

someContainerisacontaineroftypeC1.

anotherContainerisacontaineroftypeC2.

someContainerandanotherContainercontainthesametypeofitems.

TheitemsinsomeContainercanbecheckedwiththenotequaloperator(!=)toseeifthey’redifferentfromeachother.

ThethirdandfourthrequirementscombinetomeanthattheitemsinanotherContainercanalsobecheckedwiththe!=operator,becausethey’reexactlythesametypeastheitemsinsomeContainer.

TheserequirementsenabletheallItemsMatch(_:_:)functiontocomparethetwocontainers,evenifthey’reofadifferentcontainertype.

TheallItemsMatch(_:_:)functionstartsbycheckingthatbothcontainerscontainthesamenumberofitems.Iftheycontainadifferentnumberofitems,there’snowaythattheycanmatch,andthefunctionreturnsfalse.

Aftermakingthischeck,thefunctioniteratesoveralloftheitemsinsomeContainerwithafor-inloopandthehalf-openrangeoperator(..<).Foreachitem,thefunctioncheckswhethertheitemfromsomeContainerisn’tequaltothecorrespondingiteminanotherContainer.Ifthetwoitemsaren’tequal,thenthetwocontainersdon’tmatch,andthefunctionreturnsfalse.

Iftheloopfinisheswithoutfindingamismatch,thetwocontainersmatch,and

thefunctionreturnstrue.

Here’showtheallItemsMatch(_:_:)functionlooksinaction:

1 varstackOfStrings=Stack<String>()

2 stackOfStrings.push("uno")

3 stackOfStrings.push("dos")

4 stackOfStrings.push("tres")

5

6 vararrayOfStrings=["uno","dos","tres"]

7

8 ifallItemsMatch(stackOfStrings,arrayOfStrings){

9 print("Allitemsmatch.")

10 }else{

11 print("Notallitemsmatch.")

12 }

13 //Prints"Allitemsmatch."

TheexampleabovecreatesaStackinstancetostoreStringvalues,andpushesthreestringsontothestack.TheexamplealsocreatesanArrayinstanceinitializedwithanarrayliteralcontainingthesamethreestringsasthestack.Eventhoughthestackandthearrayareofadifferenttype,theybothconformtotheContainerprotocol,andbothcontainthesametypeofvalues.YoucanthereforecalltheallItemsMatch(_:_:)functionwiththesetwocontainersasitsarguments.Intheexampleabove,theallItemsMatch(_:_:)functioncorrectlyreportsthatalloftheitemsinthetwocontainersmatch.

ExtensionswithaGenericWhereClause

Youcanalsouseagenericwhereclauseaspartofanextension.TheexamplebelowextendsthegenericStackstructurefromthepreviousexamplestoaddanisTop(_:)method.

1 extensionStackwhereElement:Equatable{

2 funcisTop(_item:Element)->Bool{

3 guardlettopItem=items.lastelse{

4 returnfalse

5 }

6 returntopItem==item

7 }

8 }

ThisnewisTop(_:)methodfirstchecksthatthestackisn’tempty,andthencomparesthegivenitemagainstthestack’stopmostitem.Ifyoutriedtodothiswithoutagenericwhereclause,youwouldhaveaproblem:TheimplementationofisTop(_:)usesthe==operator,butthedefinitionofStackdoesn’trequireitsitemstobeequatable,sousingthe==operatorresultsinacompile-timeerror.Usingagenericwhereclauseletsyouaddanewrequirementtotheextension,sothattheextensionaddstheisTop(_:)methodonlywhentheitemsinthestackareequatable.

Here’showtheisTop(_:)methodlooksinaction:

1 ifstackOfStrings.isTop("tres"){

2 print("Topelementistres.")

3 }else{

4 print("Topelementissomethingelse.")

5 }

6 //Prints"Topelementistres."

IfyoutrytocalltheisTop(_:)methodonastackwhoseelementsaren’tequatable,you’llgetacompile-timeerror.

1 structNotEquatable{}

2 varnotEquatableStack=Stack<NotEquatable>()

3 letnotEquatableValue=NotEquatable()

4 notEquatableStack.push(notEquatableValue)

5 notEquatableStack.isTop(notEquatableValue)//Error

Youcanuseagenericwhereclausewithextensionstoaprotocol.TheexamplebelowextendstheContainerprotocolfromthepreviousexamplestoaddastartsWith(_:)method.

1 extensionContainerwhereItem:Equatable{

2 funcstartsWith(_item:Item)->Bool{

3 returncount>=1&&self[0]==item

4 }

5 }

ThestartsWith(_:)methodfirstmakessurethatthecontainerhasatleastoneitem,andthenitcheckswhetherthefirstiteminthecontainermatchesthegivenitem.ThisnewstartsWith(_:)methodcanbeusedwithanytypethatconformstotheContainerprotocol,includingthestacksandarraysusedabove,aslongasthecontainer’sitemsareequatable.

1 if[9,9,9].startsWith(42){

2 print("Startswith42.")

3 }else{

4 print("Startswithsomethingelse.")

5 }

6 //Prints"Startswithsomethingelse."

ThegenericwhereclauseintheexampleaboverequiresItemtoconformtoaprotocol,butyoucanalsowriteagenericwhereclausesthatrequireItemtobeaspecifictype.Forexample:

1 extensionContainerwhereItem==Double{

2 funcaverage()->Double{

3 varsum=0.0

4 forindexin0..<count{

5 sum+=self[index]

6 }

7 returnsum/Double(count)

8 }

9 }

10 print([1260.0,1200.0,98.6,37.0].average())

11 //Prints"648.9"

Thisexampleaddsanaverage()methodtocontainerswhoseItemtypeisDouble.Ititeratesovertheitemsinthecontainertoaddthemup,anddividesbythecontainer’scounttocomputetheaverage.ItexplicitlyconvertsthecountfromInttoDoubletobeabletodofloating-pointdivision.

Youcanincludemultiplerequirementsinagenericwhereclausethatispartofanextension,justlikeyoucanforagenericwhereclausethatyouwriteelsewhere.Separateeachrequirementinthelistwithacomma.

AssociatedTypeswithaGenericWhereClause

Youcanincludeagenericwhereclauseonanassociatedtype.Forexample,supposeyouwanttomakeaversionofContainerthatincludesaniterator,likewhattheSequenceprotocolusesinthestandardlibrary.Here’showyouwritethat:

1 protocolContainer{

2 associatedtypeItem

3 mutatingfuncappend(_item:Item)

4 varcount:Int{get}

5 subscript(i:Int)->Item{get}

6

7 associatedtypeIterator:IteratorProtocolwhere

Iterator.Element==Item

8 funcmakeIterator()->Iterator

9 }

ThegenericwhereclauseonIteratorrequiresthattheiteratormusttraverseoverelementsofthesameitemtypeasthecontainer’sitems,regardlessoftheiterator’stype.ThemakeIterator()functionprovidesaccesstoacontainer’siterator.

Foraprotocolthatinheritsfromanotherprotocol,youaddaconstrainttoaninheritedassociatedtypebyincludingthegenericwhereclauseintheprotocoldeclaration.Forexample,thefollowingcodedeclaresaComparableContainerprotocolthatrequiresItemtoconformtoComparable:

protocolComparableContainer:ContainerwhereItem:Comparable

{}

GenericSubscripts

Subscriptscanbegeneric,andtheycanincludegenericwhereclauses.Youwritetheplaceholdertypenameinsideanglebracketsaftersubscript,andyouwriteagenericwhereclauserightbeforetheopeningcurlybraceofthesubscript’sbody.Forexample:

1 extensionContainer{

2 subscript<Indices:Sequence>(indices:Indices)->[Item]

3 whereIndices.Iterator.Element==Int{

4 varresult=[Item]()

5 forindexinindices{

6 result.append(self[index])

7 }

8 returnresult

9 }

10 }

ThisextensiontotheContainerprotocoladdsasubscriptthattakesasequenceofindicesandreturnsanarraycontainingtheitemsateachgivenindex.Thisgenericsubscriptisconstrainedasfollows:

ThegenericparameterIndicesinanglebracketshastobeatypethatconformstotheSequenceprotocolfromthestandardlibrary.

Thesubscripttakesasingleparameter,indices,whichisaninstanceofthatIndicestype.

ThegenericwhereclauserequiresthattheiteratorforthesequencemusttraverseoverelementsoftypeInt.Thisensuresthattheindicesinthesequencearethesametypeastheindicesusedforacontainer.

Takentogether,theseconstraintsmeanthatthevaluepassedfortheindicesparameterisasequenceofintegers.

OpaqueTypes

Afunctionormethodwithanopaquereturntypehidesitsreturnvalue’stypeinformation.Insteadofprovidingaconcretetypeasthefunction’sreturntype,thereturnvalueisdescribedintermsoftheprotocolsitsupports.Hidingtypeinformationisusefulatboundariesbetweenamoduleandcodethatcallsintothemodule,becausetheunderlyingtypeofthereturnvaluecanremainprivate.Unlikereturningavaluewhosetypeisaprotocoltype,opaquetypespreservetypeidentity—thecompilerhasaccesstothetypeinformation,butclientsofthemoduledon’t.

TheProblemThatOpaqueTypesSolve

Forexample,supposeyou’rewritingamodulethatdrawsASCIIartshapes.ThebasiccharacteristicofanASCIIartshapeisadraw()functionthatreturnsthestringrepresentationofthatshape,whichyoucanuseastherequirementfortheShapeprotocol:

1 protocolShape{

2 funcdraw()->String

3 }

4

5 structTriangle:Shape{

6 varsize:Int

7 funcdraw()->String{

8 varresult=[String]()

9 forlengthin1...size{

10 result.append(String(repeating:"*",count:

length))

11 }

12 returnresult.joined(separator:"\n")

13 }

14 }

15 letsmallTriangle=Triangle(size:3)

16 print(smallTriangle.draw())

17 //*

18 //**

19 //***

Youcouldusegenericstoimplementoperationslikeflippingashapevertically,asshowninthecodebelow.However,there’sanimportantlimitationtothisapproach:Theflippedresultexposestheexactgenerictypesthatwereusedtocreateit.

1 structFlippedShape<T:Shape>:Shape{

2 varshape:T

3 funcdraw()->String{

4 letlines=shape.draw().split(separator:"\n")

5 returnlines.reversed().joined(separator:"\n")

6 }

7 }

8 letflippedTriangle=FlippedShape(shape:smallTriangle)

9 print(flippedTriangle.draw())

10 //***

11 //**

12 //*

ThisapproachtodefiningaJoinedShape<T:Shape,U:Shape>structurethatjoinstwoshapestogethervertically,likethecodebelowshows,resultsintypeslikeJoinedShape<FlippedShape<Triangle>,Triangle>fromjoiningaflippedtrianglewithanothertriangle.

1 structJoinedShape<T:Shape,U:Shape>:Shape{

2 vartop:T

3 varbottom:U

4 funcdraw()->String{

5 returntop.draw()+"\n"+bottom.draw()

6 }

7 }

8 letjoinedTriangles=JoinedShape(top:smallTriangle,bottom:

flippedTriangle)

9 print(joinedTriangles.draw())

10 //*

11 //**

12 //***

13 //***

14 //**

15 //*

Exposingdetailedinformationaboutthecreationofashapeallowstypesthataren’tmeanttobepartoftheASCIIartmodule’spublicinterfacetoleakoutbecauseoftheneedtostatethefullreturntype.Thecodeinsidethemodulecouldbuildupthesameshapeinavarietyofways,andothercodeoutsidethemodulethatusestheshapeshouldn’thavetoaccountfortheimplementationdetailsaboutthelistoftransformations.WrappertypeslikeJoinedShapeandFlippedShapedon’tmattertothemodule’susers,andtheyshouldn’tbevisible.Themodule’spublicinterfaceconsistsofoperationslikejoiningandflippingashape,andthoseoperationsreturnanotherShapevalue.

ReturninganOpaqueType

Youcanthinkofanopaquetypelikebeingthereverseofagenerictype.Generictypesletthecodethatcallsafunctionpickthetypeforthatfunction’s

parametersandreturnvalueinawaythat’sabstractedawayfromthefunctionimplementation.Forexample,thefunctioninthefollowingcodereturnsatypethatdependsonitscaller:

funcmax<T>(_x:T,_y:T)->TwhereT:Comparable{...}

Thecodethatcallsmax(_:_:)choosesthevaluesforxandy,andthetypeofthosevaluesdeterminestheconcretetypeofT.ThecallingcodecanuseanytypethatconformstotheComparableprotocol.Thecodeinsidethefunctioniswritteninageneralwaysoitcanhandlewhatevertypethecallerprovides.Theimplementationofmax(_:_:)usesonlyfunctionalitythatallComparabletypesshare.

Thoserolesarereversedforafunctionwithanopaquereturntype.Anopaquetypeletsthefunctionimplementationpickthetypeforthevalueitreturnsinawaythat’sabstractedawayfromthecodethatcallsthefunction.Forexample,thefunctioninthefollowingexamplereturnsatrapezoidwithoutexposingtheunderlyingtypeofthatshape.

1 structSquare:Shape{

2 varsize:Int

3 funcdraw()->String{

4 letline=String(repeating:"*",count:size)

5 letresult=Array<String>(repeating:line,count:

size)

6 returnresult.joined(separator:"\n")

7 }

8 }

9

10 funcmakeTrapezoid()->someShape{

11 lettop=Triangle(size:2)

12 letmiddle=Square(size:2)

13 letbottom=FlippedShape(shape:top)

14 lettrapezoid=JoinedShape(

15 top:top,

16 bottom:JoinedShape(top:middle,bottom:bottom)

17 )

18 returntrapezoid

19 }

20 lettrapezoid=makeTrapezoid()

21 print(trapezoid.draw())

22 //*

23 //**

24 //**

25 //**

26 //**

27 //*

ThemakeTrapezoid()functioninthisexampledeclaresitsreturntypeassomeShape;asaresult,thefunctionreturnsavalueofsomegiventypethatconformstotheShapeprotocol,withoutspecifyinganyparticularconcretetype.WritingmakeTrapezoid()thiswayletsitexpressthefundamentalaspectofitspublicinterface—thevalueitreturnsisashape—withoutmakingthespecifictypesthattheshapeismadefromapartofitspublicinterface.Thisimplementationusestwotrianglesandasquare,butthefunctioncouldberewrittentodrawatrapezoidinavarietyofotherwayswithoutchangingitsreturntype.

Thisexamplehighlightsthewaythatanopaquereturntypeislikethereverseofagenerictype.ThecodeinsidemakeTrapezoid()canreturnanytypeitneedsto,aslongasthattypeconformstotheShapeprotocol,likethecallingcodedoesforagenericfunction.Thecodethatcallsthefunctionneedstobewritteninageneralway,liketheimplementationofagenericfunction,sothatitcanworkwithanyShapevaluethat’sreturnedbymakeTrapezoid().

Youcanalsocombineopaquereturntypeswithgenerics.ThefunctionsinthefollowingcodebothreturnavalueofsometypethatconformstotheShapeprotocol.

1 funcflip<T:Shape>(_shape:T)->someShape{

2 returnFlippedShape(shape:shape)

3 }

4 funcjoin<T:Shape,U:Shape>(_top:T,_bottom:U)->some

Shape{

5 JoinedShape(top:top,bottom:bottom)

6 }

7

8 letopaqueJoinedTriangles=join(smallTriangle,

flip(smallTriangle))

9 print(opaqueJoinedTriangles.draw())

10 //*

11 //**

12 //***

13 //***

14 //**

15 //*

ThevalueofopaqueJoinedTrianglesinthisexampleisthesameasjoinedTrianglesinthegenericsexampleintheTheProblemThatOpaqueTypesSolvesectionearlierinthischapter.However,unlikethevalueinthatexample,flip(_:)andjoin(_:_:)wraptheunderlyingtypesthatthegenericshapeoperationsreturninanopaquereturntype,whichpreventsthosetypesfrombeingvisible.Bothfunctionsaregenericbecausethetypestheyrelyonaregeneric,andthetypeparameterstothefunctionpassalongthetypeinformationneededbyFlippedShapeandJoinedShape.

Ifafunctionwithanopaquereturntypereturnsfrommultipleplaces,allofthepossiblereturnvaluesmusthavethesametype.Foragenericfunction,thatreturntypecanusethefunction’sgenerictypeparameters,butitmuststillbeasingletype.Forexample,here’saninvalidversionoftheshape-flippingfunctionthatincludesaspecialcaseforsquares:

1 funcinvalidFlip<T:Shape>(_shape:T)->someShape{

2 ifshapeisSquare{

3 returnshape//Error:returntypesdon'tmatch

4 }

5 returnFlippedShape(shape:shape)//Error:returntypes

don'tmatch

6 }

IfyoucallthisfunctionwithaSquare,itreturnsaSquare;otherwise,itreturnsaFlippedShape.ThisviolatestherequirementtoreturnvaluesofonlyonetypeandmakesinvalidFlip(_:)invalidcode.OnewaytofixinvalidFlip(_:)istomovethespecialcaseforsquaresintotheimplementationofFlippedShape,whichletsthisfunctionalwaysreturnaFlippedShapevalue:

1 structFlippedShape<T:Shape>:Shape{

2 varshape:T

3 funcdraw()->String{

4 ifshapeisSquare{

5 returnshape.draw()

6 }

7 letlines=shape.draw().split(separator:"\n")

8 returnlines.reversed().joined(separator:"\n")

9 }

10 }

Therequirementtoalwaysreturnasingletypedoesn’tpreventyoufromusinggenericsinanopaquereturntype.Here’sanexampleofafunctionthatincorporatesitstypeparameterintotheunderlyingtypeofthevalueitreturns:

1 func`repeat`<T:Shape>(shape:T,count:Int)->some

Collection{

2 returnArray<T>(repeating:shape,count:count)

3 }

Inthiscase,theunderlyingtypeofthereturnvaluevariesdependingonT:Whatevershapeispassedit,repeat(shape:count:)createsandreturnsanarrayofthatshape.Nevertheless,thereturnvaluealwayshasthesameunderlyingtypeof[T],soitfollowstherequirementthatfunctionswithopaquereturntypesmustreturnvaluesofonlyasingletype.

DifferencesBetweenOpaqueTypesandProtocolTypes

Returninganopaquetypelooksverysimilartousingaprotocoltypeasthereturntypeofafunction,butthesetwokindsofreturntypedifferinwhethertheypreservetypeidentity.Anopaquetypereferstoonespecifictype,althoughthecallerofthefunctionisn’tabletoseewhichtype;aprotocoltypecanrefertoanytypethatconformstotheprotocol.Generallyspeaking,protocoltypesgiveyoumoreflexibilityabouttheunderlyingtypesofthevaluestheystore,andopaquetypesletyoumakestrongerguaranteesaboutthoseunderlyingtypes.

Forexample,here’saversionofflip(_:)thatreturnsavalueofprotocoltypeinsteadofusinganopaquereturntype:

1 funcprotoFlip<T:Shape>(_shape:T)->Shape{

2 returnFlippedShape(shape:shape)

3 }

ThisversionofprotoFlip(_:)hasthesamebodyasflip(_:),anditalwaysreturnsavalueofthesametype.Unlikeflip(_:),thevaluethatprotoFlip(_:)returnsisn’trequiredtoalwayshavethesametype—itjusthastoconformtotheShapeprotocol.Putanotherway,protoFlip(_:)makesamuchlooserAPIcontractwithitscallerthanflip(_:)makes.Itreservestheflexibilitytoreturnvaluesofmultipletypes:

1 funcprotoFlip<T:Shape>(_shape:T)->Shape{

2 ifshapeisSquare{

3 returnshape

4 }

5

6 returnFlippedShape(shape:shape)

7 }

TherevisedversionofthecodereturnsaninstanceofSquareoraninstanceofFlippedShape,dependingonwhatshapeispassedin.Twoflippedshapesreturnedbythisfunctionmighthavecompletelydifferenttypes.Othervalidversionsofthisfunctioncouldreturnvaluesofdifferenttypeswhenflippingmultipleinstancesofthesameshape.ThelessspecificreturntypeinformationfromprotoFlip(_:)meansthatmanyoperationsthatdependontypeinformationaren’tavailableonthereturnedvalue.Forexample,it’snotpossibletowritean==operatorcomparingresultsreturnedbythisfunction.

1 letprotoFlippedTriangle=protoFlip(smallTriangle)

2 letsameThing=protoFlip(smallTriangle)

3 protoFlippedTriangle==sameThing//Error

Theerroronthelastlineoftheexampleoccursforseveralreasons.TheimmediateissueisthattheShapedoesn’tincludean==operatoraspartofitsprotocolrequirements.Ifyoutryaddingone,thenextissueyou’llencounteristhatthe==operatorneedstoknowthetypesofitsleft-handandright-handarguments.ThissortofoperatorusuallytakesargumentsoftypeSelf,matchingwhateverconcretetypeadoptstheprotocol,butaddingaSelfrequirementtotheprotocoldoesn’tallowforthetypeerasurethathappenswhenyouusetheprotocolasatype.

Usingaprotocoltypeasthereturntypeforafunctiongivesyoutheflexibilitytoreturnanytypethatconformstotheprotocol.However,thecostofthatflexibilityisthatsomeoperationsaren’tpossibleonthereturnedvalues.Theexampleshowshowthe==operatorisn’tavailable—itdependsonspecifictypeinformationthatisn’tpreservedbyusingaprotocoltype.

Anotherproblemwiththisapproachisthattheshapetransformationsdon’tnest.

TheresultofflippingatriangleisavalueoftypeShape,andtheprotoFlip(_:)functiontakesanargumentofsometypethatconformstotheShapeprotocol.However,avalueofaprotocoltypedoesn’tconformtothatprotocol;thevaluereturnedbyprotoFlip(_:)doesn’tconformtoShape.ThismeanscodelikeprotoFlip(protoFlip(smallTriange))thatappliesmultipletransformationsisinvalidbecausetheflippedshapeisn’tavalidargumenttoprotoFlip(_:).

Incontrast,opaquetypespreservetheidentityoftheunderlyingtype.Swiftcaninferassociatedtypes,whichletsyouuseanopaquereturnvalueinplaceswhereaprotocoltypecan’tbeusedasareturnvalue.Forexample,here’saversionoftheContainerprotocolfromGenerics:

1 protocolContainer{

2 associatedtypeItem

3 varcount:Int{get}

4 subscript(i:Int)->Item{get}

5 }

6 extensionArray:Container{}

Youcan’tuseContainerasthereturntypeofafunctionbecausethatprotocolhasanassociatedtype.Youalsocan’tuseitasconstraintagenericreturntypebecausethereisn’tenoughinformationoutsidethefunctionbodytoinferwhatthegenerictypeneedstobe.

1 //Error:Protocolwithassociatedtypescan'tbeusedasa

returntype.

2 funcmakeProtocolContainer<T>(item:T)->Container{

3 return[item]

4 }

5

6 //Error:NotenoughinformationtoinferC.

7 funcmakeProtocolContainer<T,C:Container>(item:T)->C{

8 return[item]

9 }

UsingtheopaquetypesomeContainerasareturntypeexpressesthedesiredAPIcontract—thefunctionreturnsacontainer,butdeclinestospecifythecontainer’stype:

1 funcmakeOpaqueContainer<T>(item:T)->someContainer{

2 return[item]

3 }

4 letopaqueContainer=makeOpaqueContainer(item:12)

5 lettwelve=opaqueContainer[0]

6 print(type(of:twelve))

7 //Prints"Int"

ThetypeoftwelveisinferredtobeInt,whichillustratesthefactthattypeinferenceworkswithopaquetypes.IntheimplementationofmakeOpaqueContainer(item:),theunderlyingtypeoftheopaquecontaineris[T].Inthiscase,TisInt,sothereturnvalueisanarrayofintegersandtheItemassociatedtypeisinferredtobeInt.ThesubscriptonContainerreturnsItem,whichmeansthatthetypeoftwelveisalsoinferredtobeInt.

AutomaticReferenceCounting

SwiftusesAutomaticReferenceCounting(ARC)totrackandmanageyourapp’smemoryusage.Inmostcases,thismeansthatmemorymanagement“justworks”inSwift,andyoudonotneedtothinkaboutmemorymanagementyourself.ARCautomaticallyfreesupthememoryusedbyclassinstanceswhenthoseinstancesarenolongerneeded.

However,inafewcasesARCrequiresmoreinformationabouttherelationshipsbetweenpartsofyourcodeinordertomanagememoryforyou.ThischapterdescribesthosesituationsandshowshowyouenableARCtomanageallofyourapp’smemory.UsingARCinSwiftisverysimilartotheapproachdescribedinTransitioningtoARCReleaseNotesforusingARCwithObjective-C.

Referencecountingappliesonlytoinstancesofclasses.Structuresandenumerationsarevaluetypes,notreferencetypes,andarenotstoredandpassedbyreference.

HowARCWorks

Everytimeyoucreateanewinstanceofaclass,ARCallocatesachunkofmemorytostoreinformationaboutthatinstance.Thismemoryholdsinformationaboutthetypeoftheinstance,togetherwiththevaluesofanystoredpropertiesassociatedwiththatinstance.

Additionally,whenaninstanceisnolongerneeded,ARCfreesupthememoryusedbythatinstancesothatthememorycanbeusedforotherpurposesinstead.Thisensuresthatclassinstancesdonottakeupspaceinmemorywhentheyarenolongerneeded.

However,ifARCweretodeallocateaninstancethatwasstillinuse,itwouldnolongerbepossibletoaccessthatinstance’sproperties,orcallthatinstance’smethods.Indeed,ifyoutriedtoaccesstheinstance,yourappwouldmostlikelycrash.

Tomakesurethatinstancesdon’tdisappearwhiletheyarestillneeded,ARCtrackshowmanyproperties,constants,andvariablesarecurrentlyreferringtoeachclassinstance.ARCwillnotdeallocateaninstanceaslongasatleastoneactivereferencetothatinstancestillexists.

Tomakethispossible,wheneveryouassignaclassinstancetoaproperty,constant,orvariable,thatproperty,constant,orvariablemakesastrongreferencetotheinstance.Thereferenceiscalleda“strong”referencebecauseitkeepsafirmholdonthatinstance,anddoesnotallowittobedeallocatedforaslongasthatstrongreferenceremains.

ARCinAction

Here’sanexampleofhowAutomaticReferenceCountingworks.ThisexamplestartswithasimpleclasscalledPerson,whichdefinesastoredconstantpropertycalledname:

1 classPerson{

2 letname:String

3 init(name:String){

4 self.name=name

5 print("\(name)isbeinginitialized")

6 }

7 deinit{

8 print("\(name)isbeingdeinitialized")

9 }

10 }

ThePersonclasshasaninitializerthatsetstheinstance’snamepropertyandprintsamessagetoindicatethatinitializationisunderway.ThePersonclassalsohasadeinitializerthatprintsamessagewhenaninstanceoftheclassisdeallocated.

ThenextcodesnippetdefinesthreevariablesoftypePerson?,whichareusedtosetupmultiplereferencestoanewPersoninstanceinsubsequentcodesnippets.Becausethesevariablesareofanoptionaltype(Person?,notPerson),theyareautomaticallyinitializedwithavalueofnil,anddonotcurrentlyreferenceaPersoninstance.

1 varreference1:Person?

2 varreference2:Person?

3 varreference3:Person?

YoucannowcreateanewPersoninstanceandassignittooneofthesethreevariables:

1 reference1=Person(name:"JohnAppleseed")

2 //Prints"JohnAppleseedisbeinginitialized"

Notethatthemessage"JohnAppleseedisbeinginitialized"isprintedatthepointthatyoucallthePersonclass’sinitializer.Thisconfirmsthatinitializationhastakenplace.

BecausethenewPersoninstancehasbeenassignedtothereference1variable,thereisnowastrongreferencefromreference1tothenewPersoninstance.Becausethereisatleastonestrongreference,ARCmakessurethatthisPersoniskeptinmemoryandisnotdeallocated.

IfyouassignthesamePersoninstancetotwomorevariables,twomorestrongreferencestothatinstanceareestablished:

1 reference2=reference1

2 reference3=reference1

TherearenowthreestrongreferencestothissinglePersoninstance.

Ifyoubreaktwoofthesestrongreferences(includingtheoriginalreference)byassigningniltotwoofthevariables,asinglestrongreferenceremains,andthePersoninstanceisnotdeallocated:

1 reference1=nil

2 reference2=nil

ARCdoesnotdeallocatethePersoninstanceuntilthethirdandfinalstrongreferenceisbroken,atwhichpointit’sclearthatyouarenolongerusingthePersoninstance:

1 reference3=nil

2 //Prints"JohnAppleseedisbeingdeinitialized"

StrongReferenceCyclesBetweenClassInstances

Intheexamplesabove,ARCisabletotrackthenumberofreferencestothenewPersoninstanceyoucreateandtodeallocatethatPersoninstancewhenit’snolongerneeded.

However,it’spossibletowritecodeinwhichaninstanceofaclassnevergetstoapointwhereithaszerostrongreferences.Thiscanhappeniftwoclassinstancesholdastrongreferencetoeachother,suchthateachinstancekeepstheotheralive.Thisisknownasastrongreferencecycle.

Youresolvestrongreferencecyclesbydefiningsomeoftherelationshipsbetweenclassesasweakorunownedreferencesinsteadofasstrongreferences.ThisprocessisdescribedinResolvingStrongReferenceCyclesBetweenClassInstances.However,beforeyoulearnhowtoresolveastrongreferencecycle,it’susefultounderstandhowsuchacycleiscaused.

Here’sanexampleofhowastrongreferencecyclecanbecreatedbyaccident.ThisexampledefinestwoclassescalledPersonandApartment,whichmodelablockofapartmentsanditsresidents:

1 classPerson{

2 letname:String

3 init(name:String){self.name=name}

4 varapartment:Apartment?

5 deinit{print("\(name)isbeingdeinitialized")}

6 }

7

8 classApartment{

9 letunit:String

10 init(unit:String){self.unit=unit}

11 vartenant:Person?

12 deinit{print("Apartment\(unit)isbeingdeinitialized")

}

13 }

EveryPersoninstancehasanamepropertyoftypeStringandanoptionalapartmentpropertythatisinitiallynil.Theapartmentpropertyisoptional,becauseapersonmaynotalwayshaveanapartment.

Similarly,everyApartmentinstancehasaunitpropertyoftypeStringandhasanoptionaltenantpropertythatisinitiallynil.Thetenantpropertyisoptionalbecauseanapartmentmaynotalwayshaveatenant.

Bothoftheseclassesalsodefineadeinitializer,whichprintsthefactthataninstanceofthatclassisbeingdeinitialized.ThisenablesyoutoseewhetherinstancesofPersonandApartmentarebeingdeallocatedasexpected.

Thisnextcodesnippetdefinestwovariablesofoptionaltypecalledjohnandunit4A,whichwillbesettoaspecificApartmentandPersoninstancebelow.Bothofthesevariableshaveaninitialvalueofnil,byvirtueofbeingoptional:

1 varjohn:Person?

2 varunit4A:Apartment?

YoucannowcreateaspecificPersoninstanceandApartmentinstanceandassignthesenewinstancestothejohnandunit4Avariables:

1 john=Person(name:"JohnAppleseed")

2 unit4A=Apartment(unit:"4A")

Here’showthestrongreferenceslookaftercreatingandassigningthesetwoinstances.ThejohnvariablenowhasastrongreferencetothenewPersoninstance,andtheunit4AvariablehasastrongreferencetothenewApartmentinstance:

Youcannowlinkthetwoinstancestogethersothatthepersonhasanapartment,andtheapartmenthasatenant.Notethatanexclamationmark(!)isusedtounwrapandaccesstheinstancesstoredinsidethejohnandunit4Aoptionalvariables,sothatthepropertiesofthoseinstancescanbeset:

1 john!.apartment=unit4A

2 unit4A!.tenant=john

Here’showthestrongreferenceslookafteryoulinkthetwoinstancestogether:

Unfortunately,linkingthesetwoinstancescreatesastrongreferencecycle

betweenthem.ThePersoninstancenowhasastrongreferencetotheApartmentinstance,andtheApartmentinstancehasastrongreferencetothePersoninstance.Therefore,whenyoubreakthestrongreferencesheldbythejohnandunit4Avariables,thereferencecountsdonotdroptozero,andtheinstancesarenotdeallocatedbyARC:

1 john=nil

2 unit4A=nil

Notethatneitherdeinitializerwascalledwhenyousetthesetwovariablestonil.ThestrongreferencecyclepreventsthePersonandApartmentinstancesfromeverbeingdeallocated,causingamemoryleakinyourapp.

Here’showthestrongreferenceslookafteryousetthejohnandunit4Avariablestonil:

ThestrongreferencesbetweenthePersoninstanceandtheApartmentinstanceremainandcannotbebroken.

ResolvingStrongReferenceCyclesBetweenClassInstances

Swiftprovidestwowaystoresolvestrongreferencecycleswhenyouworkwithpropertiesofclasstype:weakreferencesandunownedreferences.

Weakandunownedreferencesenableoneinstanceinareferencecycletorefertotheotherinstancewithoutkeepingastrongholdonit.Theinstancescanthen

refertoeachotherwithoutcreatingastrongreferencecycle.

Useaweakreferencewhentheotherinstancehasashorterlifetime—thatis,whentheotherinstancecanbedeallocatedfirst.IntheApartmentexampleabove,it’sappropriateforanapartmenttobeabletohavenotenantatsomepointinitslifetime,andsoaweakreferenceisanappropriatewaytobreakthereferencecycleinthiscase.Incontrast,useanunownedreferencewhentheotherinstancehasthesamelifetimeoralongerlifetime.

WeakReferencesAweakreferenceisareferencethatdoesnotkeepastrongholdontheinstanceitrefersto,andsodoesnotstopARCfromdisposingofthereferencedinstance.Thisbehaviorpreventsthereferencefrombecomingpartofastrongreferencecycle.Youindicateaweakreferencebyplacingtheweakkeywordbeforeapropertyorvariabledeclaration.

Becauseaweakreferencedoesnotkeepastrongholdontheinstanceitrefersto,it’spossibleforthatinstancetobedeallocatedwhiletheweakreferenceisstillreferringtoit.Therefore,ARCautomaticallysetsaweakreferencetonilwhentheinstancethatitreferstoisdeallocated.And,becauseweakreferencesneedtoallowtheirvaluetobechangedtonilatruntime,theyarealwaysdeclaredasvariables,ratherthanconstants,ofanoptionaltype.

Youcancheckfortheexistenceofavalueintheweakreference,justlikeanyotheroptionalvalue,andyouwillneverendupwithareferencetoaninvalidinstancethatnolongerexists.

NOTE

Propertyobserversaren’tcalledwhenARCsetsaweakreferencetonil.

TheexamplebelowisidenticaltothePersonandApartmentexamplefromabove,withoneimportantdifference.Thistimearound,theApartmenttype’stenantpropertyisdeclaredasaweakreference:

1 classPerson{

2 letname:String

3 init(name:String){self.name=name}

4 varapartment:Apartment?

5 deinit{print("\(name)isbeingdeinitialized")}

6 }

7

8 classApartment{

9 letunit:String

10 init(unit:String){self.unit=unit}

11 weakvartenant:Person?

12 deinit{print("Apartment\(unit)isbeingdeinitialized")

}

13 }

Thestrongreferencesfromthetwovariables(johnandunit4A)andthelinksbetweenthetwoinstancesarecreatedasbefore:

1 varjohn:Person?

2 varunit4A:Apartment?

3

4 john=Person(name:"JohnAppleseed")

5 unit4A=Apartment(unit:"4A")

6

7 john!.apartment=unit4A

8 unit4A!.tenant=john

Here’showthereferenceslooknowthatyou’velinkedthetwoinstancestogether:

ThePersoninstancestillhasastrongreferencetotheApartmentinstance,buttheApartmentinstancenowhasaweakreferencetothePersoninstance.Thismeansthatwhenyoubreakthestrongreferenceheldbythejohnvariablebysettingittonil,therearenomorestrongreferencestothePersoninstance:

1 john=nil

2 //Prints"JohnAppleseedisbeingdeinitialized"

BecausetherearenomorestrongreferencestothePersoninstance,it’sdeallocatedandthetenantpropertyissettonil:

TheonlyremainingstrongreferencetotheApartmentinstanceisfromtheunit4Avariable.Ifyoubreakthatstrongreference,therearenomorestrongreferencestotheApartmentinstance:

1 unit4A=nil

2 //Prints"Apartment4Aisbeingdeinitialized"

BecausetherearenomorestrongreferencestotheApartmentinstance,ittoois

deallocated:

NOTE

Insystemsthatusegarbagecollection,weakpointersaresometimesusedtoimplementasimplecachingmechanismbecauseobjectswithnostrongreferencesaredeallocatedonlywhenmemorypressuretriggersgarbagecollection.However,withARC,valuesaredeallocatedassoonastheirlaststrongreferenceisremoved,makingweakreferencesunsuitableforsuchapurpose.

UnownedReferencesLikeaweakreference,anunownedreferencedoesnotkeepastrongholdontheinstanceitrefersto.Unlikeaweakreference,however,anunownedreferenceisusedwhentheotherinstancehasthesamelifetimeoralongerlifetime.Youindicateanunownedreferencebyplacingtheunownedkeywordbeforeapropertyorvariabledeclaration.

Anunownedreferenceisexpectedtoalwayshaveavalue.Asaresult,ARCneversetsanunownedreference’svaluetonil,whichmeansthatunownedreferencesaredefinedusingnon-optionaltypes.

IMPORTANT

Useanunownedreferenceonlywhenyouaresurethatthereferencealwaysreferstoaninstancethathasnotbeendeallocated.

Ifyoutrytoaccessthevalueofanunownedreferenceafterthatinstancehasbeendeallocated,you’llgetaruntimeerror.

Thefollowingexampledefinestwoclasses,CustomerandCreditCard,whichmodelabankcustomerandapossiblecreditcardforthatcustomer.Thesetwo

classeseachstoreaninstanceoftheotherclassasaproperty.Thisrelationshiphasthepotentialtocreateastrongreferencecycle.

TherelationshipbetweenCustomerandCreditCardisslightlydifferentfromtherelationshipbetweenApartmentandPersonseenintheweakreferenceexampleabove.Inthisdatamodel,acustomermayormaynothaveacreditcard,butacreditcardwillalwaysbeassociatedwithacustomer.ACreditCardinstanceneveroutlivestheCustomerthatitrefersto.Torepresentthis,theCustomerclasshasanoptionalcardproperty,buttheCreditCardclasshasanunowned(andnon-optional)customerproperty.

Furthermore,anewCreditCardinstancecanonlybecreatedbypassinganumbervalueandacustomerinstancetoacustomCreditCardinitializer.ThisensuresthataCreditCardinstancealwayshasacustomerinstanceassociatedwithitwhentheCreditCardinstanceiscreated.

Becauseacreditcardwillalwayshaveacustomer,youdefineitscustomerpropertyasanunownedreference,toavoidastrongreferencecycle:

1 classCustomer{

2 letname:String

3 varcard:CreditCard?

4 init(name:String){

5 self.name=name

6 }

7 deinit{print("\(name)isbeingdeinitialized")}

8 }

9

10 classCreditCard{

11 letnumber:UInt64

12 unownedletcustomer:Customer

13 init(number:UInt64,customer:Customer){

14 self.number=number

15 self.customer=customer

16 }

17 deinit{print("Card#\(number)isbeingdeinitialized")}

18 }

NOTE

ThenumberpropertyoftheCreditCardclassisdefinedwithatypeofUInt64ratherthanInt,toensurethatthenumberproperty’scapacityislargeenoughtostorea16-digitcardnumberonboth32-bitand64-bitsystems.

ThisnextcodesnippetdefinesanoptionalCustomervariablecalledjohn,whichwillbeusedtostoreareferencetoaspecificcustomer.Thisvariablehasaninitialvalueofnil,byvirtueofbeingoptional:

varjohn:Customer?

YoucannowcreateaCustomerinstance,anduseittoinitializeandassignanewCreditCardinstanceasthatcustomer’scardproperty:

1 john=Customer(name:"JohnAppleseed")

2 john!.card=CreditCard(number:1234_5678_9012_3456,customer:

john!)

Here’showthereferenceslook,nowthatyou’velinkedthetwoinstances:

TheCustomerinstancenowhasastrongreferencetotheCreditCardinstance,andtheCreditCardinstancehasanunownedreferencetotheCustomerinstance.

Becauseoftheunownedcustomerreference,whenyoubreakthestrongreferenceheldbythejohnvariable,therearenomorestrongreferencestotheCustomerinstance:

BecausetherearenomorestrongreferencestotheCustomerinstance,it’sdeallocated.Afterthishappens,therearenomorestrongreferencestotheCreditCardinstance,andittooisdeallocated:

1 john=nil

2 //Prints"JohnAppleseedisbeingdeinitialized"

3 //Prints"Card#1234567890123456isbeingdeinitialized"

ThefinalcodesnippetaboveshowsthatthedeinitializersfortheCustomerinstanceandCreditCardinstancebothprinttheir“deinitialized”messagesafterthejohnvariableissettonil.

NOTE

Theexamplesaboveshowhowtousesafeunownedreferences.Swiftalsoprovidesunsafeunownedreferencesforcaseswhereyouneedtodisableruntimesafetychecks—forexample,forperformancereasons.Aswithallunsafeoperations,youtakeontheresponsibilityforcheckingthatcodeforsafety.

Youindicateanunsafeunownedreferencebywritingunowned(unsafe).Ifyoutrytoaccessanunsafeunownedreferenceaftertheinstancethatitreferstoisdeallocated,yourprogramwilltrytoaccessthememorylocationwheretheinstanceusedtobe,whichisanunsafeoperation.

UnownedReferencesandImplicitlyUnwrappedOptional

PropertiesTheexamplesforweakandunownedreferencesabovecovertwoofthemorecommonscenariosinwhichit’snecessarytobreakastrongreferencecycle.

ThePersonandApartmentexampleshowsasituationwheretwoproperties,bothofwhichareallowedtobenil,havethepotentialtocauseastrongreferencecycle.Thisscenarioisbestresolvedwithaweakreference.

TheCustomerandCreditCardexampleshowsasituationwhereonepropertythatisallowedtobenilandanotherpropertythatcannotbenilhavethepotentialtocauseastrongreferencecycle.Thisscenarioisbestresolvedwithanunownedreference.

However,thereisathirdscenario,inwhichbothpropertiesshouldalwayshaveavalue,andneitherpropertyshouldeverbenilonceinitializationiscomplete.Inthisscenario,it’susefultocombineanunownedpropertyononeclasswithanimplicitlyunwrappedoptionalpropertyontheotherclass.

Thisenablesbothpropertiestobeaccesseddirectly(withoutoptionalunwrapping)onceinitializationiscomplete,whilestillavoidingareferencecycle.Thissectionshowsyouhowtosetupsucharelationship.

Theexamplebelowdefinestwoclasses,CountryandCity,eachofwhichstoresaninstanceoftheotherclassasaproperty.Inthisdatamodel,everycountrymustalwayshaveacapitalcity,andeverycitymustalwaysbelongtoacountry.Torepresentthis,theCountryclasshasacapitalCityproperty,andtheCityclasshasacountryproperty:

1 classCountry{

2 letname:String

3 varcapitalCity:City!

4 init(name:String,capitalName:String){

5 self.name=name

6 self.capitalCity=City(name:capitalName,country:

self)

7 }

8 }

9

10 classCity{

11 letname:String

12 unownedletcountry:Country

13 init(name:String,country:Country){

14 self.name=name

15 self.country=country

16 }

17 }

Tosetuptheinterdependencybetweenthetwoclasses,theinitializerforCitytakesaCountryinstance,andstoresthisinstanceinitscountryproperty.

TheinitializerforCityiscalledfromwithintheinitializerforCountry.However,theinitializerforCountrycannotpassselftotheCityinitializeruntilanewCountryinstanceisfullyinitialized,asdescribedinTwo-PhaseInitialization.

Tocopewiththisrequirement,youdeclarethecapitalCitypropertyofCountryasanimplicitlyunwrappedoptionalproperty,indicatedbytheexclamationmarkattheendofitstypeannotation(City!).ThismeansthatthecapitalCitypropertyhasadefaultvalueofnil,likeanyotheroptional,butcanbeaccessedwithouttheneedtounwrapitsvalueasdescribedinImplicitlyUnwrappedOptionals.

BecausecapitalCityhasadefaultnilvalue,anewCountryinstanceisconsideredfullyinitializedassoonastheCountryinstancesetsitsnamepropertywithinitsinitializer.ThismeansthattheCountryinitializercanstarttoreferenceandpassaroundtheimplicitselfpropertyassoonasthenamepropertyisset.TheCountryinitializercanthereforepassselfasoneoftheparametersfortheCityinitializerwhentheCountryinitializerissettingitsowncapitalCityproperty.

AllofthismeansthatyoucancreatetheCountryandCityinstancesinasinglestatement,withoutcreatingastrongreferencecycle,andthecapitalCity

propertycanbeaccesseddirectly,withoutneedingtouseanexclamationmarktounwrapitsoptionalvalue:

1 varcountry=Country(name:"Canada",capitalName:"Ottawa")

2 print("\(country.name)'scapitalcityiscalled\

(country.capitalCity.name)")

3 //Prints"Canada'scapitalcityiscalledOttawa"

Intheexampleabove,theuseofanimplicitlyunwrappedoptionalmeansthatallofthetwo-phaseclassinitializerrequirementsaresatisfied.ThecapitalCitypropertycanbeusedandaccessedlikeanon-optionalvalueonceinitializationiscomplete,whilestillavoidingastrongreferencecycle.

StrongReferenceCyclesforClosures

Yousawabovehowastrongreferencecyclecanbecreatedwhentwoclassinstancepropertiesholdastrongreferencetoeachother.Youalsosawhowtouseweakandunownedreferencestobreakthesestrongreferencecycles.

Astrongreferencecyclecanalsooccurifyouassignaclosuretoapropertyofaclassinstance,andthebodyofthatclosurecapturestheinstance.Thiscapturemightoccurbecausetheclosure’sbodyaccessesapropertyoftheinstance,suchasself.someProperty,orbecausetheclosurecallsamethodontheinstance,suchasself.someMethod().Ineithercase,theseaccessescausetheclosureto“capture”self,creatingastrongreferencecycle.

Thisstrongreferencecycleoccursbecauseclosures,likeclasses,arereferencetypes.Whenyouassignaclosuretoaproperty,youareassigningareferencetothatclosure.Inessence,it’sthesameproblemasabove—twostrongreferencesarekeepingeachotheralive.However,ratherthantwoclassinstances,thistimeit’saclassinstanceandaclosurethatarekeepingeachotheralive.

Swiftprovidesanelegantsolutiontothisproblem,knownasaclosurecapturelist.However,beforeyoulearnhowtobreakastrongreferencecyclewitha

closurecapturelist,it’susefultounderstandhowsuchacyclecanbecaused.

Theexamplebelowshowshowyoucancreateastrongreferencecyclewhenusingaclosurethatreferencesself.ThisexampledefinesaclasscalledHTMLElement,whichprovidesasimplemodelforanindividualelementwithinanHTMLdocument:

1 classHTMLElement{

2

3 letname:String

4 lettext:String?

5

6 lazyvarasHTML:()->String={

7 iflettext=self.text{

8 return"<\(self.name)>\(text)</\(self.name)>"

9 }else{

10 return"<\(self.name)/>"

11 }

12 }

13

14 init(name:String,text:String?=nil){

15 self.name=name

16 self.text=text

17 }

18

19 deinit{

20 print("\(name)isbeingdeinitialized")

21 }

22

23 }

TheHTMLElementclassdefinesanameproperty,whichindicatesthenameoftheelement,suchas"h1"foraheadingelement,"p"foraparagraphelement,or"br"foralinebreakelement.HTMLElementalsodefinesanoptionaltextproperty,whichyoucansettoastringthatrepresentsthetexttoberenderedwithinthatHTMLelement.

Inadditiontothesetwosimpleproperties,theHTMLElementclassdefinesalazypropertycalledasHTML.ThispropertyreferencesaclosurethatcombinesnameandtextintoanHTMLstringfragment.TheasHTMLpropertyisoftype()->String,or“afunctionthattakesnoparameters,andreturnsaStringvalue”.

Bydefault,theasHTMLpropertyisassignedaclosurethatreturnsastringrepresentationofanHTMLtag.Thistagcontainstheoptionaltextvalueifitexists,ornotextcontentiftextdoesnotexist.Foraparagraphelement,theclosurewouldreturn"<p>sometext</p>"or"<p/>",dependingonwhetherthetextpropertyequals"sometext"ornil.

TheasHTMLpropertyisnamedandusedsomewhatlikeaninstancemethod.However,becauseasHTMLisaclosurepropertyratherthananinstancemethod,youcanreplacethedefaultvalueoftheasHTMLpropertywithacustomclosure,ifyouwanttochangetheHTMLrenderingforaparticularHTMLelement.

Forexample,theasHTMLpropertycouldbesettoaclosurethatdefaultstosometextifthetextpropertyisnil,inordertopreventtherepresentationfromreturninganemptyHTMLtag:

1 letheading=HTMLElement(name:"h1")

2 letdefaultText="somedefaulttext"

3 heading.asHTML={

4 return"<\(heading.name)>\(heading.text??defaultText)</\

(heading.name)>"

5 }

6 print(heading.asHTML())

7 //Prints"<h1>somedefaulttext</h1>"

NOTE

TheasHTMLpropertyisdeclaredasalazyproperty,becauseit’sonlyneededifandwhentheelementactuallyneedstoberenderedasastringvalueforsomeHTMLoutputtarget.ThefactthatasHTMLisalazypropertymeansthatyoucanrefertoselfwithinthedefaultclosure,becausethelazypropertywillnotbeaccesseduntilafterinitializationhasbeencompletedandselfisknowntoexist.

TheHTMLElementclassprovidesasingleinitializer,whichtakesanameargumentand(ifdesired)atextargumenttoinitializeanewelement.Theclassalsodefinesadeinitializer,whichprintsamessagetoshowwhenanHTMLElementinstanceisdeallocated.

Here’showyouusetheHTMLElementclasstocreateandprintanewinstance:

1 varparagraph:HTMLElement?=HTMLElement(name:"p",text:

"hello,world")

2 print(paragraph!.asHTML())

3 //Prints"<p>hello,world</p>"

NOTE

TheparagraphvariableaboveisdefinedasanoptionalHTMLElement,sothatitcanbesettonilbelowtodemonstratethepresenceofastrongreferencecycle.

Unfortunately,theHTMLElementclass,aswrittenabove,createsastrongreferencecyclebetweenanHTMLElementinstanceandtheclosureusedforitsdefaultasHTMLvalue.Here’showthecyclelooks:

Theinstance’sasHTMLpropertyholdsastrongreferencetoitsclosure.However,becausetheclosurereferstoselfwithinitsbody(asawaytoreferenceself.nameandself.text),theclosurecapturesself,whichmeansthatitholdsastrongreferencebacktotheHTMLElementinstance.Astrongreferencecycleiscreatedbetweenthetwo.(Formoreinformationaboutcapturingvaluesinaclosure,seeCapturingValues.)

NOTE

Eventhoughtheclosurereferstoselfmultipletimes,itonlycapturesonestrongreferencetotheHTMLElementinstance.

IfyousettheparagraphvariabletonilandbreakitsstrongreferencetotheHTMLElementinstance,neithertheHTMLElementinstancenoritsclosurearedeallocated,becauseofthestrongreferencecycle:

paragraph=nil

NotethatthemessageintheHTMLElementdeinitializerisnotprinted,whichshowsthattheHTMLElementinstanceisnotdeallocated.

ResolvingStrongReferenceCyclesforClosures

Youresolveastrongreferencecyclebetweenaclosureandaclassinstancebydefiningacapturelistaspartoftheclosure’sdefinition.Acapturelistdefinestherulestousewhencapturingoneormorereferencetypeswithintheclosure’sbody.Aswithstrongreferencecyclesbetweentwoclassinstances,youdeclareeachcapturedreferencetobeaweakorunownedreferenceratherthanastrongreference.Theappropriatechoiceofweakorunowneddependsontherelationshipsbetweenthedifferentpartsofyourcode.

NOTE

Swiftrequiresyoutowriteself.somePropertyorself.someMethod()(ratherthanjustsomePropertyorsomeMethod())wheneveryourefertoamemberofselfwithinaclosure.Thishelpsyourememberthatit’spossibletocaptureselfbyaccident.

DefiningaCaptureListEachiteminacapturelistisapairingoftheweakorunownedkeywordwithareferencetoaclassinstance(suchasself)oravariableinitializedwithsomevalue(suchasdelegate=self.delegate).Thesepairingsarewrittenwithinapairofsquarebraces,separatedbycommas.

Placethecapturelistbeforeaclosure’sparameterlistandreturntypeiftheyareprovided:

1 lazyvarsomeClosure={

2 [unownedself,weakdelegate=self.delegate]

3 (index:Int,stringToProcess:String)->Stringin

4 //closurebodygoeshere

5 }

Ifaclosuredoesnotspecifyaparameterlistorreturntypebecausetheycanbeinferredfromcontext,placethecapturelistattheverystartoftheclosure,followedbytheinkeyword:

1 lazyvarsomeClosure={

2 [unownedself,weakdelegate=self.delegate]in

3 //closurebodygoeshere

4 }

WeakandUnownedReferencesDefineacaptureinaclosureasanunownedreferencewhentheclosureandtheinstanceitcaptureswillalwaysrefertoeachother,andwillalwaysbedeallocatedatthesametime.

Conversely,defineacaptureasaweakreferencewhenthecapturedreferencemaybecomenilatsomepointinthefuture.Weakreferencesarealwaysofanoptionaltype,andautomaticallybecomenilwhentheinstancetheyreferenceisdeallocated.Thisenablesyoutocheckfortheirexistencewithintheclosure’s

body.

NOTE

Ifthecapturedreferencewillneverbecomenil,itshouldalwaysbecapturedasanunownedreference,ratherthanaweakreference.

AnunownedreferenceistheappropriatecapturemethodtousetoresolvethestrongreferencecycleintheHTMLElementexamplefromStrongReferenceCyclesforClosuresabove.Here’showyouwritetheHTMLElementclasstoavoidthecycle:

1 classHTMLElement{

2

3 letname:String

4 lettext:String?

5

6 lazyvarasHTML:()->String={

7 [unownedself]in

8 iflettext=self.text{

9 return"<\(self.name)>\(text)</\(self.name)>"

10 }else{

11 return"<\(self.name)/>"

12 }

13 }

14

15 init(name:String,text:String?=nil){

16 self.name=name

17 self.text=text

18 }

19

20 deinit{

21 print("\(name)isbeingdeinitialized")

22 }

23

24 }

ThisimplementationofHTMLElementisidenticaltothepreviousimplementation,apartfromtheadditionofacapturelistwithintheasHTMLclosure.Inthiscase,thecapturelistis[unownedself],whichmeans“captureselfasanunownedreferenceratherthanastrongreference”.

YoucancreateandprintanHTMLElementinstanceasbefore:

1 varparagraph:HTMLElement?=HTMLElement(name:"p",text:

"hello,world")

2 print(paragraph!.asHTML())

3 //Prints"<p>hello,world</p>"

Here’showthereferenceslookwiththecapturelistinplace:

Thistime,thecaptureofselfbytheclosureisanunownedreference,anddoesnotkeepastrongholdontheHTMLElementinstanceithascaptured.Ifyousetthestrongreferencefromtheparagraphvariabletonil,theHTMLElementinstanceisdeallocated,ascanbeseenfromtheprintingofitsdeinitializermessageintheexamplebelow:

1 paragraph=nil

2 //Prints"pisbeingdeinitialized"

Formoreinformationaboutcapturelists,seeCaptureLists.

MemorySafety

Bydefault,Swiftpreventsunsafebehaviorfromhappeninginyourcode.Forexample,Swiftensuresthatvariablesareinitializedbeforethey’reused,memoryisn’taccessedafterit’sbeendeallocated,andarrayindicesarecheckedforout-of-boundserrors.

Swiftalsomakessurethatmultipleaccessestothesameareaofmemorydon’tconflict,byrequiringcodethatmodifiesalocationinmemorytohaveexclusiveaccesstothatmemory.BecauseSwiftmanagesmemoryautomatically,mostofthetimeyoudon’thavetothinkaboutaccessingmemoryatall.However,it’simportanttounderstandwherepotentialconflictscanoccur,soyoucanavoidwritingcodethathasconflictingaccesstomemory.Ifyourcodedoescontainconflicts,you’llgetacompile-timeorruntimeerror.

UnderstandingConflictingAccesstoMemory

Accesstomemoryhappensinyourcodewhenyoudothingslikesetthevalueofavariableorpassanargumenttoafunction.Forexample,thefollowingcodecontainsbothareadaccessandawriteaccess:

1 //Awriteaccesstothememorywhereoneisstored.

2 varone=1

3

4 //Areadaccessfromthememorywhereoneisstored.

5 print("We'renumber\(one)!")

Aconflictingaccesstomemorycanoccurwhendifferentpartsofyourcodearetryingtoaccessthesamelocationinmemoryatthesametime.Multipleaccessestoalocationinmemoryatthesametimecanproduceunpredictableorinconsistentbehavior.InSwift,therearewaystomodifyavaluethatspanseverallinesofcode,makingitpossibletoattempttoaccessavalueinthe

middleofitsownmodification.

Youcanseeasimilarproblembythinkingabouthowyouupdateabudgetthat’swrittenonapieceofpaper.Updatingthebudgetisatwo-stepprocess:Firstyouaddtheitems’namesandprices,andthenyouchangethetotalamounttoreflecttheitemscurrentlyonthelist.Beforeandaftertheupdate,youcanreadanyinformationfromthebudgetandgetacorrectanswer,asshowninthefigurebelow.

Whileyou’readdingitemstothebudget,it’sinatemporary,invalidstatebecausethetotalamounthasn’tbeenupdatedtoreflectthenewlyaddeditems.Readingthetotalamountduringtheprocessofaddinganitemgivesyouincorrectinformation.

Thisexamplealsodemonstratesachallengeyoumayencounterwhenfixingconflictingaccesstomemory:Therearesometimesmultiplewaystofixtheconflictthatproducedifferentanswers,andit’snotalwaysobviouswhichansweriscorrect.Inthisexample,dependingonwhetheryouwantedtheoriginaltotalamountortheupdatedtotalamount,either$5or$320couldbethecorrectanswer.Beforeyoucanfixtheconflictingaccess,youhavetodeterminewhatitwasintendedtodo.

NOTE

Ifyou’vewrittenconcurrentormultithreadedcode,conflictingaccesstomemorymightbeafamiliarproblem.However,theconflictingaccessdiscussedherecanhappenonasinglethreadanddoesn’tinvolveconcurrentormultithreadedcode.

Ifyouhaveconflictingaccesstomemoryfromwithinasinglethread,Swiftguaranteesthatyou’llgetanerrorateithercompiletimeorruntime.Formultithreadedcode,useThreadSanitizertohelpdetectconflictingaccessacrossthreads.

CharacteristicsofMemoryAccessTherearethreecharacteristicsofmemoryaccesstoconsiderinthecontextofconflictingaccess:whethertheaccessisareadorawrite,thedurationoftheaccess,andthelocationinmemorybeingaccessed.Specifically,aconflictoccursifyouhavetwoaccessesthatmeetallofthefollowingconditions:

Atleastoneisawriteaccess.

Theyaccessthesamelocationinmemory.

Theirdurationsoverlap.

Thedifferencebetweenareadandwriteaccessisusuallyobvious:awriteaccesschangesthelocationinmemory,butareadaccessdoesn’t.Thelocationinmemoryreferstowhatisbeingaccessed—forexample,avariable,constant,orproperty.Thedurationofamemoryaccessiseitherinstantaneousorlong-term.

Anaccessisinstantaneousifit’snotpossibleforothercodetorunafterthataccessstartsbutbeforeitends.Bytheirnature,twoinstantaneousaccessescan’thappenatthesametime.Mostmemoryaccessisinstantaneous.Forexample,allthereadandwriteaccessesinthecodelistingbelowareinstantaneous:

1 funconeMore(thannumber:Int)->Int{

2 returnnumber+1

3 }

4

5 varmyNumber=1

6 myNumber=oneMore(than:myNumber)

7 print(myNumber)

8 //Prints"2"

However,thereareseveralwaystoaccessmemory,calledlong-termaccesses,thatspantheexecutionofothercode.Thedifferencebetweeninstantaneousaccessandlong-termaccessisthatit’spossibleforothercodetorunafteralong-termaccessstartsbutbeforeitends,whichiscalledoverlap.Along-termaccesscanoverlapwithotherlong-termaccessesandinstantaneousaccesses.

Overlappingaccessesappearprimarilyincodethatusesin-outparametersinfunctionsandmethodsormutatingmethodsofastructure.ThespecifickindsofSwiftcodethatuselong-termaccessesarediscussedinthesectionsbelow.

ConflictingAccesstoIn-OutParameters

Afunctionhaslong-termwriteaccesstoallofitsin-outparameters.Thewriteaccessforanin-outparameterstartsafterallofthenon-in-outparametershavebeenevaluatedandlastsfortheentiredurationofthatfunctioncall.Iftherearemultiplein-outparameters,thewriteaccessesstartinthesameorderastheparametersappear.

Oneconsequenceofthislong-termwriteaccessisthatyoucan’taccesstheoriginalvariablethatwaspassedasin-out,evenifscopingrulesandaccesscontrolwouldotherwisepermitit—anyaccesstotheoriginalcreatesaconflict.Forexample:

1 varstepSize=1

2

3 funcincrement(_number:inoutInt){

4 number+=stepSize

5 }

6

7 increment(&stepSize)

8 //Error:conflictingaccessestostepSize

Inthecodeabove,stepSizeisaglobalvariable,anditisnormallyaccessiblefromwithinincrement(_:).However,thereadaccesstostepSizeoverlapswiththewriteaccesstonumber.Asshowninthefigurebelow,bothnumberandstepSizerefertothesamelocationinmemory.Thereadandwriteaccessesrefertothesamememoryandtheyoverlap,producingaconflict.

OnewaytosolvethisconflictistomakeanexplicitcopyofstepSize:

1 //Makeanexplicitcopy.

2 varcopyOfStepSize=stepSize

3 increment(&copyOfStepSize)

4

5 //Updatetheoriginal.

6 stepSize=copyOfStepSize

7 //stepSizeisnow2

WhenyoumakeacopyofstepSizebeforecallingincrement(_:),it’sclearthatthevalueofcopyOfStepSizeisincrementedbythecurrentstepsize.Thereadaccessendsbeforethewriteaccessstarts,sothereisn’taconflict.

Anotherconsequenceoflong-termwriteaccesstoin-outparametersisthatpassingasinglevariableastheargumentformultiplein-outparametersofthesamefunctionproducesaconflict.Forexample:

1 funcbalance(_x:inoutInt,_y:inoutInt){

2 letsum=x+y

3 x=sum/2

4 y=sum-x

5 }

6 varplayerOneScore=42

7 varplayerTwoScore=30

8 balance(&playerOneScore,&playerTwoScore)//OK

9 balance(&playerOneScore,&playerOneScore)

10 //Error:conflictingaccessestoplayerOneScore

Thebalance(_:_:)functionabovemodifiesitstwoparameterstodividethetotalvalueevenlybetweenthem.CallingitwithplayerOneScoreandplayerTwoScoreasargumentsdoesn’tproduceaconflict—therearetwowriteaccessesthatoverlapintime,buttheyaccessdifferentlocationsinmemory.Incontrast,passingplayerOneScoreasthevalueforbothparametersproducesaconflictbecauseittriestoperformtwowriteaccessestothesamelocationinmemoryatthesametime.

NOTE

Becauseoperatorsarefunctions,theycanalsohavelong-termaccessestotheirin-outparameters.Forexample,ifbalance(_:_:)wasanoperatorfunctionnamed<^>,writingplayerOneScore<^>playerOneScorewouldresultinthesameconflictasbalance(&playerOneScore,&playerOneScore).

ConflictingAccesstoselfinMethods

Amutatingmethodonastructurehaswriteaccesstoselfforthedurationofthemethodcall.Forexample,consideragamewhereeachplayerhasahealthamount,whichdecreaseswhentakingdamage,andanenergyamount,whichdecreaseswhenusingspecialabilities.

1 structPlayer{

2 varname:String

3 varhealth:Int

4 varenergy:Int

5

6 staticletmaxHealth=10

7 mutatingfuncrestoreHealth(){

8 health=Player.maxHealth

9 }

10 }

IntherestoreHealth()methodabove,awriteaccesstoselfstartsatthebeginningofthemethodandlastsuntilthemethodreturns.Inthiscase,there’snoothercodeinsiderestoreHealth()thatcouldhaveanoverlappingaccesstothepropertiesofaPlayerinstance.TheshareHealth(with:)methodbelowtakesanotherPlayerinstanceasanin-outparameter,creatingthepossibilityofoverlappingaccesses.

1 extensionPlayer{

2 mutatingfuncshareHealth(withteammate:inoutPlayer){

3 balance(&teammate.health,&health)

4 }

5 }

6

7 varoscar=Player(name:"Oscar",health:10,energy:10)

8 varmaria=Player(name:"Maria",health:5,energy:10)

9 oscar.shareHealth(with:&maria)//OK

Intheexampleabove,callingtheshareHealth(with:)methodforOscar’splayertosharehealthwithMaria’splayerdoesn’tcauseaconflict.There’sawriteaccesstooscarduringthemethodcallbecauseoscaristhevalueofselfinamutatingmethod,andthere’sawriteaccesstomariaforthesamedurationbecausemariawaspassedasanin-outparameter.Asshowninthefigurebelow,theyaccessdifferentlocationsinmemory.Eventhoughthetwowriteaccessesoverlapintime,theydon’tconflict.

However,ifyoupassoscarastheargumenttoshareHealth(with:),there’saconflict:

1 oscar.shareHealth(with:&oscar)

2 //Error:conflictingaccessestooscar

Themutatingmethodneedswriteaccesstoselfforthedurationofthemethod,andthein-outparameterneedswriteaccesstoteammateforthesameduration.Withinthemethod,bothselfandteammaterefertothesamelocationinmemory—asshowninthefigurebelow.Thetwowriteaccessesrefertothesamememoryandtheyoverlap,producingaconflict.

ConflictingAccesstoProperties

Typeslikestructures,tuples,andenumerationsaremadeupofindividualconstituentvalues,suchasthepropertiesofastructureortheelementsofatuple.Becausethesearevaluetypes,mutatinganypieceofthevaluemutatesthewholevalue,meaningreadorwriteaccesstooneofthepropertiesrequiresreadorwriteaccesstothewholevalue.Forexample,overlappingwriteaccessestotheelementsofatupleproducesaconflict:

1 varplayerInformation=(health:10,energy:20)

2 balance(&playerInformation.health,&playerInformation.energy)

3 //Error:conflictingaccesstopropertiesofplayerInformation

Intheexampleabove,callingbalance(_:_:)ontheelementsofatupleproduces

aconflictbecausethereareoverlappingwriteaccessestoplayerInformation.BothplayerInformation.healthandplayerInformation.energyarepassedasin-outparameters,whichmeansbalance(_:_:)needswriteaccesstothemforthedurationofthefunctioncall.Inbothcases,awriteaccesstothetupleelementrequiresawriteaccesstotheentiretuple.ThismeanstherearetwowriteaccessestoplayerInformationwithdurationsthatoverlap,causingaconflict.

Thecodebelowshowsthatthesameerrorappearsforoverlappingwriteaccessestothepropertiesofastructurethat’sstoredinaglobalvariable.

1 varholly=Player(name:"Holly",health:10,energy:10)

2 balance(&holly.health,&holly.energy)//Error

Inpractice,mostaccesstothepropertiesofastructurecanoverlapsafely.Forexample,ifthevariablehollyintheexampleaboveischangedtoalocalvariableinsteadofaglobalvariable,thecompilercanprovethatoverlappingaccesstostoredpropertiesofthestructureissafe:

1 funcsomeFunction(){

2 varoscar=Player(name:"Oscar",health:10,energy:10)

3 balance(&oscar.health,&oscar.energy)//OK

4 }

Intheexampleabove,Oscar’shealthandenergyarepassedasthetwoin-outparameterstobalance(_:_:).Thecompilercanprovethatmemorysafetyispreservedbecausethetwostoredpropertiesdon’tinteractinanyway.

Therestrictionagainstoverlappingaccesstopropertiesofastructureisn’talwaysnecessarytopreservememorysafety.Memorysafetyisthedesiredguarantee,butexclusiveaccessisastricterrequirementthanmemorysafety—whichmeanssomecodepreservesmemorysafety,eventhoughitviolatesexclusiveaccesstomemory.Swiftallowsthismemory-safecodeifthecompilercanprovethatthenonexclusiveaccesstomemoryisstillsafe.Specifically,itcanprovethatoverlappingaccesstopropertiesofastructureissafeifthefollowingconditionsapply:

You’reaccessingonlystoredpropertiesofaninstance,notcomputedpropertiesorclassproperties.

Thestructureisthevalueofalocalvariable,notaglobalvariable.

Thestructureiseithernotcapturedbyanyclosures,orit’scapturedonlybynonescapingclosures.

Ifthecompilercan’tprovetheaccessissafe,itdoesn’tallowtheaccess.

AccessControl

Accesscontrolrestrictsaccesstopartsofyourcodefromcodeinothersourcefilesandmodules.Thisfeatureenablesyoutohidetheimplementationdetailsofyourcode,andtospecifyapreferredinterfacethroughwhichthatcodecanbeaccessedandused.

Youcanassignspecificaccesslevelstoindividualtypes(classes,structures,andenumerations),aswellastoproperties,methods,initializers,andsubscriptsbelongingtothosetypes.Protocolscanberestrictedtoacertaincontext,ascanglobalconstants,variables,andfunctions.

Inadditiontoofferingvariouslevelsofaccesscontrol,Swiftreducestheneedtospecifyexplicitaccesscontrollevelsbyprovidingdefaultaccesslevelsfortypicalscenarios.Indeed,ifyouarewritingasingle-targetapp,youmaynotneedtospecifyexplicitaccesscontrollevelsatall.

NOTE

Thevariousaspectsofyourcodethatcanhaveaccesscontrolappliedtothem(properties,types,functions,andsoon)arereferredtoas“entities”inthesectionsbelow,forbrevity.

ModulesandSourceFiles

Swift’saccesscontrolmodelisbasedontheconceptofmodulesandsourcefiles.

Amoduleisasingleunitofcodedistribution—aframeworkorapplicationthatisbuiltandshippedasasingleunitandthatcanbeimportedbyanothermodulewithSwift’simportkeyword.

Eachbuildtarget(suchasanappbundleorframework)inXcodeistreatedasaseparatemoduleinSwift.Ifyougrouptogetheraspectsofyourapp’scodeasastand-aloneframework—perhapstoencapsulateandreusethatcodeacrossmultipleapplications—theneverythingyoudefinewithinthatframeworkwillbe

partofaseparatemodulewhenit’simportedandusedwithinanapp,orwhenit’susedwithinanotherframework.

AsourcefileisasingleSwiftsourcecodefilewithinamodule(ineffect,asinglefilewithinanapporframework).Althoughit’scommontodefineindividualtypesinseparatesourcefiles,asinglesourcefilecancontaindefinitionsformultipletypes,functions,andsoon.

AccessLevels

Swiftprovidesfivedifferentaccesslevelsforentitieswithinyourcode.Theseaccesslevelsarerelativetothesourcefileinwhichanentityisdefined,andalsorelativetothemodulethatsourcefilebelongsto.

Openaccessandpublicaccessenableentitiestobeusedwithinanysourcefilefromtheirdefiningmodule,andalsoinasourcefilefromanothermodulethatimportsthedefiningmodule.Youtypicallyuseopenorpublicaccesswhenspecifyingthepublicinterfacetoaframework.Thedifferencebetweenopenandpublicaccessisdescribedbelow.

Internalaccessenablesentitiestobeusedwithinanysourcefilefromtheirdefiningmodule,butnotinanysourcefileoutsideofthatmodule.Youtypicallyuseinternalaccesswhendefininganapp’soraframework’sinternalstructure.

File-privateaccessrestrictstheuseofanentitytoitsowndefiningsourcefile.Usefile-privateaccesstohidetheimplementationdetailsofaspecificpieceoffunctionalitywhenthosedetailsareusedwithinanentirefile.

Privateaccessrestrictstheuseofanentitytotheenclosingdeclaration,andtoextensionsofthatdeclarationthatareinthesamefile.Useprivateaccesstohidetheimplementationdetailsofaspecificpieceoffunctionalitywhenthosedetailsareusedonlywithinasingledeclaration.

Openaccessisthehighest(leastrestrictive)accesslevelandprivateaccessisthelowest(mostrestrictive)accesslevel.

Openaccessappliesonlytoclassesandclassmembers,anditdiffersfrompublicaccessbyallowingcodeoutsidethemoduletosubclassandoverride,asdiscussedbelowinSubclassing.Markingaclassasopenexplicitlyindicatesthatyou’veconsideredtheimpactofcodefromothermodulesusingthatclassasasuperclass,andthatyou’vedesignedyourclass’scodeaccordingly.

GuidingPrincipleofAccessLevelsAccesslevelsinSwiftfollowanoverallguidingprinciple:Noentitycanbedefinedintermsofanotherentitythathasalower(morerestrictive)accesslevel.

Forexample:

Apublicvariablecan’tbedefinedashavinganinternal,file-private,orprivatetype,becausethetypemightnotbeavailableeverywherethatthepublicvariableisused.

Afunctioncan’thaveahigheraccesslevelthanitsparametertypesandreturntype,becausethefunctioncouldbeusedinsituationswhereitsconstituenttypesareunavailabletothesurroundingcode.

Thespecificimplicationsofthisguidingprinciplefordifferentaspectsofthelanguagearecoveredindetailbelow.

DefaultAccessLevelsAllentitiesinyourcode(withafewspecificexceptions,asdescribedlaterinthischapter)haveadefaultaccesslevelofinternalifyoudon’tspecifyanexplicitaccesslevelyourself.Asaresult,inmanycasesyoudon’tneedtospecifyanexplicitaccesslevelinyourcode.

AccessLevelsforSingle-TargetAppsWhenyouwriteasimplesingle-targetapp,thecodeinyourappistypicallyself-containedwithintheappanddoesn’tneedtobemadeavailableoutsideofthe

app’smodule.Thedefaultaccesslevelofinternalalreadymatchesthisrequirement.Therefore,youdon’tneedtospecifyacustomaccesslevel.Youmay,however,wanttomarksomepartsofyourcodeasfileprivateorprivateinordertohidetheirimplementationdetailsfromothercodewithintheapp’smodule.

AccessLevelsforFrameworksWhenyoudevelopaframework,markthepublic-facinginterfacetothatframeworkasopenorpublicsothatitcanbeviewedandaccessedbyothermodules,suchasanappthatimportstheframework.Thispublic-facinginterfaceistheapplicationprogramminginterface(orAPI)fortheframework.

NOTE

Anyinternalimplementationdetailsofyourframeworkcanstillusethedefaultaccesslevelofinternal,orcanbemarkedasprivateorfileprivateifyouwanttohidethemfromotherpartsoftheframework’sinternalcode.Youneedtomarkanentityasopenorpubliconlyifyouwantittobecomepartofyourframework’sAPI.

AccessLevelsforUnitTestTargetsWhenyouwriteanappwithaunittesttarget,thecodeinyourappneedstobemadeavailabletothatmoduleinordertobetested.Bydefault,onlyentitiesmarkedasopenorpublicareaccessibletoothermodules.However,aunittesttargetcanaccessanyinternalentity,ifyoumarktheimportdeclarationforaproductmodulewiththe@testableattributeandcompilethatproductmodulewithtestingenabled.

AccessControlSyntax

Definetheaccesslevelforanentitybyplacingoneoftheopen,public,internal,fileprivate,orprivatemodifiersatthebeginningoftheentity’sdeclaration.

1 publicclassSomePublicClass{}

2 internalclassSomeInternalClass{}

3 fileprivateclassSomeFilePrivateClass{}

4 privateclassSomePrivateClass{}

5

6 publicvarsomePublicVariable=0

7 internalletsomeInternalConstant=0

8 fileprivatefuncsomeFilePrivateFunction(){}

9 privatefuncsomePrivateFunction(){}

Unlessotherwisespecified,thedefaultaccesslevelisinternal,asdescribedinDefaultAccessLevels.ThismeansthatSomeInternalClassandsomeInternalConstantcanbewrittenwithoutanexplicitaccess-levelmodifier,andwillstillhaveanaccesslevelofinternal:

1 classSomeInternalClass{}//implicitlyinternal

2 letsomeInternalConstant=0//implicitlyinternal

CustomTypes

Ifyouwanttospecifyanexplicitaccesslevelforacustomtype,dosoatthepointthatyoudefinethetype.Thenewtypecanthenbeusedwhereveritsaccesslevelpermits.Forexample,ifyoudefineafile-privateclass,thatclasscanonlybeusedasthetypeofaproperty,orasafunctionparameterorreturntype,inthesourcefileinwhichthefile-privateclassisdefined.

Theaccesscontrollevelofatypealsoaffectsthedefaultaccesslevelofthattype’smembers(itsproperties,methods,initializers,andsubscripts).Ifyoudefineatype’saccesslevelasprivateorfileprivate,thedefaultaccesslevelofitsmemberswillalsobeprivateorfileprivate.Ifyoudefineatype’saccesslevelasinternalorpublic(orusethedefaultaccesslevelofinternalwithoutspecifyinganaccesslevelexplicitly),thedefaultaccesslevelofthetype’s

memberswillbeinternal.

IMPORTANT

Apublictypedefaultstohavinginternalmembers,notpublicmembers.Ifyouwantatypemembertobepublic,youmustexplicitlymarkitassuch.Thisrequirementensuresthatthepublic-facingAPIforatypeissomethingyouoptintopublishing,andavoidspresentingtheinternalworkingsofatypeaspublicAPIbymistake.

1 publicclassSomePublicClass{//explicitly

publicclass

2 publicvarsomePublicProperty=0//explicitly

publicclassmember

3 varsomeInternalProperty=0//implicitly

internalclassmember

4 fileprivatefuncsomeFilePrivateMethod(){}//explicitly

file-privateclassmember

5 privatefuncsomePrivateMethod(){}//explicitly

privateclassmember

6 }

7

8 classSomeInternalClass{//implicitly

internalclass

9 varsomeInternalProperty=0//implicitly

internalclassmember

10 fileprivatefuncsomeFilePrivateMethod(){}//explicitly

file-privateclassmember

11 privatefuncsomePrivateMethod(){}//explicitly

privateclassmember

12 }

13

14 fileprivateclassSomeFilePrivateClass{//explicitly

file-privateclass

15 funcsomeFilePrivateMethod(){}//implicitly

file-privateclassmember

16 privatefuncsomePrivateMethod(){}//explicitly

privateclassmember

17 }

18

19 privateclassSomePrivateClass{//explicitly

privateclass

20 funcsomePrivateMethod(){}//implicitly

privateclassmember

21 }

TupleTypesTheaccesslevelforatupletypeisthemostrestrictiveaccesslevelofalltypesusedinthattuple.Forexample,ifyoucomposeatuplefromtwodifferenttypes,onewithinternalaccessandonewithprivateaccess,theaccesslevelforthatcompoundtupletypewillbeprivate.

NOTE

Tupletypesdon’thaveastandalonedefinitioninthewaythatclasses,structures,enumerations,andfunctionsdo.Atupletype’saccesslevelisdeterminedautomaticallyfromthetypesthatmakeupthetupletype,andcan’tbespecifiedexplicitly.

FunctionTypesTheaccesslevelforafunctiontypeiscalculatedasthemostrestrictiveaccesslevelofthefunction’sparametertypesandreturntype.Youmustspecifytheaccesslevelexplicitlyaspartofthefunction’sdefinitionifthefunction’scalculatedaccessleveldoesn’tmatchthecontextualdefault.

TheexamplebelowdefinesaglobalfunctioncalledsomeFunction(),withoutprovidingaspecificaccess-levelmodifierforthefunctionitself.Youmightexpectthisfunctiontohavethedefaultaccesslevelof“internal”,butthisisn’tthecase.Infact,someFunction()won’tcompileaswrittenbelow:

1 funcsomeFunction()->(SomeInternalClass,SomePrivateClass){

2 //functionimplementationgoeshere

3 }

Thefunction’sreturntypeisatupletypecomposedfromtwoofthecustomclassesdefinedaboveinCustomTypes.Oneoftheseclassesisdefinedasinternal,andtheotherisdefinedasprivate.Therefore,theoverallaccesslevelofthecompoundtupletypeisprivate(theminimumaccesslevelofthetuple’sconstituenttypes).

Becausethefunction’sreturntypeisprivate,youmustmarkthefunction’soverallaccesslevelwiththeprivatemodifierforthefunctiondeclarationtobevalid:

1 privatefuncsomeFunction()->(SomeInternalClass,

SomePrivateClass){

2 //functionimplementationgoeshere

3 }

It’snotvalidtomarkthedefinitionofsomeFunction()withthepublicorinternalmodifiers,ortousethedefaultsettingofinternal,becausepublicorinternalusersofthefunctionmightnothaveappropriateaccesstotheprivateclassusedinthefunction’sreturntype.

EnumerationTypesTheindividualcasesofanenumerationautomaticallyreceivethesameaccesslevelastheenumerationtheybelongto.Youcan’tspecifyadifferentaccesslevelforindividualenumerationcases.

Intheexamplebelow,theCompassPointenumerationhasanexplicitaccesslevelofpublic.Theenumerationcasesnorth,south,east,andwestthereforealsohaveanaccesslevelofpublic:

1 publicenumCompassPoint{

2 casenorth

3 casesouth

4 caseeast

5 casewest

6 }

RawValuesandAssociatedValues

Thetypesusedforanyrawvaluesorassociatedvaluesinanenumerationdefinitionmusthaveanaccesslevelatleastashighastheenumeration’saccesslevel.Forexample,youcan’tuseaprivatetypeastheraw-valuetypeofanenumerationwithaninternalaccesslevel.

NestedTypesTheaccesslevelofanestedtypeisthesameasitscontainingtype,unlessthecontainingtypeispublic.Nestedtypesdefinedwithinapublictypehaveanautomaticaccesslevelofinternal.Ifyouwantanestedtypewithinapublictypetobepubliclyavailable,youmustexplicitlydeclarethenestedtypeaspublic.

Subclassing

Youcansubclassanyclassthatcanbeaccessedinthecurrentaccesscontextandthat’sdefinedinthesamemoduleasthesubclass.Youcanalsosubclassanyopenclassthat’sdefinedinadifferentmodule.Asubclasscan’thaveahigheraccesslevelthanitssuperclass—forexample,youcan’twriteapublicsubclassofaninternalsuperclass.

Inaddition,forclassesthataredefinedinthesamemodule,youcanoverrideanyclassmember(method,property,initializer,orsubscript)that’svisibleinacertainaccesscontext.Forclassesthataredefinedinanothermodule,youcanoverrideanyopenclassmember.

Anoverridecanmakeaninheritedclassmembermoreaccessiblethanitssuperclassversion.Intheexamplebelow,classAisapublicclasswithafile-privatemethodcalledsomeMethod().ClassBisasubclassofA,withareducedaccesslevelof“internal”.Nonetheless,classBprovidesanoverrideofsomeMethod()withanaccesslevelof“internal”,whichishigherthantheoriginalimplementationofsomeMethod():

1 publicclassA{

2 fileprivatefuncsomeMethod(){}

3 }

4

5 internalclassB:A{

6 overrideinternalfuncsomeMethod(){}

7 }

It’sevenvalidforasubclassmembertocallasuperclassmemberthathasloweraccesspermissionsthanthesubclassmember,aslongasthecalltothesuperclass’smembertakesplacewithinanallowedaccesslevelcontext(thatis,withinthesamesourcefileasthesuperclassforafile-privatemembercall,orwithinthesamemoduleasthesuperclassforaninternalmembercall):

1 publicclassA{

2 fileprivatefuncsomeMethod(){}

3 }

4

5 internalclassB:A{

6 overrideinternalfuncsomeMethod(){

7 super.someMethod()

8 }

9 }

BecausesuperclassAandsubclassBaredefinedinthesamesourcefile,it’svalidfortheBimplementationofsomeMethod()tocallsuper.someMethod().

Constants,Variables,Properties,andSubscripts

Aconstant,variable,orpropertycan’tbemorepublicthanitstype.It’snotvalidtowriteapublicpropertywithaprivatetype,forexample.Similarly,asubscriptcan’tbemorepublicthaneitheritsindextypeorreturntype.

Ifaconstant,variable,property,orsubscriptmakesuseofaprivatetype,theconstant,variable,property,orsubscriptmustalsobemarkedasprivate:

privatevarprivateInstance=SomePrivateClass()

GettersandSettersGettersandsettersforconstants,variables,properties,andsubscriptsautomaticallyreceivethesameaccesslevelastheconstant,variable,property,orsubscripttheybelongto.

Youcangiveasetteraloweraccesslevelthanitscorrespondinggetter,torestricttheread-writescopeofthatvariable,property,orsubscript.Youassignaloweraccesslevelbywritingfileprivate(set),private(set),orinternal(set)beforethevarorsubscriptintroducer.

NOTE

Thisruleappliestostoredpropertiesaswellascomputedproperties.Eventhoughyoudon’twriteanexplicitgetterandsetterforastoredproperty,Swiftstillsynthesizesanimplicitgetterandsetterforyoutoprovideaccesstothestoredproperty’sbackingstorage.Usefileprivate(set),private(set),andinternal(set)tochangetheaccesslevelofthissynthesizedsetterinexactlythesamewayasforanexplicitsetterinacomputedproperty.

TheexamplebelowdefinesastructurecalledTrackedString,whichkeepstrackofthenumberoftimesastringpropertyismodified:

1 structTrackedString{

2 private(set)varnumberOfEdits=0

3 varvalue:String=""{

4 didSet{

5 numberOfEdits+=1

6 }

7 }

8 }

TheTrackedStringstructuredefinesastoredstringpropertycalledvalue,withaninitialvalueof""(anemptystring).ThestructurealsodefinesastoredintegerpropertycallednumberOfEdits,whichisusedtotrackthenumberoftimesthatvalueismodified.ThismodificationtrackingisimplementedwithadidSetpropertyobserveronthevalueproperty,whichincrementsnumberOfEditseverytimethevaluepropertyissettoanewvalue.

TheTrackedStringstructureandthevaluepropertydon’tprovideanexplicitaccess-levelmodifier,andsotheybothreceivethedefaultaccesslevelofinternal.However,theaccesslevelforthenumberOfEditspropertyismarkedwithaprivate(set)modifiertoindicatethattheproperty’sgetterstillhasthedefaultaccesslevelofinternal,butthepropertyissettableonlyfromwithincodethat’spartoftheTrackedStringstructure.ThisenablesTrackedStringtomodifythenumberOfEditspropertyinternally,buttopresentthepropertyasaread-onlypropertywhenit’susedoutsidethestructure’sdefinition.

IfyoucreateaTrackedStringinstanceandmodifyitsstringvalueafewtimes,youcanseethenumberOfEditspropertyvalueupdatetomatchthenumberofmodifications:

1 varstringToEdit=TrackedString()

2 stringToEdit.value="Thisstringwillbetracked."

3 stringToEdit.value+="Thiseditwillincrement

numberOfEdits."

4 stringToEdit.value+="Sowillthisone."

5 print("Thenumberofeditsis\(stringToEdit.numberOfEdits)")

6 //Prints"Thenumberofeditsis3"

AlthoughyoucanquerythecurrentvalueofthenumberOfEditspropertyfromwithinanothersourcefile,youcan’tmodifythepropertyfromanothersourcefile.ThisrestrictionprotectstheimplementationdetailsoftheTrackedStringedit-trackingfunctionality,whilestillprovidingconvenientaccesstoanaspectofthatfunctionality.

Notethatyoucanassignanexplicitaccesslevelforbothagetterandasetterifrequired.TheexamplebelowshowsaversionoftheTrackedStringstructureinwhichthestructureisdefinedwithanexplicitaccesslevelofpublic.Thestructure’smembers(includingthenumberOfEditsproperty)thereforehaveaninternalaccesslevelbydefault.Youcanmakethestructure’snumberOfEditspropertygetterpublic,anditspropertysetterprivate,bycombiningthepublicandprivate(set)access-levelmodifiers:

1 publicstructTrackedString{

2 publicprivate(set)varnumberOfEdits=0

3 publicvarvalue:String=""{

4 didSet{

5 numberOfEdits+=1

6 }

7 }

8 publicinit(){}

9 }

Initializers

Custominitializerscanbeassignedanaccesslevellessthanorequaltothetype

thattheyinitialize.Theonlyexceptionisforrequiredinitializers(asdefinedinRequiredInitializers).Arequiredinitializermusthavethesameaccesslevelastheclassitbelongsto.

Aswithfunctionandmethodparameters,thetypesofaninitializer’sparameterscan’tbemoreprivatethantheinitializer’sownaccesslevel.

DefaultInitializersAsdescribedinDefaultInitializers,Swiftautomaticallyprovidesadefaultinitializerwithoutanyargumentsforanystructureorbaseclassthatprovidesdefaultvaluesforallofitspropertiesanddoesn’tprovideatleastoneinitializeritself.

Adefaultinitializerhasthesameaccesslevelasthetypeitinitializes,unlessthattypeisdefinedaspublic.Foratypethatisdefinedaspublic,thedefaultinitializerisconsideredinternal.Ifyouwantapublictypetobeinitializablewithano-argumentinitializerwhenusedinanothermodule,youmustexplicitlyprovideapublicno-argumentinitializeryourselfaspartofthetype’sdefinition.

DefaultMemberwiseInitializersforStructureTypesThedefaultmemberwiseinitializerforastructuretypeisconsideredprivateifanyofthestructure’sstoredpropertiesareprivate.Likewise,ifanyofthestructure’sstoredpropertiesarefileprivate,theinitializerisfileprivate.Otherwise,theinitializerhasanaccesslevelofinternal.

Aswiththedefaultinitializerabove,ifyouwantapublicstructuretypetobeinitializablewithamemberwiseinitializerwhenusedinanothermodule,youmustprovideapublicmemberwiseinitializeryourselfaspartofthetype’sdefinition.

Protocols

Ifyouwanttoassignanexplicitaccessleveltoaprotocoltype,dosoatthepointthatyoudefinetheprotocol.Thisenablesyoutocreateprotocolsthatcanonlybeadoptedwithinacertainaccesscontext.

Theaccesslevelofeachrequirementwithinaprotocoldefinitionisautomaticallysettothesameaccesslevelastheprotocol.Youcan’tsetaprotocolrequirementtoadifferentaccesslevelthantheprotocolitsupports.Thisensuresthatalloftheprotocol’srequirementswillbevisibleonanytypethatadoptstheprotocol.

NOTE

Ifyoudefineapublicprotocol,theprotocol’srequirementsrequireapublicaccesslevelforthoserequirementswhenthey’reimplemented.Thisbehaviorisdifferentfromothertypes,whereapublictypedefinitionimpliesanaccesslevelofinternalforthetype’smembers.

ProtocolInheritanceIfyoudefineanewprotocolthatinheritsfromanexistingprotocol,thenewprotocolcanhaveatmostthesameaccesslevelastheprotocolitinheritsfrom.Forexample,youcan’twriteapublicprotocolthatinheritsfromaninternalprotocol.

ProtocolConformanceAtypecanconformtoaprotocolwithaloweraccesslevelthanthetypeitself.Forexample,youcandefineapublictypethatcanbeusedinothermodules,butwhoseconformancetoaninternalprotocolcanonlybeusedwithintheinternalprotocol’sdefiningmodule.

Thecontextinwhichatypeconformstoaparticularprotocolistheminimumofthetype’saccesslevelandtheprotocol’saccesslevel.Forexample,ifatypeispublic,butaprotocolitconformstoisinternal,thetype’sconformancetothatprotocolisalsointernal.

Whenyouwriteorextendatypetoconformtoaprotocol,youmustensurethat

thetype’simplementationofeachprotocolrequirementhasatleastthesameaccesslevelasthetype’sconformancetothatprotocol.Forexample,ifapublictypeconformstoaninternalprotocol,thetype’simplementationofeachprotocolrequirementmustbeatleastinternal.

NOTE

InSwift,asinObjective-C,protocolconformanceisglobal—itisn’tpossibleforatypetoconformtoaprotocolintwodifferentwayswithinthesameprogram.

Extensions

Youcanextendaclass,structure,orenumerationinanyaccesscontextinwhichtheclass,structure,orenumerationisavailable.Anytypemembersaddedinanextensionhavethesamedefaultaccesslevelastypemembersdeclaredintheoriginaltypebeingextended.Ifyouextendapublicorinternaltype,anynewtypemembersyouaddhaveadefaultaccesslevelofinternal.Ifyouextendafile-privatetype,anynewtypemembersyouaddhaveadefaultaccessleveloffileprivate.Ifyouextendaprivatetype,anynewtypemembersyouaddhaveadefaultaccesslevelofprivate.

Alternatively,youcanmarkanextensionwithanexplicitaccess-levelmodifier(forexample,private)tosetanewdefaultaccesslevelforallmembersdefinedwithintheextension.Thisnewdefaultcanstillbeoverriddenwithintheextensionforindividualtypemembers.

Youcan’tprovideanexplicitaccess-levelmodifierforanextensionifyou’reusingthatextensiontoaddprotocolconformance.Instead,theprotocol’sownaccesslevelisusedtoprovidethedefaultaccesslevelforeachprotocolrequirementimplementationwithintheextension.

PrivateMembersinExtensionsExtensionsthatareinthesamefileastheclass,structure,orenumerationthattheyextendbehaveasifthecodeintheextensionhadbeenwrittenaspartofthe

originaltype’sdeclaration.Asaresult,youcan:

Declareaprivatememberintheoriginaldeclaration,andaccessthatmemberfromextensionsinthesamefile.

Declareaprivatememberinoneextension,andaccessthatmemberfromanotherextensioninthesamefile.

Declareaprivatememberinanextension,andaccessthatmemberfromtheoriginaldeclarationinthesamefile.

Thisbehaviormeansyoucanuseextensionsinthesamewaytoorganizeyourcode,whetherornotyourtypeshaveprivateentities.Forexample,giventhefollowingsimpleprotocol:

1 protocolSomeProtocol{

2 funcdoSomething()

3 }

Youcanuseanextensiontoaddprotocolconformance,likethis:

1 structSomeStruct{

2 privatevarprivateVariable=12

3 }

4

5 extensionSomeStruct:SomeProtocol{

6 funcdoSomething(){

7 print(privateVariable)

8 }

9 }

Generics

Theaccesslevelforagenerictypeorgenericfunctionistheminimumoftheaccesslevelofthegenerictypeorfunctionitselfandtheaccesslevelofanytypeconstraintsonitstypeparameters.

TypeAliases

Anytypealiasesyoudefinearetreatedasdistincttypesforthepurposesofaccesscontrol.Atypealiascanhaveanaccesslevellessthanorequaltotheaccesslevelofthetypeitaliases.Forexample,aprivatetypealiascanaliasaprivate,file-private,internal,public,oropentype,butapublictypealiascan’taliasaninternal,file-private,orprivatetype.

NOTE

Thisrulealsoappliestotypealiasesforassociatedtypesusedtosatisfyprotocolconformances.

AdvancedOperators

InadditiontotheoperatorsdescribedinBasicOperators,Swiftprovidesseveraladvancedoperatorsthatperformmorecomplexvaluemanipulation.TheseincludeallofthebitwiseandbitshiftingoperatorsyouwillbefamiliarwithfromCandObjective-C.

UnlikearithmeticoperatorsinC,arithmeticoperatorsinSwiftdonotoverflowbydefault.Overflowbehavioristrappedandreportedasanerror.Tooptintooverflowbehavior,useSwift’ssecondsetofarithmeticoperatorsthatoverflowbydefault,suchastheoverflowadditionoperator(&+).Alloftheseoverflowoperatorsbeginwithanampersand(&).

Whenyoudefineyourownstructures,classes,andenumerations,itcanbeusefultoprovideyourownimplementationsofthestandardSwiftoperatorsforthesecustomtypes.Swiftmakesiteasytoprovidetailoredimplementationsoftheseoperatorsandtodetermineexactlywhattheirbehaviorshouldbeforeachtypeyoucreate.

You’renotlimitedtothepredefinedoperators.Swiftgivesyouthefreedomtodefineyourowncustominfix,prefix,postfix,andassignmentoperators,withcustomprecedenceandassociativityvalues.Theseoperatorscanbeusedandadoptedinyourcodelikeanyofthepredefinedoperators,andyoucanevenextendexistingtypestosupportthecustomoperatorsyoudefine.

BitwiseOperators

Bitwiseoperatorsenableyoutomanipulatetheindividualrawdatabitswithinadatastructure.Theyareoftenusedinlow-levelprogramming,suchasgraphicsprogramminganddevicedrivercreation.Bitwiseoperatorscanalsobeusefulwhenyouworkwithrawdatafromexternalsources,suchasencodinganddecodingdataforcommunicationoveracustomprotocol.

SwiftsupportsallofthebitwiseoperatorsfoundinC,asdescribedbelow.

BitwiseNOTOperatorThebitwiseNOToperator(~)invertsallbitsinanumber:

ThebitwiseNOToperatorisaprefixoperator,andappearsimmediatelybeforethevalueitoperateson,withoutanywhitespace:

1 letinitialBits:UInt8=0b00001111

2 letinvertedBits=~initialBits//equals11110000

UInt8integershaveeightbitsandcanstoreanyvaluebetween0and255.ThisexampleinitializesaUInt8integerwiththebinaryvalue00001111,whichhasitsfirstfourbitssetto0,anditssecondfourbitssetto1.Thisisequivalenttoadecimalvalueof15.

ThebitwiseNOToperatoristhenusedtocreateanewconstantcalledinvertedBits,whichisequaltoinitialBits,butwithallofthebitsinverted.Zerosbecomeones,andonesbecomezeros.ThevalueofinvertedBitsis11110000,whichisequaltoanunsigneddecimalvalueof240.

BitwiseANDOperatorThebitwiseANDoperator(&)combinesthebitsoftwonumbers.Itreturnsanewnumberwhosebitsaresetto1onlyifthebitswereequalto1inbothinputnumbers:

Intheexamplebelow,thevaluesoffirstSixBitsandlastSixBitsbothhavefourmiddlebitsequalto1.ThebitwiseANDoperatorcombinesthemtomakethenumber00111100,whichisequaltoanunsigneddecimalvalueof60:

1 letfirstSixBits:UInt8=0b11111100

2 letlastSixBits:UInt8=0b00111111

3 letmiddleFourBits=firstSixBits&lastSixBits//equals

00111100

BitwiseOROperatorThebitwiseORoperator(|)comparesthebitsoftwonumbers.Theoperatorreturnsanewnumberwhosebitsaresetto1ifthebitsareequalto1ineitherinputnumber:

Intheexamplebelow,thevaluesofsomeBitsandmoreBitshavedifferentbitsset

to1.ThebitwiseORoperatorcombinesthemtomakethenumber11111110,whichequalsanunsigneddecimalof254:

1 letsomeBits:UInt8=0b10110010

2 letmoreBits:UInt8=0b01011110

3 letcombinedbits=someBits|moreBits//equals11111110

BitwiseXOROperatorThebitwiseXORoperator,or“exclusiveORoperator”(^),comparesthebitsoftwonumbers.Theoperatorreturnsanewnumberwhosebitsaresetto1wheretheinputbitsaredifferentandaresetto0wheretheinputbitsarethesame:

Intheexamplebelow,thevaluesoffirstBitsandotherBitseachhaveabitsetto1inalocationthattheotherdoesnot.ThebitwiseXORoperatorsetsbothofthesebitsto1initsoutputvalue.AlloftheotherbitsinfirstBitsandotherBitsmatchandaresetto0intheoutputvalue:

1 letfirstBits:UInt8=0b00010100

2 letotherBits:UInt8=0b00000101

3 letoutputBits=firstBits^otherBits//equals00010001

BitwiseLeftandRightShiftOperators

Thebitwiseleftshiftoperator(<<)andbitwiserightshiftoperator(>>)moveallbitsinanumbertotheleftortherightbyacertainnumberofplaces,accordingtotherulesdefinedbelow.

Bitwiseleftandrightshiftshavetheeffectofmultiplyingordividinganintegerbyafactoroftwo.Shiftinganinteger’sbitstotheleftbyonepositiondoublesitsvalue,whereasshiftingittotherightbyonepositionhalvesitsvalue.

ShiftingBehaviorforUnsignedIntegers

Thebit-shiftingbehaviorforunsignedintegersisasfollows:

1. Existingbitsaremovedtotheleftorrightbytherequestednumberofplaces.

2. Anybitsthataremovedbeyondtheboundsoftheinteger’sstoragearediscarded.

3. Zerosareinsertedinthespacesleftbehindaftertheoriginalbitsaremovedtotheleftorright.

Thisapproachisknownasalogicalshift.

Theillustrationbelowshowstheresultsof11111111<<1(whichis11111111shiftedtotheleftby1place),and11111111>>1(whichis11111111shiftedtotherightby1place).Bluenumbersareshifted,graynumbersarediscarded,andorangezerosareinserted:

Here’showbitshiftinglooksinSwiftcode:

1 letshiftBits:UInt8=4//00000100inbinary

2 shiftBits<<1//00001000

3 shiftBits<<2//00010000

4 shiftBits<<5//10000000

5 shiftBits<<6//00000000

6 shiftBits>>2//00000001

Youcanusebitshiftingtoencodeanddecodevalueswithinotherdatatypes:

1 letpink:UInt32=0xCC6699

2 letredComponent=(pink&0xFF0000)>>16//redComponent

is0xCC,or204

3 letgreenComponent=(pink&0x00FF00)>>8//greenComponent

is0x66,or102

4 letblueComponent=pink&0x0000FF//blueComponent

is0x99,or153

ThisexampleusesaUInt32constantcalledpinktostoreaCascadingStyleSheetscolorvalueforthecolorpink.TheCSScolorvalue#CC6699iswrittenas0xCC6699inSwift’shexadecimalnumberrepresentation.Thiscoloristhendecomposedintoitsred(CC),green(66),andblue(99)componentsbythebitwiseANDoperator(&)andthebitwiserightshiftoperator(>>).

TheredcomponentisobtainedbyperformingabitwiseANDbetweenthenumbers0xCC6699and0xFF0000.Thezerosin0xFF0000effectively“mask”thesecondandthirdbytesof0xCC6699,causingthe6699tobeignoredandleaving0xCC0000astheresult.

Thisnumberisthenshifted16placestotheright(>>16).Eachpairofcharactersinahexadecimalnumberuses8bits,soamove16placestotherightwillconvert0xCC0000into0x0000CC.Thisisthesameas0xCC,whichhasadecimalvalueof204.

Similarly,thegreencomponentisobtainedbyperformingabitwiseANDbetweenthenumbers0xCC6699and0x00FF00,whichgivesanoutputvalueof0x006600.Thisoutputvalueisthenshiftedeightplacestotheright,givinga

valueof0x66,whichhasadecimalvalueof102.

Finally,thebluecomponentisobtainedbyperformingabitwiseANDbetweenthenumbers0xCC6699and0x0000FF,whichgivesanoutputvalueof0x000099.There’snoneedtoshiftthistotheright,as0x000099alreadyequals0x99,whichhasadecimalvalueof153.

ShiftingBehaviorforSignedIntegers

Theshiftingbehaviorismorecomplexforsignedintegersthanforunsignedintegers,becauseofthewaysignedintegersarerepresentedinbinary.(Theexamplesbelowarebasedon8-bitsignedintegersforsimplicity,butthesameprinciplesapplyforsignedintegersofanysize.)

Signedintegersusetheirfirstbit(knownasthesignbit)toindicatewhethertheintegerispositiveornegative.Asignbitof0meanspositive,andasignbitof1meansnegative.

Theremainingbits(knownasthevaluebits)storetheactualvalue.Positivenumbersarestoredinexactlythesamewayasforunsignedintegers,countingupwardsfrom0.Here’showthebitsinsideanInt8lookforthenumber4:

Thesignbitis0(meaning“positive”),andthesevenvaluebitsarejustthenumber4,writteninbinarynotation.

Negativenumbers,however,arestoreddifferently.Theyarestoredbysubtractingtheirabsolutevaluefrom2tothepowerofn,wherenisthenumberofvaluebits.Aneight-bitnumberhassevenvaluebits,sothismeans2tothepowerof7,or128.

Here’showthebitsinsideanInt8lookforthenumber-4:

Thistime,thesignbitis1(meaning“negative”),andthesevenvaluebitshaveabinaryvalueof124(whichis128-4):

Thisencodingfornegativenumbersisknownasatwo’scomplementrepresentation.Itmayseemanunusualwaytorepresentnegativenumbers,butithasseveraladvantages.

First,youcanadd-1to-4,simplybyperformingastandardbinaryadditionofalleightbits(includingthesignbit),anddiscardinganythingthatdoesn’tfitintheeightbitsonceyou’redone:

Second,thetwo’scomplementrepresentationalsoletsyoushiftthebitsofnegativenumberstotheleftandrightlikepositivenumbers,andstillendupdoublingthemforeveryshiftyoumaketotheleft,orhalvingthemforeveryshiftyoumaketotheright.Toachievethis,anextraruleisusedwhensignedintegersareshiftedtotheright:Whenyoushiftsignedintegerstotheright,applythesamerulesasforunsignedintegers,butfillanyemptybitsontheleftwiththesignbit,ratherthanwithazero.

Thisactionensuresthatsignedintegershavethesamesignaftertheyareshiftedtotheright,andisknownasanarithmeticshift.

Becauseofthespecialwaythatpositiveandnegativenumbersarestored,shiftingeitherofthemtotherightmovesthemclosertozero.Keepingthesignbitthesameduringthisshiftmeansthatnegativeintegersremainnegativeastheirvaluemovesclosertozero.

OverflowOperators

Ifyoutrytoinsertanumberintoanintegerconstantorvariablethatcannotholdthatvalue,bydefaultSwiftreportsanerrorratherthanallowinganinvalidvaluetobecreated.Thisbehaviorgivesextrasafetywhenyouworkwithnumbersthataretoolargeortoosmall.

Forexample,theInt16integertypecanholdanysignedintegerbetween-32768and32767.TryingtosetanInt16constantorvariabletoanumberoutsideofthisrangecausesanerror:

1 varpotentialOverflow=Int16.max

2 //potentialOverflowequals32767,whichisthemaximumvalue

anInt16canhold

3 potentialOverflow+=1

4 //thiscausesanerror

Providingerrorhandlingwhenvaluesgettoolargeortoosmallgivesyoumuchmoreflexibilitywhencodingforboundaryvalueconditions.

However,whenyouspecificallywantanoverflowconditiontotruncatethenumberofavailablebits,youcanoptintothisbehaviorratherthantriggeringanerror.Swiftprovidesthreearithmeticoverflowoperatorsthatoptintotheoverflowbehaviorforintegercalculations.Theseoperatorsallbeginwithanampersand(&):

Overflowaddition(&+)

Overflowsubtraction(&-)

Overflowmultiplication(&*)

ValueOverflowNumberscanoverflowinboththepositiveandnegativedirection.

Here’sanexampleofwhathappenswhenanunsignedintegerisallowedtooverflowinthepositivedirection,usingtheoverflowadditionoperator(&+):

1 varunsignedOverflow=UInt8.max

2 //unsignedOverflowequals255,whichisthemaximumvaluea

UInt8canhold

3 unsignedOverflow=unsignedOverflow&+1

4 //unsignedOverflowisnowequalto0

ThevariableunsignedOverflowisinitializedwiththemaximumvalueaUInt8canhold(255,or11111111inbinary).Itisthenincrementedby1usingtheoverflowadditionoperator(&+).ThispushesitsbinaryrepresentationjustoverthesizethataUInt8canhold,causingittooverflowbeyonditsbounds,asshowninthediagrambelow.ThevaluethatremainswithintheboundsoftheUInt8aftertheoverflowadditionis00000000,orzero.

Somethingsimilarhappenswhenanunsignedintegerisallowedtooverflowinthenegativedirection.Here’sanexampleusingtheoverflowsubtractionoperator(&-):

1 varunsignedOverflow=UInt8.min

2 //unsignedOverflowequals0,whichistheminimumvaluea

UInt8canhold

3 unsignedOverflow=unsignedOverflow&-1

4 //unsignedOverflowisnowequalto255

TheminimumvaluethataUInt8canholdiszero,or00000000inbinary.Ifyousubtract1from00000000usingtheoverflowsubtractionoperator(&-),thenumberwilloverflowandwraparoundto11111111,or255indecimal.

Overflowalsooccursforsignedintegers.Alladditionandsubtractionforsignedintegersisperformedinbitwisefashion,withthesignbitincludedaspartofthenumbersbeingaddedorsubtracted,asdescribedinBitwiseLeftandRightShiftOperators.

1 varsignedOverflow=Int8.min

2 //signedOverflowequals-128,whichistheminimumvaluean

Int8canhold

3 signedOverflow=signedOverflow&-1

4 //signedOverflowisnowequalto127

TheminimumvaluethatanInt8canholdis-128,or10000000inbinary.Subtracting1fromthisbinarynumberwiththeoverflowoperatorgivesabinaryvalueof01111111,whichtogglesthesignbitandgivespositive127,themaximumpositivevaluethatanInt8canhold.

Forbothsignedandunsignedintegers,overflowinthepositivedirectionwrapsaroundfromthemaximumvalidintegervaluebacktotheminimum,andoverflowinthenegativedirectionwrapsaroundfromtheminimumvaluetothemaximum.

PrecedenceandAssociativity

Operatorprecedencegivessomeoperatorshigherprioritythanothers;theseoperatorsareappliedfirst.

Operatorassociativitydefineshowoperatorsofthesameprecedencearegroupedtogether—eithergroupedfromtheleft,orgroupedfromtheright.Thinkofitasmeaning“theyassociatewiththeexpressiontotheirleft,”or“theyassociatewiththeexpressiontotheirright.”

Itisimportanttoconsidereachoperator’sprecedenceandassociativitywhenworkingouttheorderinwhichacompoundexpressionwillbecalculated.Forexample,operatorprecedenceexplainswhythefollowingexpressionequals17.

1 2+3%4*5

2 //thisequals17

Ifyoureadstrictlyfromlefttoright,youmightexpecttheexpressiontobecalculatedasfollows:

2plus3equals5

5remainder4equals1

1times5equals5

However,theactualansweris17,not5.Higher-precedenceoperatorsareevaluatedbeforelower-precedenceones.InSwift,asinC,theremainderoperator(%)andthemultiplicationoperator(*)haveahigherprecedencethantheadditionoperator(+).Asaresult,theyarebothevaluatedbeforetheadditionisconsidered.

However,remainderandmultiplicationhavethesameprecedenceaseachother.Toworkouttheexactevaluationordertouse,youalsoneedtoconsidertheirassociativity.Remainderandmultiplicationbothassociatewiththeexpressiontotheirleft.Thinkofthisasaddingimplicitparenthesesaroundthesepartsoftheexpression,startingfromtheirleft:

2+((3%4)*5)

(3%4)is3,sothisisequivalentto:

2+(3*5)

(3*5)is15,sothisisequivalentto:

2+15

Thiscalculationyieldsthefinalanswerof17.

ForinformationabouttheoperatorsprovidedbytheSwiftstandardlibrary,includingacompletelistoftheoperatorprecedencegroupsandassociativitysettings,seeOperatorDeclarations.

NOTE

Swift’soperatorprecedencesandassociativityrulesaresimplerandmorepredictablethanthosefoundinCandObjective-C.However,thismeansthattheyarenotexactlythesameasinC-basedlanguages.BecarefultoensurethatoperatorinteractionsstillbehaveinthewayyouintendwhenportingexistingcodetoSwift.

OperatorMethods

Classesandstructurescanprovidetheirownimplementationsofexistingoperators.Thisisknownasoverloadingtheexistingoperators.

Theexamplebelowshowshowtoimplementthearithmeticadditionoperator(+)foracustomstructure.Thearithmeticadditionoperatorisabinaryoperatorbecauseitoperatesontwotargetsandissaidtobeinfixbecauseitappearsinbetweenthosetwotargets.

TheexampledefinesaVector2Dstructureforatwo-dimensionalpositionvector(x,y),followedbyadefinitionofanoperatormethodtoaddtogetherinstancesoftheVector2Dstructure:

1 structVector2D{

2 varx=0.0,y=0.0

3 }

4

5 extensionVector2D{

6 staticfunc+(left:Vector2D,right:Vector2D)->Vector2D

{

7 returnVector2D(x:left.x+right.x,y:left.y+

right.y)

8 }

9 }

TheoperatormethodisdefinedasatypemethodonVector2D,withamethodnamethatmatchestheoperatortobeoverloaded(+).Becauseadditionisn’tpartoftheessentialbehaviorforavector,thetypemethodisdefinedinanextensionofVector2DratherthaninthemainstructuredeclarationofVector2D.Becausethearithmeticadditionoperatorisabinaryoperator,thisoperatormethodtakestwoinputparametersoftypeVector2Dandreturnsasingleoutputvalue,alsooftypeVector2D.

Inthisimplementation,theinputparametersarenamedleftandrighttorepresenttheVector2Dinstancesthatwillbeontheleftsideandrightsideofthe+operator.ThemethodreturnsanewVector2Dinstance,whosexandypropertiesareinitializedwiththesumofthexandypropertiesfromthetwoVector2Dinstancesthatareaddedtogether.

ThetypemethodcanbeusedasaninfixoperatorbetweenexistingVector2Dinstances:

1 letvector=Vector2D(x:3.0,y:1.0)

2 letanotherVector=Vector2D(x:2.0,y:4.0)

3 letcombinedVector=vector+anotherVector

4 //combinedVectorisaVector2Dinstancewithvaluesof(5.0,

5.0)

Thisexampleaddstogetherthevectors(3.0,1.0)and(2.0,4.0)tomakethevector(5.0,5.0),asillustratedbelow.

PrefixandPostfixOperatorsTheexampleshownabovedemonstratesacustomimplementationofabinaryinfixoperator.Classesandstructurescanalsoprovideimplementationsofthestandardunaryoperators.Unaryoperatorsoperateonasingletarget.Theyareprefixiftheyprecedetheirtarget(suchas-a)andpostfixoperatorsiftheyfollowtheirtarget(suchasb!).

Youimplementaprefixorpostfixunaryoperatorbywritingtheprefixorpostfixmodifierbeforethefunckeywordwhendeclaringtheoperatormethod:

1 extensionVector2D{

2 staticprefixfunc-(vector:Vector2D)->Vector2D{

3 returnVector2D(x:-vector.x,y:-vector.y)

4 }

5 }

Theexampleaboveimplementstheunaryminusoperator(-a)forVector2D

instances.Theunaryminusoperatorisaprefixoperator,andsothismethodhastobequalifiedwiththeprefixmodifier.

Forsimplenumericvalues,theunaryminusoperatorconvertspositivenumbersintotheirnegativeequivalentandviceversa.ThecorrespondingimplementationforVector2Dinstancesperformsthisoperationonboththexandyproperties:

1 letpositive=Vector2D(x:3.0,y:4.0)

2 letnegative=-positive

3 //negativeisaVector2Dinstancewithvaluesof(-3.0,-4.0)

4 letalsoPositive=-negative

5 //alsoPositiveisaVector2Dinstancewithvaluesof(3.0,

4.0)

CompoundAssignmentOperatorsCompoundassignmentoperatorscombineassignment(=)withanotheroperation.Forexample,theadditionassignmentoperator(+=)combinesadditionandassignmentintoasingleoperation.Youmarkacompoundassignmentoperator’sleftinputparametertypeasinout,becausetheparameter’svaluewillbemodifieddirectlyfromwithintheoperatormethod.

TheexamplebelowimplementsanadditionassignmentoperatormethodforVector2Dinstances:

1 extensionVector2D{

2 staticfunc+=(left:inoutVector2D,right:Vector2D){

3 left=left+right

4 }

5 }

Becauseanadditionoperatorwasdefinedearlier,youdon’tneedtoreimplementtheadditionprocesshere.Instead,theadditionassignmentoperatormethodtakesadvantageoftheexistingadditionoperatormethod,andusesittosettheleft

valuetobetheleftvalueplustherightvalue:

1 varoriginal=Vector2D(x:1.0,y:2.0)

2 letvectorToAdd=Vector2D(x:3.0,y:4.0)

3 original+=vectorToAdd

4 //originalnowhasvaluesof(4.0,6.0)

NOTE

Itisn’tpossibletooverloadthedefaultassignmentoperator(=).Onlythecompoundassignmentoperatorscanbeoverloaded.Similarly,theternaryconditionaloperator(a?b:c)can’tbeoverloaded.

EquivalenceOperatorsBydefault,customclassesandstructuresdon’thaveanimplementationoftheequivalenceoperators,knownastheequaltooperator(==)andnotequaltooperator(!=).Youusuallyimplementthe==operator,andusethestandardlibrary’sdefaultimplementationofthe!=operatorthatnegatestheresultofthe==operator.Therearetwowaystoimplementthe==operator:Youcanimplementityourself,orformanytypes,youcanaskSwifttosynthesizeanimplementationforyou.Inbothcases,youaddconformancetothestandardlibrary’sEquatableprotocol.

Youprovideanimplementationofthe==operatorinthesamewayasyouimplementotherinfixoperators:

1 extensionVector2D:Equatable{

2 staticfunc==(left:Vector2D,right:Vector2D)->Bool{

3 return(left.x==right.x)&&(left.y==right.y)

4 }

5 }

Theexampleaboveimplementsan==operatortocheckwhethertwoVector2Dinstanceshaveequivalentvalues.InthecontextofVector2D,itmakessenseto

consider“equal”asmeaning“bothinstanceshavethesamexvaluesandyvalues”,andsothisisthelogicusedbytheoperatorimplementation.

YoucannowusethisoperatortocheckwhethertwoVector2Dinstancesareequivalent:

1 lettwoThree=Vector2D(x:2.0,y:3.0)

2 letanotherTwoThree=Vector2D(x:2.0,y:3.0)

3 iftwoThree==anotherTwoThree{

4 print("Thesetwovectorsareequivalent.")

5 }

6 //Prints"Thesetwovectorsareequivalent."

Inmanysimplecases,youcanaskSwifttoprovidesynthesizedimplementationsoftheequivalenceoperatorsforyou.Swiftprovidessynthesizedimplementationsforthefollowingkindsofcustomtypes:

StructuresthathaveonlystoredpropertiesthatconformtotheEquatableprotocol

EnumerationsthathaveonlyassociatedtypesthatconformtotheEquatableprotocol

Enumerationsthathavenoassociatedtypes

Toreceiveasynthesizedimplementationof==,declareEquatableconformanceinthefilethatcontainstheoriginaldeclaration,withoutimplementingan==operatoryourself.

TheexamplebelowdefinesaVector3Dstructureforathree-dimensionalpositionvector(x,y,z),similartotheVector2Dstructure.Becausethex,y,andzpropertiesareallofanEquatabletype,Vector3Dreceivessynthesizedimplementationsoftheequivalenceoperators.

1 structVector3D:Equatable{

2 varx=0.0,y=0.0,z=0.0

3 }

4

5 lettwoThreeFour=Vector3D(x:2.0,y:3.0,z:4.0)

6 letanotherTwoThreeFour=Vector3D(x:2.0,y:3.0,z:4.0)

7 iftwoThreeFour==anotherTwoThreeFour{

8 print("Thesetwovectorsarealsoequivalent.")

9 }

10 //Prints"Thesetwovectorsarealsoequivalent."

CustomOperators

YoucandeclareandimplementyourowncustomoperatorsinadditiontothestandardoperatorsprovidedbySwift.Foralistofcharactersthatcanbeusedtodefinecustomoperators,seeOperators.

Newoperatorsaredeclaredatagloballevelusingtheoperatorkeyword,andaremarkedwiththeprefix,infixorpostfixmodifiers:

prefixoperator+++

Theexampleabovedefinesanewprefixoperatorcalled+++.ThisoperatordoesnothaveanexistingmeaninginSwift,andsoitisgivenitsowncustommeaningbelowinthespecificcontextofworkingwithVector2Dinstances.Forthepurposesofthisexample,+++istreatedasanew“prefixdoubling”operator.ItdoublesthexandyvaluesofaVector2Dinstance,byaddingthevectortoitselfwiththeadditionassignmentoperatordefinedearlier.Toimplementthe+++operator,youaddatypemethodcalled+++toVector2Dasfollows:

1 extensionVector2D{

2 staticprefixfunc+++(vector:inoutVector2D)->Vector2D

{

3 vector+=vector

4 returnvector

5 }

6 }

7

8 vartoBeDoubled=Vector2D(x:1.0,y:4.0)

9 letafterDoubling=+++toBeDoubled

10 //toBeDoublednowhasvaluesof(2.0,8.0)

11 //afterDoublingalsohasvaluesof(2.0,8.0)

PrecedenceforCustomInfixOperatorsCustominfixoperatorseachbelongtoaprecedencegroup.Aprecedencegroupspecifiesanoperator’sprecedencerelativetootherinfixoperators,aswellastheoperator’sassociativity.SeePrecedenceandAssociativityforanexplanationofhowthesecharacteristicsaffectaninfixoperator’sinteractionwithotherinfixoperators.

Acustominfixoperatorthatisnotexplicitlyplacedintoaprecedencegroupisgivenadefaultprecedencegroupwithaprecedenceimmediatelyhigherthantheprecedenceoftheternaryconditionaloperator.

Thefollowingexampledefinesanewcustominfixoperatorcalled+-,whichbelongstotheprecedencegroupAdditionPrecedence:

1 infixoperator+-:AdditionPrecedence

2 extensionVector2D{

3 staticfunc+-(left:Vector2D,right:Vector2D)->

Vector2D{

4 returnVector2D(x:left.x+right.x,y:left.y-

right.y)

5 }

6 }

7 letfirstVector=Vector2D(x:1.0,y:2.0)

8 letsecondVector=Vector2D(x:3.0,y:4.0)

9 letplusMinusVector=firstVector+-secondVector

10 //plusMinusVectorisaVector2Dinstancewithvaluesof(4.0,

-2.0)

Thisoperatoraddstogetherthexvaluesoftwovectors,andsubtractstheyvalueofthesecondvectorfromthefirst.Becauseitisinessencean“additive”operator,ithasbeengiventhesameprecedencegroupasadditiveinfixoperatorssuchas+and-.ForinformationabouttheoperatorsprovidedbytheSwiftstandardlibrary,includingacompletelistoftheoperatorprecedencegroupsandassociativitysettings,seeOperatorDeclarations.Formoreinformationaboutprecedencegroupsandtoseethesyntaxfordefiningyourownoperatorsandprecedencegroups,seeOperatorDeclaration.

NOTE

Youdonotspecifyaprecedencewhendefiningaprefixorpostfixoperator.However,ifyouapplybothaprefixandapostfixoperatortothesameoperand,thepostfixoperatorisappliedfirst.

LanguageReference

AbouttheLanguageReference

ThispartofthebookdescribestheformalgrammaroftheSwiftprogramminglanguage.Thegrammardescribedhereisintendedtohelpyouunderstandthelanguageinmoredetail,ratherthantoallowyoutodirectlyimplementaparserorcompiler.

TheSwiftlanguageisrelativelysmall,becausemanycommontypes,functions,andoperatorsthatappearvirtuallyeverywhereinSwiftcodeareactuallydefinedintheSwiftstandardlibrary.Althoughthesetypes,functions,andoperatorsarenotpartoftheSwiftlanguageitself,theyareusedextensivelyinthediscussionsandcodeexamplesinthispartofthebook.

HowtoReadtheGrammar

ThenotationusedtodescribetheformalgrammaroftheSwiftprogramminglanguagefollowsafewconventions:

Anarrow(→)isusedtomarkgrammarproductionsandcanbereadas“canconsistof.”

Syntacticcategoriesareindicatedbyitalictextandappearonbothsidesofagrammarproductionrule.

Literalwordsandpunctuationareindicatedbyboldfaceconstantwidthtextandappearonlyontheright-handsideofagrammarproductionrule.

Alternativegrammarproductionsareseparatedbyverticalbars(|).Whenalternativeproductionsaretoolongtoreadeasily,theyarebrokenintomultiplegrammarproductionrulesonnewlines.

Inafewcases,regularfonttextisusedtodescribetheright-handsideofagrammarproductionrule.

Optionalsyntacticcategoriesandliteralsaremarkedbyatrailingsubscript,opt.

Asanexample,thegrammarofagetter-setterblockisdefinedasfollows:

GRAMMAR OF AGETTER -SETTER BLOCK

getter-setter-block → { getter-clause setter-clause opt } | { setter-clause getter-clause }

Thisdefinitionindicatesthatagetter-setterblockcanconsistofagetterclausefollowedbyanoptionalsetterclause,enclosedinbraces,orasetterclausefollowedbyagetterclause,enclosedinbraces.Thegrammarproductionaboveisequivalenttothefollowingtwoproductions,wherethealternativesarespelledoutexplicitly:

GRAMMAR OF AGETTER -SETTER BLOCK

getter-setter-block → { getter-clause setter-clause opt }getter-setter-block → { setter-clause getter-clause }

LexicalStructure

ThelexicalstructureofSwiftdescribeswhatsequenceofcharactersformvalidtokensofthelanguage.Thesevalidtokensformthelowest-levelbuildingblocksofthelanguageandareusedtodescribetherestofthelanguageinsubsequentchapters.Atokenconsistsofanidentifier,keyword,punctuation,literal,oroperator.

Inmostcases,tokensaregeneratedfromthecharactersofaSwiftsourcefilebyconsideringthelongestpossiblesubstringfromtheinputtext,withintheconstraintsofthegrammarthatarespecifiedbelow.Thisbehaviorisreferredtoaslongestmatchormaximalmunch.

WhitespaceandComments

Whitespacehastwouses:toseparatetokensinthesourcefileandtohelpdeterminewhetheranoperatorisaprefixorpostfix(seeOperators),butisotherwiseignored.Thefollowingcharactersareconsideredwhitespace:space(U+0020),linefeed(U+000A),carriagereturn(U+000D),horizontaltab(U+0009),verticaltab(U+000B),formfeed(U+000C)andnull(U+0000).

Commentsaretreatedaswhitespacebythecompiler.Singlelinecommentsbeginwith//andcontinueuntilalinefeed(U+000A)orcarriagereturn(U+000D).Multilinecommentsbeginwith/*andendwith*/.Nestingmultilinecommentsisallowed,butthecommentmarkersmustbebalanced.

Commentscancontainadditionalformattingandmarkup,asdescribedinMarkupFormattingReference.

GRAMMAR OF WH I TESPACE

whitespace → whitespace-item whitespace optwhitespace-item → line-breakwhitespace-item → commentwhitespace-item → multiline-commentwhitespace-item → U+0000,U+0009,U+000B,U+000C,orU+0020

line-break → U+000Aline-break → U+000Dline-break → U+000DfollowedbyU+000Acomment → // comment-text line-breakmultiline-comment → /* multiline-comment-text */comment-text → comment-text-item comment-text optcomment-text-item → AnyUnicodescalarvalueexceptU+000AorU+000Dmultiline-comment-text → multiline-comment-text-item multiline-comment-text optmultiline-comment-text-item → multiline-commentmultiline-comment-text-item → comment-text-itemmultiline-comment-text-item → AnyUnicodescalarvalueexcept /* or */

Identifiers

IdentifiersbeginwithanuppercaseorlowercaseletterAthroughZ,anunderscore(_),anoncombiningalphanumericUnicodecharacterintheBasicMultilingualPlane,oracharacteroutsidetheBasicMultilingualPlanethatisn’tinaPrivateUseArea.Afterthefirstcharacter,digitsandcombiningUnicodecharactersarealsoallowed.

Touseareservedwordasanidentifier,putabacktick(`)beforeandafterit.Forexample,classisnotavalididentifier,but`class`isvalid.Thebackticksaren’tconsideredpartoftheidentifier;`x`andxhavethesamemeaning.

Insideaclosurewithnoexplicitparameternames,theparametersareimplicitlynamed$0,$1,$2,andsoon.Thesenamesarevalididentifierswithinthescopeoftheclosure.

Thecompilersynthesizesidentifiersthatbeginwithadollarsign($)forpropertiesthathaveapropertywrapperprojection.Yourcodecaninteractwiththeseidentifiers,butyoucan’tdeclareidentifierswiththatprefix.Formoreinformation,seethepropertyWrappersectionoftheAttributeschapter.

GRAMMAR OF AN I DENT I F I ER

identifier → identifier-head identifier-characters optidentifier → ` identifier-head identifier-characters opt `identifier → implicit-parameter-nameidentifier → property-wrapper-projection

identifier-list → identifier | identifier , identifier-listidentifier-head → Upper-orlowercaseletterAthroughZidentifier-head → _identifier-head → U+00A8,U+00AA,U+00AD,U+00AF,U+00B2–U+00B5,orU+00B7–

U+00BAidentifier-head → U+00BC–U+00BE,U+00C0–U+00D6,U+00D8–U+00F6,orU+00F8–

U+00FFidentifier-head → U+0100–U+02FF,U+0370–U+167F,U+1681–U+180D,orU+180F–

U+1DBFidentifier-head → U+1E00–U+1FFFidentifier-head → U+200B–U+200D,U+202A–U+202E,U+203F–U+2040,U+2054,or

U+2060–U+206Fidentifier-head → U+2070–U+20CF,U+2100–U+218F,U+2460–U+24FF,orU+2776–

U+2793identifier-head → U+2C00–U+2DFForU+2E80–U+2FFFidentifier-head → U+3004–U+3007,U+3021–U+302F,U+3031–U+303F,orU+3040–

U+D7FFidentifier-head → U+F900–U+FD3D,U+FD40–U+FDCF,U+FDF0–U+FE1F,orU+FE30–

U+FE44identifier-head → U+FE47–U+FFFDidentifier-head → U+10000–U+1FFFD,U+20000–U+2FFFD,U+30000–U+3FFFD,or

U+40000–U+4FFFDidentifier-head → U+50000–U+5FFFD,U+60000–U+6FFFD,U+70000–U+7FFFD,or

U+80000–U+8FFFDidentifier-head → U+90000–U+9FFFD,U+A0000–U+AFFFD,U+B0000–U+BFFFD,or

U+C0000–U+CFFFDidentifier-head → U+D0000–U+DFFFDorU+E0000–U+EFFFDidentifier-character → Digit0through9identifier-character → U+0300–U+036F,U+1DC0–U+1DFF,U+20D0–U+20FF,or

U+FE20–U+FE2Fidentifier-character → identifier-headidentifier-characters → identifier-character identifier-characters optimplicit-parameter-name → $ decimal-digitsproperty-wrapper-projection → $ identifier-characters

KeywordsandPunctuation

Thefollowingkeywordsarereservedandcan’tbeusedasidentifiers,unlessthey’reescapedwithbackticks,asdescribedaboveinIdentifiers.Keywordsotherthaninout,var,andletcanbeusedasparameternamesinafunctiondeclarationorfunctioncallwithoutbeingescapedwithbackticks.Whenamemberhasthesamenameasakeyword,referencestothatmemberdon’tneedtobeescapedwithbackticks,exceptwhenthere’sambiguitybetweenreferring

tothememberandusingthekeyword—forexample,self,Type,andProtocolhavespecialmeaninginanexplicitmemberexpression,sotheymustbeescapedwithbackticksinthatcontext.

Keywordsusedindeclarations:associatedtype,class,deinit,enum,extension,fileprivate,func,import,init,inout,internal,let,open,operator,private,protocol,public,rethrows,static,struct,subscript,typealias,andvar.

Keywordsusedinstatements:break,case,continue,default,defer,do,else,fallthrough,for,guard,if,in,repeat,return,switch,where,andwhile.

Keywordsusedinexpressionsandtypes:as,Any,catch,false,is,nil,super,self,Self,throw,throws,true,andtry.

Keywordsusedinpatterns:_.

Keywordsthatbeginwithanumbersign(#):#available,#colorLiteral,#column,#else,#elseif,#endif,#error,#file,#fileLiteral,#function,#if,#imageLiteral,#line,#selector,#sourceLocation,and#warning.

Keywordsreservedinparticularcontexts:associativity,convenience,dynamic,didSet,final,get,infix,indirect,lazy,left,mutating,none,nonmutating,optional,override,postfix,precedence,prefix,Protocol,required,right,set,Type,unowned,weak,andwillSet.Outsidethecontextinwhichtheyappearinthegrammar,theycanbeusedasidentifiers.

Thefollowingtokensarereservedaspunctuationandcan’tbeusedascustomoperators:(,),{,},[,],.,,,:,;,=,@,#,&(asaprefixoperator),->,`,?,and!(asapostfixoperator).

Literals

Aliteralisthesourcecoderepresentationofavalueofatype,suchasanumberorstring.

Thefollowingareexamplesofliterals:

1 42//Integerliteral

2 3.14159//Floating-pointliteral

3 "Hello,world!"//Stringliteral

4 true//Booleanliteral

Aliteraldoesn’thaveatypeonitsown.Instead,aliteralisparsedashavinginfiniteprecisionandSwift’stypeinferenceattemptstoinferatypefortheliteral.Forexample,inthedeclarationletx:Int8=42,Swiftusestheexplicittypeannotation(:Int8)toinferthatthetypeoftheintegerliteral42isInt8.Ifthereisn’tsuitabletypeinformationavailable,Swiftinfersthattheliteral’stypeisoneofthedefaultliteraltypesdefinedintheSwiftstandardlibrary.ThedefaulttypesareIntforintegerliterals,Doubleforfloating-pointliterals,Stringforstringliterals,andBoolforBooleanliterals.Forexample,inthedeclarationletstr="Hello,world",thedefaultinferredtypeofthestringliteral"Hello,world"isString.

Whenspecifyingthetypeannotationforaliteralvalue,theannotation’stypemustbeatypethatcanbeinstantiatedfromthatliteralvalue.Thatis,thetypemustconformtooneofthefollowingSwiftstandardlibraryprotocols:ExpressibleByIntegerLiteralforintegerliterals,ExpressibleByFloatLiteralforfloating-pointliterals,ExpressibleByStringLiteralforstringliterals,ExpressibleByBooleanLiteralforBooleanliterals,ExpressibleByUnicodeScalarLiteralforstringliteralsthatcontainonlyasingleUnicodescalar,andExpressibleByExtendedGraphemeClusterLiteralforstringliteralsthatcontainonlyasingleextendedgraphemecluster.Forexample,Int8conformstotheExpressibleByIntegerLiteralprotocol,andthereforeitcanbeusedinthetypeannotationfortheintegerliteral42inthedeclarationletx:Int8=42.

GRAMMAR OF A L I TERAL

literal → numeric-literal | string-literal | boolean-literal | nil-literalnumeric-literal → -opt integer-literal | -opt floating-point-literalboolean-literal → true | falsenil-literal → nil

IntegerLiteralsIntegerliteralsrepresentintegervaluesofunspecifiedprecision.Bydefault,integerliteralsareexpressedindecimal;youcanspecifyanalternatebaseusingaprefix.Binaryliteralsbeginwith0b,octalliteralsbeginwith0o,andhexadecimalliteralsbeginwith0x.

Decimalliteralscontainthedigits0through9.Binaryliteralscontain0and1,octalliteralscontain0through7,andhexadecimalliteralscontain0through9aswellasAthroughFinupper-orlowercase.

Negativeintegersliteralsareexpressedbyprependingaminussign(-)toanintegerliteral,asin-42.

Underscores(_)areallowedbetweendigitsforreadability,butthey’reignoredandthereforedon’taffectthevalueoftheliteral.Integerliteralscanbeginwithleadingzeros(0),butthey’relikewiseignoredanddon’taffectthebaseorvalueoftheliteral.

Unlessotherwisespecified,thedefaultinferredtypeofanintegerliteralistheSwiftstandardlibrarytypeInt.TheSwiftstandardlibraryalsodefinestypesforvarioussizesofsignedandunsignedintegers,asdescribedinIntegers.

GRAMMAR OF AN I NTEGER L I TERAL

integer-literal → binary-literalinteger-literal → octal-literalinteger-literal → decimal-literalinteger-literal → hexadecimal-literalbinary-literal → 0b binary-digit binary-literal-characters optbinary-digit → Digit0or1binary-literal-character → binary-digit | _binary-literal-characters → binary-literal-character binary-literal-characters optoctal-literal → 0o octal-digit octal-literal-characters optoctal-digit → Digit0through7octal-literal-character → octal-digit | _octal-literal-characters → octal-literal-character octal-literal-characters optdecimal-literal → decimal-digit decimal-literal-characters optdecimal-digit → Digit0through9decimal-digits → decimal-digit decimal-digits optdecimal-literal-character → decimal-digit | _decimal-literal-characters → decimal-literal-character decimal-literal-characters opthexadecimal-literal → 0x hexadecimal-digit hexadecimal-literal-characters opt

hexadecimal-digit → Digit0through9,athroughf,orAthroughFhexadecimal-literal-character → hexadecimal-digit | _hexadecimal-literal-characters → hexadecimal-literal-character hexadecimal-literal-

characters opt

Floating-PointLiteralsFloating-pointliteralsrepresentfloating-pointvaluesofunspecifiedprecision.

Bydefault,floating-pointliteralsareexpressedindecimal(withnoprefix),buttheycanalsobeexpressedinhexadecimal(witha0xprefix).

Decimalfloating-pointliteralsconsistofasequenceofdecimaldigitsfollowedbyeitheradecimalfraction,adecimalexponent,orboth.Thedecimalfractionconsistsofadecimalpoint(.)followedbyasequenceofdecimaldigits.Theexponentconsistsofanupper-orlowercaseeprefixfollowedbyasequenceofdecimaldigitsthatindicateswhatpowerof10thevalueprecedingtheeismultipliedby.Forexample,1.25e2represents1.25x102,whichevaluatesto125.0.Similarly,1.25e-2represents1.25x10-2,whichevaluatesto0.0125.

Hexadecimalfloating-pointliteralsconsistofa0xprefix,followedbyanoptionalhexadecimalfraction,followedbyahexadecimalexponent.Thehexadecimalfractionconsistsofadecimalpointfollowedbyasequenceofhexadecimaldigits.Theexponentconsistsofanupper-orlowercasepprefixfollowedbyasequenceofdecimaldigitsthatindicateswhatpowerof2thevalueprecedingthepismultipliedby.Forexample,0xFp2represents15x22,whichevaluatesto60.Similarly,0xFp-2represents15x2-2,whichevaluatesto3.75.

Negativefloating-pointliteralsareexpressedbyprependingaminussign(-)toafloating-pointliteral,asin-42.5.

Underscores(_)areallowedbetweendigitsforreadability,butthey’reignoredandthereforedon’taffectthevalueoftheliteral.Floating-pointliteralscanbeginwithleadingzeros(0),butthey’relikewiseignoredanddon’taffectthebaseorvalueoftheliteral.

Unlessotherwisespecified,thedefaultinferredtypeofafloating-pointliteralistheSwiftstandardlibrarytypeDouble,whichrepresentsa64-bitfloating-pointnumber.TheSwiftstandardlibraryalsodefinesaFloattype,whichrepresentsa32-bitfloating-pointnumber.

GRAMMAR OF A F LOAT ING -PO INT L I TERAL

floating-point-literal → decimal-literal decimal-fraction opt decimal-exponent optfloating-point-literal → hexadecimal-literal hexadecimal-fraction opt hexadecimal-

exponentdecimal-fraction → . decimal-literaldecimal-exponent → floating-point-e sign opt decimal-literalhexadecimal-fraction → . hexadecimal-digit hexadecimal-literal-characters opthexadecimal-exponent → floating-point-p sign opt decimal-literalfloating-point-e → e | Efloating-point-p → p | Psign → + | -

StringLiteralsAstringliteralisasequenceofcharacterssurroundedbyquotationmarks.Asingle-linestringliteralissurroundedbydoublequotationmarksandhasthefollowingform:

" characters "

Stringliteralscan’tcontainanunescapeddoublequotationmark("),anunescapedbackslash(\),acarriagereturn,oralinefeed.

Amultilinestringliteralissurroundedbythreedoublequotationmarksandhasthefollowingform:

"""

characters

"""

Unlikeasingle-linestringliteral,amultilinestringliteralcancontainunescapeddoublequotationmarks("),carriagereturns,andlinefeeds.Itcan’tcontainthree

unescapeddoublequotationmarksnexttoeachother.

Thelinebreakafterthe"""thatbeginsthemultilinestringliteralisnotpartofthestring.Thelinebreakbeforethe"""thatendstheliteralisalsonotpartofthestring.Tomakeamultilinestringliteralthatbeginsorendswithalinefeed,writeablanklineasitsfirstorlastline.

Amultilinestringliteralcanbeindentedusinganycombinationofspacesandtabs;thisindentationisnotincludedinthestring.The"""thatendstheliteraldeterminestheindentation:Everynonblanklineintheliteralmustbeginwithexactlythesameindentationthatappearsbeforetheclosing""";there’snoconversionbetweentabsandspaces.Youcanincludeadditionalspacesandtabsafterthatindentation;thosespacesandtabsappearinthestring.

Linebreaksinamultilinestringliteralarenormalizedtousethelinefeedcharacter.Evenifyoursourcefilehasamixofcarriagereturnsandlinefeeds,allofthelinebreaksinthestringwillbethesame.

Inamultilinestringliteral,writingabackslash(\)attheendofalineomitsthatlinebreakfromthestring.Anywhitespacebetweenthebackslashandthelinebreakisalsoomitted.Youcanusethissyntaxtohardwrapamultilinestringliteralinyoursourcecode,withoutchangingthevalueoftheresultingstring.

Specialcharacterscanbeincludedinstringliteralsofboththesingle-lineandmultilineformsusingthefollowingescapesequences:

Nullcharacter(\0)

Backslash(\\)

Horizontaltab(\t)

Linefeed(\n)

Carriagereturn(\r)

Doublequotationmark(\")

Singlequotationmark(\')

Unicodescalar(\u{n}),wherenisahexadecimalnumberthathasonetoeightdigits

Thevalueofanexpressioncanbeinsertedintoastringliteralbyplacingtheexpressioninparenthesesafterabackslash(\).Theinterpolatedexpressioncancontainastringliteral,butcan’tcontainanunescapedbackslash,acarriagereturn,oralinefeed.

Forexample,allofthefollowingstringliteralshavethesamevalue:

1 "123"

2 "12\("3")"

3 "12\(3)"

4 "12\(1+2)"

5 letx=3;"12\(x)"

Astringdelimitedbyextendeddelimitersisasequenceofcharacterssurroundedbyquotationmarksandabalancedsetofoneormorenumbersigns(#).Astringdelimitedbyextendeddelimitershasthefollowingforms:

#" characters "#

#"""

characters

"""#

Specialcharactersinastringdelimitedbyextendeddelimitersappearintheresultingstringasnormalcharactersratherthanasspecialcharacters.Youcanuseextendeddelimiterstocreatestringswithcharactersthatwouldordinarilyhaveaspecialeffectsuchasgeneratingastringinterpolation,startinganescapesequence,orterminatingthestring.

Thefollowingexampleshowsastringliteralandastringdelimitedbyextendeddelimitersthatcreateequivalentstringvalues:

1 letstring=#"\(x)\"\u{2603}"#

2 letescaped="\\(x)\\\"\\u{2603}"

3 print(string)

4 //Prints"\(x)\"\u{2603}"

5 print(string==escaped)

6 //Prints"true"

Ifyouusemorethanonenumbersigntoformastringdelimitedbyextendeddelimiters,don’tplacewhitespaceinbetweenthenumbersigns:

1 print(###"Line1\###nLine2"###)//OK

2 print(###"Line1\###nLine2"###)//Error

Multilinestringliteralsthatyoucreateusingextendeddelimitershavethesameindentationrequirementsasregularmultilinestringliterals.

ThedefaultinferredtypeofastringliteralisString.FormoreinformationabouttheStringtype,seeStringsandCharactersandString.

Stringliteralsthatareconcatenatedbythe+operatorareconcatenatedatcompiletime.Forexample,thevaluesoftextAandtextBintheexamplebelowareidentical—noruntimeconcatenationisperformed.

1 lettextA="Hello"+"world"

2 lettextB="Helloworld"

GRAMMAR OF A STR ING L I TERAL

string-literal → static-string-literal | interpolated-string-literalstring-literal-opening-delimiter → extended-string-literal-delimiter opt "string-literal-closing-delimiter → " extended-string-literal-delimiter optstatic-string-literal → string-literal-opening-delimiter quoted-text opt string-literal-closing-

delimiterstatic-string-literal → multiline-string-literal-opening-delimiter multiline-quoted-text opt

multiline-string-literal-closing-delimitermultiline-string-literal-opening-delimiter → extended-string-literal-delimiter """multiline-string-literal-closing-delimiter → """ extended-string-literal-delimiter

extended-string-literal-delimiter → # extended-string-literal-delimiter optquoted-text → quoted-text-item quoted-text optquoted-text-item → escaped-characterquoted-text-item → AnyUnicodescalarvalueexcept " , \ ,U+000A,orU+000Dmultiline-quoted-text → multiline-quoted-text-item multiline-quoted-text optmultiline-quoted-text-item → escaped-charactermultiline-quoted-text-item → AnyUnicodescalarvalueexcept \multiline-quoted-text-item → escaped-newlineinterpolated-string-literal → string-literal-opening-delimiter interpolated-text opt string-

literal-closing-delimiterinterpolated-string-literal → multiline-string-literal-opening-delimiter interpolated-text opt

multiline-string-literal-closing-delimiterinterpolated-text → interpolated-text-item interpolated-text optinterpolated-text-item → \( expression ) | quoted-text-itemmultiline-interpolated-text → multiline-interpolated-text-item multiline-interpolated-text optmultiline-interpolated-text-item → \( expression ) |multiline-quoted-text-itemescape-sequence → \ extended-string-literal-delimiterescaped-character → escape-sequence 0 | escape-sequence \ | escape-sequence t |

escape-sequence n | escape-sequence r | escape-sequence " | escape-sequence'

escaped-character → escape-sequence u { unicode-scalar-digits }unicode-scalar-digits → Betweenoneandeighthexadecimaldigitsescaped-newline → escape-sequence whitespace opt line-break

Operators

TheSwiftstandardlibrarydefinesanumberofoperatorsforyouruse,manyofwhicharediscussedinBasicOperatorsandAdvancedOperators.Thepresentsectiondescribeswhichcharacterscanbeusedtodefinecustomoperators.

CustomoperatorscanbeginwithoneoftheASCIIcharacters/,=,-,+,!,*,%,<,>,&,|,^,?,or~,oroneoftheUnicodecharactersdefinedinthegrammarbelow(whichincludecharactersfromtheMathematicalOperators,MiscellaneousSymbols,andDingbatsUnicodeblocks,amongothers).Afterthefirstcharacter,combiningUnicodecharactersarealsoallowed.

Youcanalsodefinecustomoperatorsthatbeginwithadot(.).Theseoperatorscancontainadditionaldots.Forexample,.+.istreatedasasingleoperator.Ifanoperatordoesn’tbeginwithadot,itcan’tcontainadotelsewhere.Forexample,

+.+istreatedasthe+operatorfollowedbythe.+operator.

Althoughyoucandefinecustomoperatorsthatcontainaquestionmark(?),theycan’tconsistofasinglequestionmarkcharacteronly.Additionally,althoughoperatorscancontainanexclamationmark(!),postfixoperatorscan’tbeginwitheitheraquestionmarkoranexclamationmark.

NOTE

Thetokens=,->,//,/*,*/,.,theprefixoperators<,&,and?,theinfixoperator?,andthepostfixoperators>,!,and?arereserved.Thesetokenscan’tbeoverloaded,norcantheybeusedascustomoperators.

Thewhitespacearoundanoperatorisusedtodeterminewhetheranoperatorisusedasaprefixoperator,apostfixoperator,orabinaryoperator.Thisbehaviorissummarizedinthefollowingrules:

Ifanoperatorhaswhitespacearoundbothsidesoraroundneitherside,it’streatedasabinaryoperator.Asanexample,the+++operatorina+++banda+++bistreatedasabinaryoperator.

Ifanoperatorhaswhitespaceontheleftsideonly,it’streatedasaprefixunaryoperator.Asanexample,the+++operatorina+++bistreatedasaprefixunaryoperator.

Ifanoperatorhaswhitespaceontherightsideonly,it’streatedasapostfixunaryoperator.Asanexample,the+++operatorina+++bistreatedasapostfixunaryoperator.

Ifanoperatorhasnowhitespaceontheleftbutisfollowedimmediatelybyadot(.),it’streatedasapostfixunaryoperator.Asanexample,the+++operatorina+++.bistreatedasapostfixunaryoperator(a+++.bratherthana+++.b).

Forthepurposesoftheserules,thecharacters(,[,and{beforeanoperator,thecharacters),],and}afteranoperator,andthecharacters,,;,and:arealsoconsideredwhitespace.

There’sonecaveattotherulesabove.Ifthe!or?predefinedoperatorhasno

whitespaceontheleft,it’streatedasapostfixoperator,regardlessofwhetherithaswhitespaceontheright.Tousethe?astheoptional-chainingoperator,itmustnothavewhitespaceontheleft.Touseitintheternaryconditional(?:)operator,itmusthavewhitespacearoundbothsides.

Incertainconstructs,operatorswithaleading<or>maybesplitintotwoormoretokens.Theremainderistreatedthesamewayandmaybesplitagain.Asaresult,there’snoneedtousewhitespacetodisambiguatebetweentheclosing>charactersinconstructslikeDictionary<String,Array<Int>>.Inthisexample,theclosing>charactersarenottreatedasasingletokenthatmaythenbemisinterpretedasabitshift>>operator.

Tolearnhowtodefinenew,customoperators,seeCustomOperatorsandOperatorDeclaration.Tolearnhowtooverloadexistingoperators,seeOperatorMethods.

GRAMMAR OF OPERATORS

operator → operator-head operator-characters optoperator → dot-operator-head dot-operator-charactersoperator-head → / | = | - | + | ! | * | % | < | > | & | | | ^ | ~ | ?operator-head → U+00A1–U+00A7operator-head → U+00A9orU+00ABoperator-head → U+00ACorU+00AEoperator-head → U+00B0–U+00B1operator-head → U+00B6,U+00BB,U+00BF,U+00D7,orU+00F7operator-head → U+2016–U+2017operator-head → U+2020–U+2027operator-head → U+2030–U+203Eoperator-head → U+2041–U+2053operator-head → U+2055–U+205Eoperator-head → U+2190–U+23FFoperator-head → U+2500–U+2775operator-head → U+2794–U+2BFFoperator-head → U+2E00–U+2E7Foperator-head → U+3001–U+3003operator-head → U+3008–U+3020operator-head → U+3030operator-character → operator-headoperator-character → U+0300–U+036Foperator-character → U+1DC0–U+1DFFoperator-character → U+20D0–U+20FFoperator-character → U+FE00–U+FE0Foperator-character → U+FE20–U+FE2Foperator-character → U+E0100–U+E01EF

operator-characters → operator-character operator-characters optdot-operator-head → .dot-operator-character → . | operator-characterdot-operator-characters → dot-operator-character dot-operator-characters optbinary-operator → operatorprefix-operator → operatorpostfix-operator → operator

Types

InSwift,therearetwokindsoftypes:namedtypesandcompoundtypes.Anamedtypeisatypethatcanbegivenaparticularnamewhenit’sdefined.Namedtypesincludeclasses,structures,enumerations,andprotocols.Forexample,instancesofauser-definedclassnamedMyClasshavethetypeMyClass.Inadditiontouser-definednamedtypes,theSwiftstandardlibrarydefinesmanycommonlyusednamedtypes,includingthosethatrepresentarrays,dictionaries,andoptionalvalues.

Datatypesthatarenormallyconsideredbasicorprimitiveinotherlanguages—suchastypesthatrepresentnumbers,characters,andstrings—areactuallynamedtypes,definedandimplementedintheSwiftstandardlibraryusingstructures.Becausethey’renamedtypes,youcanextendtheirbehaviortosuittheneedsofyourprogram,usinganextensiondeclaration,discussedinExtensionsandExtensionDeclaration.

Acompoundtypeisatypewithoutaname,definedintheSwiftlanguageitself.Therearetwocompoundtypes:functiontypesandtupletypes.Acompoundtypemaycontainnamedtypesandothercompoundtypes.Forexample,thetupletype(Int,(Int,Int))containstwoelements:ThefirstisthenamedtypeInt,andthesecondisanothercompoundtype(Int,Int).

Youcanputparenthesesaroundanamedtypeoracompoundtype.However,addingparenthesesaroundatypedoesn’thaveanyeffect.Forexample,(Int)isequivalenttoInt.

ThischapterdiscussesthetypesdefinedintheSwiftlanguageitselfanddescribesthetypeinferencebehaviorofSwift.

GRAMMAR OF ATYPE

type → function-typetype → array-typetype → dictionary-typetype → type-identifiertype → tuple-typetype → optional-typetype → implicitly-unwrapped-optional-type

type → protocol-composition-typetype → opaque-typetype → metatype-typetype → self-typetype → Anytype → ( type )

TypeAnnotation

Atypeannotationexplicitlyspecifiesthetypeofavariableorexpression.Typeannotationsbeginwithacolon(:)andendwithatype,asthefollowingexamplesshow:

1 letsomeTuple:(Double,Double)=(3.14159,2.71828)

2 funcsomeFunction(a:Int){/*...*/}

Inthefirstexample,theexpressionsomeTupleisspecifiedtohavethetupletype(Double,Double).Inthesecondexample,theparameteratothefunctionsomeFunctionisspecifiedtohavethetypeInt.

Typeannotationscancontainanoptionallistoftypeattributesbeforethetype.

GRAMMAR OF ATYPE ANNOTAT ION

type-annotation → : attributes opt inoutopt type

TypeIdentifier

Atypeidentifierreferstoeitheranamedtypeoratypealiasofanamedorcompoundtype.

Mostofthetime,atypeidentifierdirectlyreferstoanamedtypewiththesamenameastheidentifier.Forexample,IntisatypeidentifierthatdirectlyreferstothenamedtypeInt,andthetypeidentifierDictionary<String,Int>directly

referstothenamedtypeDictionary<String,Int>.

Therearetwocasesinwhichatypeidentifierdoesn’trefertoatypewiththesamename.Inthefirstcase,atypeidentifierreferstoatypealiasofanamedorcompoundtype.Forinstance,intheexamplebelow,theuseofPointinthetypeannotationreferstothetupletype(Int,Int).

1 typealiasPoint=(Int,Int)

2 letorigin:Point=(0,0)

Inthesecondcase,atypeidentifierusesdot(.)syntaxtorefertonamedtypesdeclaredinothermodulesornestedwithinothertypes.Forexample,thetypeidentifierinthefollowingcodereferencesthenamedtypeMyTypethatisdeclaredintheExampleModulemodule.

varsomeValue:ExampleModule.MyType

GRAMMAR OF ATYPE I DENT I F I ER

type-identifier → type-name generic-argument-clause opt | type-name generic-argument-clause opt . type-identifier

type-name → identifier

TupleType

Atupletypeisacomma-separatedlistoftypes,enclosedinparentheses.

Youcanuseatupletypeasthereturntypeofafunctiontoenablethefunctiontoreturnasingletuplecontainingmultiplevalues.Youcanalsonametheelementsofatupletypeandusethosenamestorefertothevaluesoftheindividualelements.Anelementnameconsistsofanidentifierfollowedimmediatelybyacolon(:).Foranexamplethatdemonstratesbothofthesefeatures,seeFunctionswithMultipleReturnValues.

Whenanelementofatupletypehasaname,thatnameispartofthetype.

1 varsomeTuple=(top:10,bottom:12)//someTupleisoftype

(top:Int,bottom:Int)

2 someTuple=(top:4,bottom:42)//OK:namesmatch

3 someTuple=(9,99)//OK:namesareinferred

4 someTuple=(left:5,right:5)//Error:namesdon'tmatch

Alltupletypescontaintwoormoretypes,exceptforVoidwhichisatypealiasfortheemptytupletype,().

GRAMMAR OF ATUPLE TYPE

tuple-type → ( ) | ( tuple-type-element , tuple-type-element-list )tuple-type-element-list → tuple-type-element | tuple-type-element , tuple-type-element-

listtuple-type-element → element-name type-annotation | typeelement-name → identifier

FunctionType

Afunctiontyperepresentsthetypeofafunction,method,orclosureandconsistsofaparameterandreturntypeseparatedbyanarrow(->):

( parametertype )-> returntype

Theparametertypeiscomma-separatedlistoftypes.Becausethereturntypecanbeatupletype,functiontypessupportfunctionsandmethodsthatreturnmultiplevalues.

Aparameterofthefunctiontype()->T(whereTisanytype)canapplytheautoclosureattributetoimplicitlycreateaclosureatitscallsites.Thisprovidesasyntacticallyconvenientwaytodefertheevaluationofanexpressionwithoutneedingtowriteanexplicitclosurewhenyoucallthefunction.Foranexampleofanautoclosurefunctiontypeparameter,seeAutoclosures.

Afunctiontypecanhaveavariadicparameterinitsparametertype.Syntactically,avariadicparameterconsistsofabasetypenamefollowed

immediatelybythreedots(...),asinInt....Avariadicparameteristreatedasanarraythatcontainselementsofthebasetypename.Forinstance,thevariadicparameterInt...istreatedas[Int].Foranexamplethatusesavariadicparameter,seeVariadicParameters.

Tospecifyanin-outparameter,prefixtheparametertypewiththeinoutkeyword.Youcan’tmarkavariadicparameterorareturntypewiththeinoutkeyword.In-outparametersarediscussedinIn-OutParameters.

Ifafunctiontypehasonlyoneparameterandthatparameter’stypeisatupletype,thenthetupletypemustbeparenthesizedwhenwritingthefunction’stype.Forexample,((Int,Int))->Voidisthetypeofafunctionthattakesasingleparameterofthetupletype(Int,Int)anddoesn’treturnanyvalue.Incontrast,withoutparentheses,(Int,Int)->VoidisthetypeofafunctionthattakestwoIntparametersanddoesn’treturnanyvalue.Likewise,becauseVoidisatypealiasfor(),thefunctiontype(Void)->Voidisthesameas(())->()—afunctionthattakesasingleargumentthatisanemptytuple.Thesetypesarenotthesameas()->()—afunctionthattakesnoarguments.

Argumentnamesinfunctionsandmethodsarenotpartofthecorrespondingfunctiontype.Forexample:

1 funcsomeFunction(left:Int,right:Int){}

2 funcanotherFunction(left:Int,right:Int){}

3 funcfunctionWithDifferentLabels(top:Int,bottom:Int){}

4

5 varf=someFunction//Thetypeoffis(Int,Int)->Void,

not(left:Int,right:Int)->Void.

6 f=anotherFunction//OK

7 f=functionWithDifferentLabels//OK

8

9 funcfunctionWithDifferentArgumentTypes(left:Int,right:

String){}

10 f=functionWithDifferentArgumentTypes//Error

11

12 funcfunctionWithDifferentNumberOfArguments(left:Int,right:

Int,top:Int){}

13 f=functionWithDifferentNumberOfArguments//Error

Becauseargumentlabelsarenotpartofafunction’stype,youomitthemwhenwritingafunctiontype.

1 varoperation:(lhs:Int,rhs:Int)->Int//Error

2 varoperation:(_lhs:Int,_rhs:Int)->Int//OK

3 varoperation:(Int,Int)->Int//OK

Ifafunctiontypeincludesmorethanasinglearrow(->),thefunctiontypesaregroupedfromrighttoleft.Forexample,thefunctiontype(Int)->(Int)->Intisunderstoodas(Int)->((Int)->Int)—thatis,afunctionthattakesanIntandreturnsanotherfunctionthattakesandreturnsanInt.

Functiontypesthatcanthroworrethrowanerrormustbemarkedwiththethrowskeyword.Thethrowskeywordispartofafunction’stype,andnonthrowingfunctionsaresubtypesofthrowingfunctions.Asaresult,youcanuseanonthrowingfunctioninthesameplacesasathrowingone.ThrowingandrethrowingfunctionsaredescribedinThrowingFunctionsandMethodsandRethrowingFunctionsandMethods.

RestrictionsforNonescapingClosuresAparameterthat’sanonescapingfunctioncan’tbestoredinaproperty,variable,orconstantoftypeAny,becausethatmightallowthevaluetoescape.

Aparameterthat’sanonescapingfunctioncan’tbepassedasanargumenttoanothernonescapingfunctionparameter.ThisrestrictionhelpsSwiftperformmoreofitschecksforconflictingaccesstomemoryatcompiletimeinsteadofatruntime.Forexample:

1 letexternal:(()->Void)->Void={_in()}

2 functakesTwoFunctions(first:(()->Void)->Void,second:(()

->Void)->Void){

3 first{first{}}//Error

4 second{second{}}//Error

5

6 first{second{}}//Error

7 second{first{}}//Error

8

9 first{external{}}//OK

10 external{first{}}//OK

11 }

Inthecodeabove,bothoftheparameterstotakesTwoFunctions(first:second:)arefunctions.Neitherparameterismarked@escaping,sothey’rebothnonescapingasaresult.

Thefourfunctioncallsmarked“Error”intheexampleabovecausecompilererrors.Becausethefirstandsecondparametersarenonescapingfunctions,theycan’tbepassedasargumentstoanothernonescapingfunctionparameter.Incontrast,thetwofunctioncallsmarked“OK”don’tcauseacompilererror.Thesefunctioncallsdon’tviolatetherestrictionbecauseexternalisn’toneoftheparametersoftakesTwoFunctions(first:second:).

Ifyouneedtoavoidthisrestriction,markoneoftheparametersasescaping,ortemporarilyconvertoneofthenonescapingfunctionparameterstoanescapingfunctionbyusingthewithoutActuallyEscaping(_:do:)function.Forinformationaboutavoidingconflictingaccesstomemory,seeMemorySafety.

GRAMMAR OF A FUNCT ION TYPE

function-type → attributes opt function-type-argument-clause throwsopt -> typefunction-type-argument-clause → ( )function-type-argument-clause → ( function-type-argument-list ...opt )function-type-argument-list → function-type-argument | function-type-argument ,

function-type-argument-list

function-type-argument → attributes opt inoutopt type | argument-label type-annotation

argument-label → identifier

ArrayType

TheSwiftlanguageprovidesthefollowingsyntacticsugarfortheSwiftstandardlibraryArray<Element>type:

[ type ]

Inotherwords,thefollowingtwodeclarationsareequivalent:

1 letsomeArray:Array<String>=["Alex","Brian","Dave"]

2 letsomeArray:[String]=["Alex","Brian","Dave"]

Inbothcases,theconstantsomeArrayisdeclaredasanarrayofstrings.Theelementsofanarraycanbeaccessedthroughsubscriptingbyspecifyingavalidindexvalueinsquarebrackets:someArray[0]referstotheelementatindex0,"Alex".

Youcancreatemultidimensionalarraysbynestingpairsofsquarebrackets,wherethenameofthebasetypeoftheelementsiscontainedintheinnermostpairofsquarebrackets.Forexample,youcancreateathree-dimensionalarrayofintegersusingthreesetsofsquarebrackets:

vararray3D:[[[Int]]]=[[[1,2],[3,4]],[[5,6],[7,8]]]

Whenaccessingtheelementsinamultidimensionalarray,theleft-mostsubscriptindexreferstotheelementatthatindexintheoutermostarray.Thenextsubscriptindextotherightreferstotheelementatthatindexinthearraythat’snestedonelevelin.Andsoon.Thismeansthatintheexampleabove,array3D[0]refersto[[1,2],[3,4]],array3D[0][1]refersto[3,4],andarray3D[0][1][1]referstothevalue4.

ForadetaileddiscussionoftheSwiftstandardlibraryArraytype,seeArrays.

GRAMMAR OF AN ARRAY TYPE

array-type → [ type ]

DictionaryType

TheSwiftlanguageprovidesthefollowingsyntacticsugarfortheSwiftstandardlibraryDictionary<Key,Value>type:

[ keytype : valuetype ]

Inotherwords,thefollowingtwodeclarationsareequivalent:

1 letsomeDictionary:[String:Int]=["Alex":31,"Paul":39]

2 letsomeDictionary:Dictionary<String,Int>=["Alex":31,

"Paul":39]

Inbothcases,theconstantsomeDictionaryisdeclaredasadictionarywithstringsaskeysandintegersasvalues.

Thevaluesofadictionarycanbeaccessedthroughsubscriptingbyspecifyingthecorrespondingkeyinsquarebrackets:someDictionary["Alex"]referstothevalueassociatedwiththekey"Alex".Thesubscriptreturnsanoptionalvalueofthedictionary’svaluetype.Ifthespecifiedkeyisn’tcontainedinthedictionary,thesubscriptreturnsnil.

ThekeytypeofadictionarymustconformtotheSwiftstandardlibraryHashableprotocol.

ForadetaileddiscussionoftheSwiftstandardlibraryDictionarytype,seeDictionaries.

GRAMMAR OF AD ICT IONARY TYPE

dictionary-type → [ type : type ]

OptionalType

TheSwiftlanguagedefinesthepostfix?assyntacticsugarforthenamedtypeOptional<Wrapped>,whichisdefinedintheSwiftstandardlibrary.Inotherwords,thefollowingtwodeclarationsareequivalent:

1 varoptionalInteger:Int?

2 varoptionalInteger:Optional<Int>

Inbothcases,thevariableoptionalIntegerisdeclaredtohavethetypeofanoptionalinteger.Notethatnowhitespacemayappearbetweenthetypeandthe?.

ThetypeOptional<Wrapped>isanenumerationwithtwocases,noneandsome(Wrapped),whichareusedtorepresentvaluesthatmayormaynotbepresent.Anytypecanbeexplicitlydeclaredtobe(orimplicitlyconvertedto)anoptionaltype.Ifyoudon’tprovideaninitialvaluewhenyoudeclareanoptionalvariableorproperty,itsvalueautomaticallydefaultstonil.

Ifaninstanceofanoptionaltypecontainsavalue,youcanaccessthatvalueusingthepostfixoperator!,asshownbelow:

1 optionalInteger=42

2 optionalInteger!//42

Usingthe!operatortounwrapanoptionalthathasavalueofnilresultsinaruntimeerror.

Youcanalsouseoptionalchainingandoptionalbindingtoconditionallyperformanoperationonanoptionalexpression.Ifthevalueisnil,nooperationisperformedandthereforenoruntimeerrorisproduced.

Formoreinformationandtoseeexamplesthatshowhowtouseoptionaltypes,

seeOptionals.

GRAMMAR OF AN OPT IONALTYPE

optional-type → type ?

ImplicitlyUnwrappedOptionalType

TheSwiftlanguagedefinesthepostfix!assyntacticsugarforthenamedtypeOptional<Wrapped>,whichisdefinedintheSwiftstandardlibrary,withtheadditionalbehaviorthatit’sautomaticallyunwrappedwhenit’saccessed.Ifyoutrytouseanimplicitlyunwrappedoptionalthathasavalueofnil,you’llgetaruntimeerror.Withtheexceptionoftheimplicitunwrappingbehavior,thefollowingtwodeclarationsareequivalent:

1 varimplicitlyUnwrappedString:String!

2 varexplicitlyUnwrappedString:Optional<String>

Notethatnowhitespacemayappearbetweenthetypeandthe!.

Becauseimplicitunwrappingchangesthemeaningofthedeclarationthatcontainsthattype,optionaltypesthatarenestedinsideatupletypeoragenerictype—suchastheelementtypesofadictionaryorarray—can’tbemarkedasimplicitlyunwrapped.Forexample:

1 lettupleOfImplicitlyUnwrappedElements:(Int!,Int!)//Error

2 letimplicitlyUnwrappedTuple:(Int,Int)!//OK

3

4 letarrayOfImplicitlyUnwrappedElements:[Int!]//Error

5 letimplicitlyUnwrappedArray:[Int]!//OK

BecauseimplicitlyunwrappedoptionalshavethesameOptional<Wrapped>typeasoptionalvalues,youcanuseimplicitlyunwrappedoptionalsinallthesameplacesinyourcodethatyoucanuseoptionals.Forexample,youcanassign

valuesofimplicitlyunwrappedoptionalstovariables,constants,andpropertiesofoptionals,andviceversa.

Aswithoptionals,ifyoudon’tprovideaninitialvaluewhenyoudeclareanimplicitlyunwrappedoptionalvariableorproperty,itsvalueautomaticallydefaultstonil.

Useoptionalchainingtoconditionallyperformanoperationonanimplicitlyunwrappedoptionalexpression.Ifthevalueisnil,nooperationisperformedandthereforenoruntimeerrorisproduced.

Formoreinformationaboutimplicitlyunwrappedoptionaltypes,seeImplicitlyUnwrappedOptionals.

GRAMMAR OF AN IMPL IC I T LY UNWRAPPED OPT IONALTYPE

implicitly-unwrapped-optional-type → type !

ProtocolCompositionType

Aprotocolcompositiontypedefinesatypethatconformstoeachprotocolinalistofspecifiedprotocols,oratypethatisasubclassofagivenclassandconformstoeachprotocolinalistofspecifiedprotocols.Protocolcompositiontypesmaybeusedonlywhenspecifyingatypeintypeannotations,ingenericparameterclauses,andingenericwhereclauses.

Protocolcompositiontypeshavethefollowingform:

Protocol1 & Protocol2

Aprotocolcompositiontypeallowsyoutospecifyavaluewhosetypeconformstotherequirementsofmultipleprotocolswithoutexplicitlydefininganew,namedprotocolthatinheritsfromeachprotocolyouwantthetypetoconformto.Forexample,youcanusetheprotocolcompositiontypeProtocolA&ProtocolB&ProtocolCinsteadofdeclaringanewprotocolthatinheritsfromProtocolA,ProtocolB,andProtocolC.Likewise,youcanuseSuperClass&ProtocolA

insteadofdeclaringanewprotocolthatisasubclassofSuperClassandconformstoProtocolA.

Eachiteminaprotocolcompositionlistisoneofthefollowing;thelistcancontainatmostoneclass:

Thenameofaclass

Thenameofaprotocol

Atypealiaswhoseunderlyingtypeisaprotocolcompositiontype,aprotocol,oraclass.

Whenaprotocolcompositiontypecontainstypealiases,it’spossibleforthesameprotocoltoappearmorethanonceinthedefinitions—duplicatesareignored.Forexample,thedefinitionofPQRinthecodebelowisequivalenttoP&Q&R.

1 typealiasPQ=P&Q

2 typealiasPQR=PQ&Q&R

GRAMMAR OF A PROTOCOLCOMPOS I T ION TYPE

protocol-composition-type → type-identifier & protocol-composition-continuationprotocol-composition-continuation → type-identifier | protocol-composition-type

OpaqueType

Anopaquetypedefinesatypethatconformstoaprotocolorprotocolcomposition,withoutspecifyingtheunderlyingconcretetype.

Opaquetypesappearasthereturntypeofafunctionorsubscript,orthetypeofaproperty.Opaquetypescan’tappearaspartofatupletypeoragenerictype,suchastheelementtypeofanarrayorthewrappedtypeofanoptional.

Opaquetypeshavethefollowingform:

some constraint

Theconstraintisaclasstype,protocoltype,protocolcompositiontype,orAny.Avaluecanbeusedasaninstanceoftheopaquetypeonlyifit’saninstanceofatypethatconformstothelistedprotocolorprotocolcomposition,orinheritsfromthelistedclass.Codethatinteractswithanopaquevaluecanusethevalueonlyinwaysthatarepartoftheinterfacedefinedbytheconstraint.

Protocoldeclarationscan’tincludeopaquetypes.Classescan’tuseanopaquetypeasthereturntypeofanonfinalmethod.

Afunctionthatusesanopaquetypeasitsreturntypemustreturnvaluesthatshareasingleunderlyingtype.Thereturntypecanincludetypesthatarepartofthefunction’sgenerictypeparameters.Forexample,afunctionsomeFunction<T>()couldreturnavalueoftypeTorDictionary<String,T>.

GRAMMAR OF AN OPAQUE TYPE

opaque-type → some type

MetatypeType

Ametatypetypereferstothetypeofanytype,includingclasstypes,structuretypes,enumerationtypes,andprotocoltypes.

Themetatypeofaclass,structure,orenumerationtypeisthenameofthattypefollowedby.Type.Themetatypeofaprotocoltype—nottheconcretetypethatconformstotheprotocolatruntime—isthenameofthatprotocolfollowedby.Protocol.Forexample,themetatypeoftheclasstypeSomeClassisSomeClass.TypeandthemetatypeoftheprotocolSomeProtocolisSomeProtocol.Protocol.

Youcanusethepostfixselfexpressiontoaccessatypeasavalue.Forexample,SomeClass.selfreturnsSomeClassitself,notaninstanceofSomeClass.AndSomeProtocol.selfreturnsSomeProtocolitself,notaninstanceofatypethatconformstoSomeProtocolatruntime.Youcancallthetype(of:)functionwith

aninstanceofatypetoaccessthatinstance’sdynamic,runtimetypeasavalue,asthefollowingexampleshows:

1 classSomeBaseClass{

2 classfuncprintClassName(){

3 print("SomeBaseClass")

4 }

5 }

6 classSomeSubClass:SomeBaseClass{

7 overrideclassfuncprintClassName(){

8 print("SomeSubClass")

9 }

10 }

11 letsomeInstance:SomeBaseClass=SomeSubClass()

12 //Thecompile-timetypeofsomeInstanceisSomeBaseClass,

13 //andtheruntimetypeofsomeInstanceisSomeSubClass

14 type(of:someInstance).printClassName()

15 //Prints"SomeSubClass"

Formoreinformation,seetype(of:)intheSwiftstandardlibrary.

Useaninitializerexpressiontoconstructaninstanceofatypefromthattype’smetatypevalue.Forclassinstances,theinitializerthat’scalledmustbemarkedwiththerequiredkeywordortheentireclassmarkedwiththefinalkeyword.

1 classAnotherSubClass:SomeBaseClass{

2 letstring:String

3 requiredinit(string:String){

4 self.string=string

5 }

6 overrideclassfuncprintClassName(){

7 print("AnotherSubClass")

8 }

9 }

10 letmetatype:AnotherSubClass.Type=AnotherSubClass.self

11 letanotherInstance=metatype.init(string:"somestring")

GRAMMAR OF AMETATYPE TYPE

metatype-type → type . Type | type . Protocol

SelfType

TheSelftypeisn’taspecifictype,butratherletsyouconvenientlyrefertothecurrenttypewithoutrepeatingorknowingthattype’sname.

Inaprotocoldeclarationoraprotocolmemberdeclaration,theSelftypereferstotheeventualtypethatconformstotheprotocol.

Inastructure,class,orenumerationdeclaration,theSelftypereferstothetypeintroducedbythedeclaration.Insidethedeclarationforamemberofatype,theSelftypereferstothattype.Inthemembersofaclassdeclaration,Selfcanappearasthereturntypeofamethodandinthebodyofamethod,butnotinanyothercontext.Forexample,thecodebelowshowsaninstancemethodfwhosereturntypeisSelf.

1 classSuperclass{

2 funcf()->Self{returnself}

3 }

4 letx=Superclass()

5 print(type(of:x.f()))

6 //Prints"Superclass"

7

8 classSubclass:Superclass{}

9 lety=Subclass()

10 print(type(of:y.f()))

11 //Prints"Subclass"

12

13 letz:Superclass=Subclass()

14 print(type(of:z.f()))

15 //Prints"Subclass"

ThelastpartoftheexampleaboveshowsthatSelfreferstotheruntimetypeSubclassofthevalueofz,notthecompile-timetypeSuperclassofthevariableitself.

Insideanestedtypedeclaration,theSelftypereferstothetypeintroducedbytheinnermosttypedeclaration.

TheSelftypereferstothesametypeasthetype(of:)functionintheSwiftstandardlibrary.WritingSelf.someStaticMembertoaccessamemberofthecurrenttypeisthesameaswritingtype(of:self).someStaticMember.

GRAMMAR OF A SELF TYPE

self-type → Self

TypeInheritanceClause

Atypeinheritanceclauseisusedtospecifywhichclassanamedtypeinheritsfromandwhichprotocolsanamedtypeconformsto.Atypeinheritanceclausebeginswithacolon(:),followedbyalistoftypeidentifiers.

Classtypescaninheritfromasinglesuperclassandconformtoanynumberofprotocols.Whendefiningaclass,thenameofthesuperclassmustappearfirstinthelistoftypeidentifiers,followedbyanynumberofprotocolstheclassmustconformto.Iftheclassdoesn’tinheritfromanotherclass,thelistcanbeginwithaprotocolinstead.Foranextendeddiscussionandseveralexamplesofclassinheritance,seeInheritance.

Othernamedtypescanonlyinheritfromorconformtoalistofprotocols.Protocoltypescaninheritfromanynumberofotherprotocols.Whenaprotocoltypeinheritsfromotherprotocols,thesetofrequirementsfromthoseotherprotocolsareaggregatedtogether,andanytypethatinheritsfromthecurrentprotocolmustconformtoallofthoserequirements.

Atypeinheritanceclauseinanenumerationdefinitioncanbeeitheralistofprotocols,orinthecaseofanenumerationthatassignsrawvaluestoitscases,asingle,namedtypethatspecifiesthetypeofthoserawvalues.Foranexampleofanenumerationdefinitionthatusesatypeinheritanceclausetospecifythetypeofitsrawvalues,seeRawValues.

GRAMMAR OF ATYPE I NHER I TANCE CLAUSE

type-inheritance-clause → : type-inheritance-listtype-inheritance-list → type-identifier | type-identifier , type-inheritance-list

TypeInference

Swiftusestypeinferenceextensively,allowingyoutoomitthetypeorpartofthetypeofmanyvariablesandexpressionsinyourcode.Forexample,insteadofwritingvarx:Int=0,youcanwritevarx=0,omittingthetypecompletely—thecompilercorrectlyinfersthatxnamesavalueoftypeInt.Similarly,youcanomitpartofatypewhenthefulltypecanbeinferredfromcontext.Forexample,ifyouwriteletdict:Dictionary=["A":1],thecompilerinfersthatdicthasthetypeDictionary<String,Int>.

Inbothoftheexamplesabove,thetypeinformationispassedupfromtheleavesoftheexpressiontreetoitsroot.Thatis,thetypeofxinvarx:Int=0isinferredbyfirstcheckingthetypeof0andthenpassingthistypeinformationuptotheroot(thevariablex).

InSwift,typeinformationcanalsoflowintheoppositedirection—fromtherootdowntotheleaves.Inthefollowingexample,forinstance,theexplicittypeannotation(:Float)ontheconstanteFloatcausesthenumericliteral2.71828tohaveaninferredtypeofFloatinsteadofDouble.

1 lete=2.71828//ThetypeofeisinferredtobeDouble.

2 leteFloat:Float=2.71828//ThetypeofeFloatisFloat.

TypeinferenceinSwiftoperatesatthelevelofasingleexpressionorstatement.Thismeansthatalloftheinformationneededtoinferanomittedtypeorpartofatypeinanexpressionmustbeaccessiblefromtype-checkingtheexpressionoroneofitssubexpressions.

Expressions

InSwift,therearefourkindsofexpressions:prefixexpressions,binaryexpressions,primaryexpressions,andpostfixexpressions.Evaluatinganexpressionreturnsavalue,causesasideeffect,orboth.

Prefixandbinaryexpressionsletyouapplyoperatorstosmallerexpressions.Primaryexpressionsareconceptuallythesimplestkindofexpression,andtheyprovideawaytoaccessvalues.Postfixexpressions,likeprefixandbinaryexpressions,letyoubuildupmorecomplexexpressionsusingpostfixessuchasfunctioncallsandmemberaccess.Eachkindofexpressionisdescribedindetailinthesectionsbelow.

GRAMMAR OF AN EXPRESS ION

expression → try-operator opt prefix-expression binary-expressions optexpression-list → expression | expression , expression-list

PrefixExpressions

Prefixexpressionscombineanoptionalprefixoperatorwithanexpression.Prefixoperatorstakeoneargument,theexpressionthatfollowsthem.

Forinformationaboutthebehavioroftheseoperators,seeBasicOperatorsandAdvancedOperators.

ForinformationabouttheoperatorsprovidedbytheSwiftstandardlibrary,seeOperatorDeclarations.

Inadditiontothestandardlibraryoperators,youuse&immediatelybeforethenameofavariablethat’sbeingpassedasanin-outargumenttoafunctioncallexpression.Formoreinformationandtoseeanexample,seeIn-OutParameters.

GRAMMAR OF A PREF IX EXPRESS ION

prefix-expression → prefix-operator opt postfix-expression

prefix-expression → in-out-expressionin-out-expression → & identifier

TryOperatorAtryexpressionconsistsofthetryoperatorfollowedbyanexpressionthatcanthrowanerror.Ithasthefollowingform:

try expression

Anoptional-tryexpressionconsistsofthetry?operatorfollowedbyanexpressionthatcanthrowanerror.Ithasthefollowingform:

try? expression

Iftheexpressiondoesnotthrowanerror,thevalueoftheoptional-tryexpressionisanoptionalcontainingthevalueoftheexpression.Otherwise,thevalueoftheoptional-tryexpressionisnil.

Aforced-tryexpressionconsistsofthetry!operatorfollowedbyanexpressionthatcanthrowanerror.Ithasthefollowingform:

try! expression

Iftheexpressionthrowsanerror,aruntimeerrorisproduced.

Whentheexpressionontheleft-handsideofabinaryoperatorismarkedwithtry,try?,ortry!,thatoperatorappliestothewholebinaryexpression.Thatsaid,youcanuseparenthesestobeexplicitaboutthescopeoftheoperator’sapplication.

1 sum=trysomeThrowingFunction()+anotherThrowingFunction()

//tryappliestobothfunctioncalls

2 sum=try(someThrowingFunction()+anotherThrowingFunction())

//tryappliestobothfunctioncalls

3 sum=(trysomeThrowingFunction())+anotherThrowingFunction()

//Error:tryappliesonlytothefirstfunctioncall

Atryexpressioncan’tappearontheright-handsideofabinaryoperator,unlessthebinaryoperatoristheassignmentoperatororthetryexpressionisenclosedinparentheses.

Formoreinformationandtoseeexamplesofhowtousetry,try?,andtry!,seeErrorHandling.

GRAMMAR OF ATRY EXPRESS ION

try-operator → try | try ? | try !

BinaryExpressions

Binaryexpressionscombineaninfixbinaryoperatorwiththeexpressionthatittakesasitsleft-handandright-handarguments.Ithasthefollowingform:

left-handargument operator right-handargument

Forinformationaboutthebehavioroftheseoperators,seeBasicOperatorsandAdvancedOperators.

ForinformationabouttheoperatorsprovidedbytheSwiftstandardlibrary,seeOperatorDeclarations.

NOTE

Atparsetime,anexpressionmadeupofbinaryoperatorsisrepresentedasaflatlist.Thislististransformedintoatreebyapplyingoperatorprecedence.Forexample,theexpression2+3*5isinitiallyunderstoodasaflatlistoffiveitems,2,+,3,*,and5.Thisprocesstransformsitintothetree(2+(3*5)).

GRAMMAR OF A B INARY EXPRESS ION

binary-expression → binary-operator prefix-expressionbinary-expression → assignment-operator try-operator opt prefix-expression

binary-expression → conditional-operator try-operator opt prefix-expressionbinary-expression → type-casting-operatorbinary-expressions → binary-expression binary-expressions opt

AssignmentOperatorTheassignmentoperatorsetsanewvalueforagivenexpression.Ithasthefollowingform:

expression = value

Thevalueoftheexpressionissettothevalueobtainedbyevaluatingthevalue.Iftheexpressionisatuple,thevaluemustbeatuplewiththesamenumberofelements.(Nestedtuplesareallowed.)Assignmentisperformedfromeachpartofthevaluetothecorrespondingpartoftheexpression.Forexample:

1 (a,_,(b,c))=("test",9.45,(12,3))

2 //ais"test",bis12,cis3,and9.45isignored

Theassignmentoperatordoesnotreturnanyvalue.

GRAMMAR OF AN ASS IGNMENT OPERATOR

assignment-operator → =

TernaryConditionalOperatorTheternaryconditionaloperatorevaluatestooneoftwogivenvaluesbasedonthevalueofacondition.Ithasthefollowingform:

condition ? expressionusediftrue :

expressionusediffalse

Iftheconditionevaluatestotrue,theconditionaloperatorevaluatesthefirstexpressionandreturnsitsvalue.Otherwise,itevaluatesthesecondexpression

andreturnsitsvalue.Theunusedexpressionisnotevaluated.

Foranexamplethatusestheternaryconditionaloperator,seeTernaryConditionalOperator.

GRAMMAR OF ACOND I T IONALOPERATOR

conditional-operator → ? expression :

Type-CastingOperatorsTherearefourtype-castingoperators:theisoperator,theasoperator,theas?operator,andtheas!operator.

Theyhavethefollowingform:

expression is type

expression as type

expression as? type

expression as! type

Theisoperatorchecksatruntimewhethertheexpressioncanbecasttothespecifiedtype.Itreturnstrueiftheexpressioncanbecasttothespecifiedtype;otherwise,itreturnsfalse.

Theasoperatorperformsacastwhenitisknownatcompiletimethatthecastalwayssucceeds,suchasupcastingorbridging.Upcastingletsyouuseanexpressionasaninstanceofitstype’ssupertype,withoutusinganintermediatevariable.Thefollowingapproachesareequivalent:

1 funcf(_any:Any){print("FunctionforAny")}

2 funcf(_int:Int){print("FunctionforInt")}

3 letx=10

4 f(x)

5 //Prints"FunctionforInt"

6

7 lety:Any=x

8 f(y)

9 //Prints"FunctionforAny"

10

11 f(xasAny)

12 //Prints"FunctionforAny"

BridgingletsyouuseanexpressionofaSwiftstandardlibrarytypesuchasStringasitscorrespondingFoundationtypesuchasNSStringwithoutneedingtocreateanewinstance.Formoreinformationonbridging,seeWorkingwithFoundationTypes.

Theas?operatorperformsaconditionalcastoftheexpressiontothespecifiedtype.Theas?operatorreturnsanoptionalofthespecifiedtype.Atruntime,ifthecastsucceeds,thevalueofexpressioniswrappedinanoptionalandreturned;otherwise,thevaluereturnedisnil.Ifcastingtothespecifiedtypeisguaranteedtofailorisguaranteedtosucceed,acompile-timeerrorisraised.

Theas!operatorperformsaforcedcastoftheexpressiontothespecifiedtype.Theas!operatorreturnsavalueofthespecifiedtype,notanoptionaltype.Ifthecastfails,aruntimeerrorisraised.Thebehaviorofxas!Tisthesameasthebehaviorof(xas?T)!.

Formoreinformationabouttypecastingandtoseeexamplesthatusethetype-castingoperators,seeTypeCasting.

GRAMMAR OF ATYPE -CAST ING OPERATOR

type-casting-operator → is typetype-casting-operator → as typetype-casting-operator → as ? typetype-casting-operator → as ! type

PrimaryExpressions

Primaryexpressionsarethemostbasickindofexpression.Theycanbeusedasexpressionsontheirown,andtheycanbecombinedwithothertokenstomakeprefixexpressions,binaryexpressions,andpostfixexpressions.

GRAMMAR OF A PR IMARY EXPRESS ION

primary-expression → identifier generic-argument-clause optprimary-expression → literal-expressionprimary-expression → self-expressionprimary-expression → superclass-expressionprimary-expression → closure-expressionprimary-expression → parenthesized-expressionprimary-expression → tuple-expressionprimary-expression → implicit-member-expressionprimary-expression → wildcard-expressionprimary-expression → key-path-expressionprimary-expression → selector-expressionprimary-expression → key-path-string-expression

LiteralExpressionAliteralexpressionconsistsofeitheranordinaryliteral(suchasastringoranumber),anarrayordictionaryliteral,aplaygroundliteral,oroneofthefollowingspecialliterals:

Literal Type Value

#file String Thenameofthefileinwhichitappears.

#line Int Thelinenumberonwhichitappears.

#column Int Thecolumnnumberinwhichitbegins.

#function String Thenameofthedeclarationinwhichitappears.

#dsohandle UnsafeRawPointer TheDSO(dynamicsharedobject)handleinusewhereitappears.

Insideafunction,thevalueof#functionisthenameofthatfunction,insideamethoditisthenameofthatmethod,insideapropertygetterorsetteritisthenameofthatproperty,insidespecialmemberslikeinitorsubscriptitisthenameofthatkeyword,andatthetoplevelofafileitisthenameofthecurrentmodule.

Whenusedasthedefaultvalueofafunctionormethodparameter,thespecialliteral’svalueisdeterminedwhenthedefaultvalueexpressionisevaluatedatthecallsite.

1 funclogFunctionName(string:String=#function){

2 print(string)

3 }

4 funcmyFunction(){

5 logFunctionName()//Prints"myFunction()".

6 }

Anarrayliteralisanorderedcollectionofvalues.Ithasthefollowingform:

[ value1 , value2 , ... ]

Thelastexpressioninthearraycanbefollowedbyanoptionalcomma.Thevalueofanarrayliteralhastype[T],whereTisthetypeoftheexpressionsinsideit.Ifthereareexpressionsofmultipletypes,Tistheirclosestcommonsupertype.Emptyarrayliteralsarewrittenusinganemptypairofsquarebracketsandcanbeusedtocreateanemptyarrayofaspecifiedtype.

varemptyArray:[Double]=[]

Adictionaryliteralisanunorderedcollectionofkey-valuepairs.Ithasthefollowingform:

[ key1 : value1 , key2 : value2 , ... ]

Thelastexpressioninthedictionarycanbefollowedbyanoptionalcomma.Thevalueofadictionaryliteralhastype[Key:Value],whereKeyisthetypeofitskeyexpressionsandValueisthetypeofitsvalueexpressions.Ifthereareexpressionsofmultipletypes,KeyandValuearetheclosestcommonsupertypefortheirrespectivevalues.Anemptydictionaryliteraliswrittenasacoloninsideapairofbrackets([:])todistinguishitfromanemptyarrayliteral.Youcanuseanemptydictionaryliteraltocreateanemptydictionaryliteralofspecifiedkeyandvaluetypes.

varemptyDictionary:[String:Double]=[:]

AplaygroundliteralisusedbyXcodetocreateaninteractiverepresentationofacolor,file,orimagewithintheprogrameditor.PlaygroundliteralsinplaintextoutsideofXcodearerepresentedusingaspecialliteralsyntax.

ForinformationonusingplaygroundliteralsinXcode,seeAddacolor,file,orimageliteralinXcodeHelp.

GRAMMAR OF A L I TERALEXPRESS ION

literal-expression → literalliteral-expression → array-literal | dictionary-literal | playground-literalliteral-expression → #file | #line | #column | #function | #dsohandlearray-literal → [ array-literal-items opt ]array-literal-items → array-literal-item ,opt | array-literal-item , array-literal-itemsarray-literal-item → expressiondictionary-literal → [ dictionary-literal-items ] | [ : ]dictionary-literal-items → dictionary-literal-item ,opt | dictionary-literal-item , dictionary-

literal-itemsdictionary-literal-item → expression : expressionplayground-literal → #colorLiteral ( red : expression , green : expression

, blue : expression , alpha : expression )playground-literal → #fileLiteral ( resourceName : expression )playground-literal → #imageLiteral ( resourceName : expression )

SelfExpression

Theselfexpressionisanexplicitreferencetothecurrenttypeorinstanceofthetypeinwhichitoccurs.Ithasthefollowingforms:

self

self. membername

self[ subscriptindex ]

self( initializerarguments )

self.init( initializerarguments )

Inaninitializer,subscript,orinstancemethod,selfreferstothecurrentinstanceofthetypeinwhichitoccurs.Inatypemethod,selfreferstothecurrenttypeinwhichitoccurs.

Theselfexpressionisusedtospecifyscopewhenaccessingmembers,providingdisambiguationwhenthereisanothervariableofthesamenameinscope,suchasafunctionparameter.Forexample:

1 classSomeClass{

2 vargreeting:String

3 init(greeting:String){

4 self.greeting=greeting

5 }

6 }

Inamutatingmethodofavaluetype,youcanassignanewinstanceofthatvaluetypetoself.Forexample:

1 structPoint{

2 varx=0.0,y=0.0

3 mutatingfuncmoveBy(xdeltaX:Double,ydeltaY:Double){

4 self=Point(x:x+deltaX,y:y+deltaY)

5 }

6 }

GRAMMAR OF A SELF EXPRESS ION

self-expression → self | self-method-expression | self-subscript-expression | self-initializer-expression

self-method-expression → self . identifierself-subscript-expression → self [ function-call-argument-list ]self-initializer-expression → self . init

SuperclassExpressionAsuperclassexpressionletsaclassinteractwithitssuperclass.Ithasoneofthefollowingforms:

super. membername

super[ subscriptindex ]

super.init( initializerarguments )

Thefirstformisusedtoaccessamemberofthesuperclass.Thesecondformisusedtoaccessthesuperclass’ssubscriptimplementation.Thethirdformisusedtoaccessaninitializerofthesuperclass.

Subclassescanuseasuperclassexpressionintheirimplementationofmembers,subscripting,andinitializerstomakeuseoftheimplementationintheirsuperclass.

GRAMMAR OF A SUPERCLASS EXPRESS ION

superclass-expression → superclass-method-expression | superclass-subscript-expression | superclass-initializer-expression

superclass-method-expression → super . identifiersuperclass-subscript-expression → super [ function-call-argument-list ]superclass-initializer-expression → super . init

ClosureExpressionAclosureexpressioncreatesaclosure,alsoknownasalambdaorananonymousfunctioninotherprogramminglanguages.Likeafunctiondeclaration,aclosurecontainsstatements,anditcapturesconstantsandvariablesfromitsenclosing

scope.Ithasthefollowingform:

{( parameters )-> returntype in

statements

}

Theparametershavethesameformastheparametersinafunctiondeclaration,asdescribedinFunctionDeclaration.

Thereareseveralspecialformsthatallowclosurestobewrittenmoreconcisely:

Aclosurecanomitthetypesofitsparameters,itsreturntype,orboth.Ifyouomittheparameternamesandbothtypes,omittheinkeywordbeforethestatements.Iftheomittedtypescan’tbeinferred,acompile-timeerrorisraised.

Aclosuremayomitnamesforitsparameters.Itsparametersarethenimplicitlynamed$followedbytheirposition:$0,$1,$2,andsoon.

Aclosurethatconsistsofonlyasingleexpressionisunderstoodtoreturnthevalueofthatexpression.Thecontentsofthisexpressionarealsoconsideredwhenperformingtypeinferenceonthesurroundingexpression.

Thefollowingclosureexpressionsareequivalent:

1 myFunction{(x:Int,y:Int)->Intin

2 returnx+y

3 }

4

5 myFunction{x,yin

6 returnx+y

7 }

8

9 myFunction{return$0+$1}

10

11 myFunction{$0+$1}

Forinformationaboutpassingaclosureasanargumenttoafunction,seeFunctionCallExpression.

Closureexpressionscanbeusedwithoutbeingstoredinavariableorconstant,suchaswhenyouimmediatelyuseaclosureaspartofafunctioncall.TheclosureexpressionspassedtomyFunctionincodeaboveareexamplesofthiskindofimmediateuse.Asaresult,whetheraclosureexpressionisescapingornonescapingdependsonthesurroundingcontextoftheexpression.Aclosureexpressionisnonescapingifitiscalledimmediatelyorpassedasanonescapingfunctionargument.Otherwise,theclosureexpressionisescaping.

Formoreinformationaboutescapingclosures,seeEscapingClosures.

CaptureLists

Bydefault,aclosureexpressioncapturesconstantsandvariablesfromitssurroundingscopewithstrongreferencestothosevalues.Youcanuseacapturelisttoexplicitlycontrolhowvaluesarecapturedinaclosure.

Acapturelistiswrittenasacomma-separatedlistofexpressionssurroundedbysquarebrackets,beforethelistofparameters.Ifyouuseacapturelist,youmustalsousetheinkeyword,evenifyouomittheparameternames,parametertypes,andreturntype.

Theentriesinthecapturelistareinitializedwhentheclosureiscreated.Foreachentryinthecapturelist,aconstantisinitializedtothevalueoftheconstantorvariablethathasthesamenameinthesurroundingscope.Forexampleinthecodebelow,aisincludedinthecapturelistbutbisnot,whichgivesthemdifferentbehavior.

1 vara=0

2 varb=0

3 letclosure={[a]in

4 print(a,b)

5 }

6

7 a=10

8 b=10

9 closure()

10 //Prints"010"

Therearetwodifferentthingsnameda,thevariableinthesurroundingscopeandtheconstantintheclosure’sscope,butonlyonevariablenamedb.Theaintheinnerscopeisinitializedwiththevalueoftheaintheouterscopewhentheclosureiscreated,buttheirvaluesarenotconnectedinanyspecialway.Thismeansthatachangetothevalueofaintheouterscopedoesnotaffectthevalueofaintheinnerscope,nordoesachangetoainsidetheclosureaffectthevalueofaoutsidetheclosure.Incontrast,thereisonlyonevariablenamedb—thebintheouterscope—sochangesfrominsideoroutsidetheclosurearevisibleinbothplaces.

Thisdistinctionisnotvisiblewhenthecapturedvariable’stypehasreferencesemantics.Forexample,therearetwothingsnamedxinthecodebelow,avariableintheouterscopeandaconstantintheinnerscope,buttheybothrefertothesameobjectbecauseofreferencesemantics.

1 classSimpleClass{

2 varvalue:Int=0

3 }

4 varx=SimpleClass()

5 vary=SimpleClass()

6 letclosure={[x]in

7 print(x.value,y.value)

8 }

9

10 x.value=10

11 y.value=10

12 closure()

13 //Prints"1010"

Ifthetypeoftheexpression’svalueisaclass,youcanmarktheexpressioninacapturelistwithweakorunownedtocaptureaweakorunownedreferencetotheexpression’svalue.

1 myFunction{print(self.title)}//implicit

strongcapture

2 myFunction{[self]inprint(self.title)}//explicit

strongcapture

3 myFunction{[weakself]inprint(self!.title)}//weak

capture

4 myFunction{[unownedself]inprint(self.title)}//unowned

capture

Youcanalsobindanarbitraryexpressiontoanamedvalueinacapturelist.Theexpressionisevaluatedwhentheclosureiscreated,andthevalueiscapturedwiththespecifiedstrength.Forexample:

1 //Weakcaptureof"self.parent"as"parent"

2 myFunction{[weakparent=self.parent]in

print(parent!.title)}

Formoreinformationandexamplesofclosureexpressions,seeClosureExpressions.Formoreinformationandexamplesofcapturelists,seeResolvingStrongReferenceCyclesforClosures.

GRAMMAR OF ACLOSURE EXPRESS ION

closure-expression → { closure-signature opt statements opt }closure-signature → capture-list opt closure-parameter-clause throwsopt function-

result opt inclosure-signature → capture-list inclosure-parameter-clause → ( ) | ( closure-parameter-list ) | identifier-listclosure-parameter-list → closure-parameter | closure-parameter , closure-parameter-

listclosure-parameter → closure-parameter-name type-annotation optclosure-parameter → closure-parameter-name type-annotation ...closure-parameter-name → identifiercapture-list → [ capture-list-items ]capture-list-items → capture-list-item | capture-list-item , capture-list-itemscapture-list-item → capture-specifier opt expressioncapture-specifier → weak | unowned | unowned(safe) | unowned(unsafe)

ImplicitMemberExpressionAnimplicitmemberexpressionisanabbreviatedwaytoaccessamemberofatype,suchasanenumerationcaseoratypemethod,inacontextwheretypeinferencecandeterminetheimpliedtype.Ithasthefollowingform:

. membername

Forexample:

1 varx=MyEnumeration.someValue

2 x=.anotherValue

GRAMMAR OF A IMPL IC I T MEMBER EXPRESS ION

implicit-member-expression → . identifier

ParenthesizedExpressionAparenthesizedexpressionconsistsofanexpressionsurroundedbyparentheses.Youcanuseparenthesestospecifytheprecedenceofoperationsbyexplicitlygroupingexpressions.Groupingparenthesesdon’tchangeanexpression’stype—forexample,thetypeof(1)issimplyInt.

GRAMMAR OF A PARENTHES I ZED EXPRESS ION

parenthesized-expression → ( expression )

TupleExpressionAtupleexpressionconsistsofacomma-separatedlistofexpressionssurroundedbyparentheses.Eachexpressioncanhaveanoptionalidentifierbeforeit,separatedbyacolon(:).Ithasthefollowingform:

( identifier1 : expression1 , identifier2 : expression2 ,

... )

Eachidentifierinatupleexpressionmustbeuniquewithinthescopeofthetupleexpression.Inanestedtupleexpression,identifiersatthesamelevelofnestingmustbeunique.Forexample,(a:10,a:20)isinvalidbecausethelabelaappearstwiceatthesamelevel.However,(a:10,b:(a:1,x:2))isvalid—althoughaappearstwice,itappearsonceintheoutertupleandonceintheinnertuple.

Atupleexpressioncancontainzeroexpressions,oritcancontaintwoormoreexpressions.Asingleexpressioninsideparenthesesisaparenthesizedexpression.

NOTE

Bothanemptytupleexpressionandanemptytupletypearewritten()inSwift.BecauseVoidisatypealiasfor(),youcanuseittowriteanemptytupletype.However,likealltypealiases,Voidisalwaysatype—youcan’tuseittowriteanemptytupleexpression.

GRAMMAR OF ATUPLE EXPRESS ION

tuple-expression → ( ) | ( tuple-element , tuple-element-list )tuple-element-list → tuple-element | tuple-element , tuple-element-listtuple-element → expression | identifier : expression

WildcardExpressionAwildcardexpressionisusedtoexplicitlyignoreavalueduringanassignment.Forexample,inthefollowingassignment10isassignedtoxand20isignored:

1 (x,_)=(10,20)

2 //xis10,and20isignored

GRAMMAR OF AW I LDCARD EXPRESS ION

wildcard-expression → _

Key-PathExpressionAkey-pathexpressionreferstoapropertyorsubscriptofatype.Youusekey-pathexpressionsindynamicprogrammingtasks,suchaskey-valueobserving.Theyhavethefollowingform:

\ typename . path

Thetypenameisthenameofaconcretetype,includinganygenericparameters,suchasString,[Int],orSet<Int>.

Thepathconsistsofpropertynames,subscripts,optional-chainingexpressions,andforcedunwrappingexpressions.Eachofthesekey-pathcomponentscanberepeatedasmanytimesasneeded,inanyorder.

Atcompiletime,akey-pathexpressionisreplacedbyaninstanceoftheKeyPathclass.

Toaccessavalueusingakeypath,passthekeypathtothesubscript(keyPath:)subscript,whichisavailableonalltypes.Forexample:

1 structSomeStructure{

2 varsomeValue:Int

3 }

4

5 lets=SomeStructure(someValue:12)

6 letpathToProperty=\SomeStructure.someValue

7

8 letvalue=s[keyPath:pathToProperty]

9 //valueis12

Thetypenamecanbeomittedincontextswheretypeinferencecandeterminetheimpliedtype.Thefollowingcodeuses\.somePropertyinsteadof\SomeClass.someProperty:

1 classSomeClass:NSObject{

2 @objcvarsomeProperty:Int

3 init(someProperty:Int){

4 self.someProperty=someProperty

5 }

6 }

7

8 letc=SomeClass(someProperty:10)

9 c.observe(\.someProperty){object,changein

10 //...

11 }

Thepathcanrefertoselftocreatetheidentitykeypath(\.self).Theidentitykeypathreferstoawholeinstance,soyoucanuseittoaccessandchangeallofthedatastoredinavariableinasinglestep.Forexample:

1 varcompoundValue=(a:1,b:2)

2 //EquivalenttocompoundValue=(a:10,b:20)

3 compoundValue[keyPath:\.self]=(a:10,b:20)

Thepathcancontainmultiplepropertynames,separatedbyperiods,torefertoapropertyofaproperty’svalue.Thiscodeusesthekeypathexpression\OuterStructure.outer.someValuetoaccessthesomeValuepropertyoftheOuterStructuretype’souterproperty:

1 structOuterStructure{

2 varouter:SomeStructure

3 init(someValue:Int){

4 self.outer=SomeStructure(someValue:someValue)

5 }

6 }

7

8 letnested=OuterStructure(someValue:24)

9 letnestedKeyPath=\OuterStructure.outer.someValue

10

11 letnestedValue=nested[keyPath:nestedKeyPath]

12 //nestedValueis24

Thepathcanincludesubscriptsusingbrackets,aslongasthesubscript’sparametertypeconformstotheHashableprotocol.Thisexampleusesasubscriptinakeypathtoaccessthesecondelementofanarray:

1 letgreetings=["hello","hola","bonjour","" ]

2 letmyGreeting=greetings[keyPath:\[String].[1]]

3 //myGreetingis'hola'

Thevalueusedinasubscriptcanbeanamedvalueoraliteral.Valuesarecapturedinkeypathsusingvaluesemantics.Thefollowingcodeusesthevariableindexinbothakey-pathexpressionandinaclosuretoaccessthethirdelementofthegreetingsarray.Whenindexismodified,thekey-pathexpressionstillreferencesthethirdelement,whiletheclosureusesthenewindex.

1 varindex=2

2 letpath=\[String].[index]

3 letfn:([String])->String={stringsinstrings[index]}

4

5 print(greetings[keyPath:path])

6 //Prints"bonjour"

7 print(fn(greetings))

8 //Prints"bonjour"

9

10 //Setting'index'toanewvaluedoesn'taffect'path'

11 index+=1

12 print(greetings[keyPath:path])

13 //Prints"bonjour"

14

15 //Because'fn'closesover'index',itusesthenewvalue

16 print(fn(greetings))

17 //Prints""

Thepathcanuseoptionalchainingandforcedunwrapping.Thiscodeusesoptionalchaininginakeypathtoaccessapropertyofanoptionalstring:

1 letfirstGreeting:String?=greetings.first

2 print(firstGreeting?.countasAny)

3 //Prints"Optional(5)"

4

5 //Dothesamethingusingakeypath.

6 letcount=greetings[keyPath:\[String].first?.count]

7 print(countasAny)

8 //Prints"Optional(5)"

Youcanmixandmatchcomponentsofkeypathstoaccessvaluesthataredeeplynestedwithinatype.Thefollowingcodeaccessesdifferentvaluesandpropertiesofadictionaryofarraysbyusingkey-pathexpressionsthatcombinethesecomponents.

1 letinterestingNumbers=["prime":[2,3,5,7,11,13,17],

2 "triangular":[1,3,6,10,15,21,

28],

3 "hexagonal":[1,6,15,28,45,66,

91]]

4 print(interestingNumbers[keyPath:\[String:[Int]].["prime"]]

asAny)

5 //Prints"Optional([2,3,5,7,11,13,17])"

6 print(interestingNumbers[keyPath:\[String:[Int]].["prime"]!

[0]])

7 //Prints"2"

8 print(interestingNumbers[keyPath:\[String:[Int]].

["hexagonal"]!.count])

9 //Prints"7"

10 print(interestingNumbers[keyPath:\[String:[Int]].

["hexagonal"]!.count.bitWidth])

11 //Prints"64"

FormoreinformationaboutusingkeypathsincodethatinteractswithObjective-CAPIs,seeUsingObjective-CRuntimeFeaturesinSwift.Forinformationaboutkey-valuecodingandkey-valueobserving,seeKey-ValueCodingProgrammingGuideandKey-ValueObservingProgrammingGuide.

GRAMMAR OF A KEY-PATH EXPRESS ION

key-path-expression → \ type opt . key-path-componentskey-path-components → key-path-component | key-path-component . key-path-

componentskey-path-component → identifier key-path-postfixes opt | key-path-postfixeskey-path-postfixes → key-path-postfix key-path-postfixes optkey-path-postfix → ? | ! | self | [ function-call-argument-list ]

SelectorExpressionAselectorexpressionletsyouaccesstheselectorusedtorefertoamethodortoaproperty’sgetterorsetterinObjective-C.Ithasthefollowingform:

#selector( methodname )

#selector(getter: propertyname )

#selector(setter: propertyname )

ThemethodnameandpropertynamemustbeareferencetoamethodorapropertythatisavailableintheObjective-Cruntime.ThevalueofaselectorexpressionisaninstanceoftheSelectortype.Forexample:

1 classSomeClass:NSObject{

2 @objcletproperty:String

3 @objc(doSomethingWithInt:)

4 funcdoSomething(_x:Int){}

5

6 init(property:String){

7 self.property=property

8 }

9 }

10 letselectorForMethod=#selector(SomeClass.doSomething(_:))

11 letselectorForPropertyGetter=#selector(getter:

SomeClass.property)

Whencreatingaselectorforaproperty’sgetter,thepropertynamecanbeareferencetoavariableorconstantproperty.Incontrast,whencreatingaselectorforaproperty’ssetter,thepropertynamemustbeareferencetoavariablepropertyonly.

Themethodnamecancontainparenthesesforgrouping,aswelltheasoperatortodisambiguatebetweenmethodsthatshareanamebuthavedifferenttypesignatures.Forexample:

1 extensionSomeClass{

2 @objc(doSomethingWithString:)

3 funcdoSomething(_x:String){}

4 }

5 letanotherSelector=#selector(SomeClass.doSomething(_:)as

(SomeClass)->(String)->Void)

Becauseaselectoriscreatedatcompiletime,notatruntime,thecompilercancheckthatamethodorpropertyexistsandthatthey’reexposedtotheObjective-Cruntime.

NOTE

Althoughthemethodnameandthepropertynameareexpressions,they’reneverevaluated.

FormoreinformationaboutusingselectorsinSwiftcodethatinteractswithObjective-CAPIs,seeUsingObjective-CRuntimeFeaturesinSwift.

GRAMMAR OF A SELECTOR EXPRESS ION

selector-expression → #selector ( expression )selector-expression → #selector ( getter: expression )selector-expression → #selector ( setter: expression )

Key-PathStringExpressionAkey-pathstringexpressionletsyouaccessthestringusedtorefertoapropertyinObjective-C,foruseinkey-valuecodingandkey-valueobservingAPIs.Ithasthefollowingform:

#keyPath( propertyname )

ThepropertynamemustbeareferencetoapropertythatisavailableintheObjective-Cruntime.Atcompiletime,thekey-pathstringexpressionisreplacedbyastringliteral.Forexample:

1 classSomeClass:NSObject{

2 @objcvarsomeProperty:Int

3 init(someProperty:Int){

4 self.someProperty=someProperty

5 }

6 }

7

8 letc=SomeClass(someProperty:12)

9 letkeyPath=#keyPath(SomeClass.someProperty)

10

11 ifletvalue=c.value(forKey:keyPath){

12 print(value)

13 }

14 //Prints"12"

Whenyouuseakey-pathstringexpressionwithinaclass,youcanrefertoapropertyofthatclassbywritingjustthepropertyname,withouttheclassname.

1 extensionSomeClass{

2 funcgetSomeKeyPath()->String{

3 return#keyPath(someProperty)

4 }

5 }

6 print(keyPath==c.getSomeKeyPath())

7 //Prints"true"

Becausethekeypathstringiscreatedatcompiletime,notatruntime,thecompilercancheckthatthepropertyexistsandthatthepropertyisexposedtotheObjective-Cruntime.

FormoreinformationaboutusingkeypathsinSwiftcodethatinteractswithObjective-CAPIs,seeUsingObjective-CRuntimeFeaturesinSwift.Forinformationaboutkey-valuecodingandkey-valueobserving,seeKey-ValueCodingProgrammingGuideandKey-ValueObservingProgrammingGuide.

NOTE

Althoughthepropertynameisanexpression,itisneverevaluated.

GRAMMAR OF A KEY-PATH STR ING EXPRESS ION

key-path-string-expression → #keyPath ( expression )

PostfixExpressions

Postfixexpressionsareformedbyapplyingapostfixoperatororotherpostfixsyntaxtoanexpression.Syntactically,everyprimaryexpressionisalsoapostfixexpression.

Forinformationaboutthebehavioroftheseoperators,seeBasicOperatorsandAdvancedOperators.

ForinformationabouttheoperatorsprovidedbytheSwiftstandardlibrary,seeOperatorDeclarations.

GRAMMAR OF A POSTF IX EXPRESS ION

postfix-expression → primary-expressionpostfix-expression → postfix-expression postfix-operatorpostfix-expression → function-call-expressionpostfix-expression → initializer-expressionpostfix-expression → explicit-member-expressionpostfix-expression → postfix-self-expressionpostfix-expression → subscript-expressionpostfix-expression → forced-value-expressionpostfix-expression → optional-chaining-expression

FunctionCallExpressionAfunctioncallexpressionconsistsofafunctionnamefollowedbyacomma-separatedlistofthefunction’sargumentsinparentheses.Functioncallexpressionshavethefollowingform:

functionname ( argumentvalue1 , argumentvalue2 )

Thefunctionnamecanbeanyexpressionwhosevalueisofafunctiontype.

Ifthefunctiondefinitionincludesnamesforitsparameters,thefunctioncallmustincludenamesbeforeitsargumentvaluesseparatedbyacolon(:).Thiskindoffunctioncallexpressionhasthefollowingform:

functionname ( argumentname1 : argumentvalue1 ,

argumentname2 : argumentvalue2 )

Afunctioncallexpressioncanincludeatrailingclosureintheformofaclosureexpressionimmediatelyaftertheclosingparenthesis.Thetrailingclosureisunderstoodasanargumenttothefunction,addedafterthelastparenthesizedargument.Thefollowingfunctioncallsareequivalent:

1 //someFunctiontakesanintegerandaclosureasitsarguments

2 someFunction(x:x,f:{$0==13})

3 someFunction(x:x){$0==13}

Ifthetrailingclosureisthefunction’sonlyargument,theparenthesescanbeomitted.

1 //someMethodtakesaclosureasitsonlyargument

2 myData.someMethod(){$0==13}

3 myData.someMethod{$0==13}

GRAMMAR OF A FUNCT ION CALL EXPRESS ION

function-call-expression → postfix-expression function-call-argument-clausefunction-call-expression → postfix-expression function-call-argument-clause opt trailing-

closurefunction-call-argument-clause → ( ) | ( function-call-argument-list )function-call-argument-list → function-call-argument | function-call-argument , function-

call-argument-listfunction-call-argument → expression | identifier : expressionfunction-call-argument → operator | identifier : operatortrailing-closure → closure-expression

InitializerExpression

Aninitializerexpressionprovidesaccesstoatype’sinitializer.Ithasthefollowingform:

expression .init( initializerarguments )

Youusetheinitializerexpressioninafunctioncallexpressiontoinitializeanewinstanceofatype.Youalsouseaninitializerexpressiontodelegatetotheinitializerofasuperclass.

1 classSomeSubClass:SomeSuperClass{

2 overrideinit(){

3 //subclassinitializationgoeshere

4 super.init()

5 }

6 }

Likeafunction,aninitializercanbeusedasavalue.Forexample:

1 //TypeannotationisrequiredbecauseStringhasmultiple

initializers.

2 letinitializer:(Int)->String=String.init

3 letoneTwoThree=[1,2,3].map(initializer).reduce("",+)

4 print(oneTwoThree)

5 //Prints"123"

Ifyouspecifyatypebyname,youcanaccessthetype’sinitializerwithoutusinganinitializerexpression.Inallothercases,youmustuseaninitializerexpression.

1 lets1=SomeType.init(data:3)//Valid

2 lets2=SomeType(data:1)//Alsovalid

3

4 lets3=type(of:someValue).init(data:7)//Valid

5 lets4=type(of:someValue)(data:5)//Error

GRAMMAR OF AN I N I T I A L I ZER EXPRESS ION

initializer-expression → postfix-expression . initinitializer-expression → postfix-expression . init ( argument-names )

ExplicitMemberExpressionAnexplicitmemberexpressionallowsaccesstothemembersofanamedtype,atuple,oramodule.Itconsistsofaperiod(.)betweentheitemandtheidentifierofitsmember.

expression . membername

Themembersofanamedtypearenamedaspartofthetype’sdeclarationorextension.Forexample:

1 classSomeClass{

2 varsomeProperty=42

3 }

4 letc=SomeClass()

5 lety=c.someProperty//Memberaccess

Themembersofatupleareimplicitlynamedusingintegersintheordertheyappear,startingfromzero.Forexample:

1 vart=(10,20,30)

2 t.0=t.1

3 //Nowtis(20,20,30)

Themembersofamoduleaccessthetop-leveldeclarationsofthatmodule.

TypesdeclaredwiththedynamicMemberLookupattributeincludemembersthatarelookedupatruntime,asdescribedinAttributes.

Todistinguishbetweenmethodsorinitializerswhosenamesdifferonlybythenamesoftheirarguments,includetheargumentnamesinparentheses,witheachargumentnamefollowedbyacolon(:).Writeanunderscore(_)foranargumentwithnoname.Todistinguishbetweenoverloadedmethods,useatypeannotation.Forexample:

1 classSomeClass{

2 funcsomeMethod(x:Int,y:Int){}

3 funcsomeMethod(x:Int,z:Int){}

4 funcoverloadedMethod(x:Int,y:Int){}

5 funcoverloadedMethod(x:Int,y:Bool){}

6 }

7 letinstance=SomeClass()

8

9 leta=instance.someMethod//Ambiguous

10 letb=instance.someMethod(x:y:)//Unambiguous

11

12 letd=instance.overloadedMethod//Ambiguous

13 letd=instance.overloadedMethod(x:y:)//Stillambiguous

14 letd:(Int,Bool)->Void=instance.overloadedMethod(x:y:)

//Unambiguous

Ifaperiodappearsatthebeginningofaline,itisunderstoodaspartofanexplicitmemberexpression,notasanimplicitmemberexpression.Forexample,thefollowinglistingshowschainedmethodcallssplitoverseverallines:

1 letx=[10,3,20,15,4]

2 .sorted()

3 .filter{$0>5}

4 .map{$0*100}

GRAMMAR OF AN EXPL IC I T MEMBER EXPRESS ION

explicit-member-expression → postfix-expression . decimal-digitsexplicit-member-expression → postfix-expression . identifier generic-argument-clause

optexplicit-member-expression → postfix-expression . identifier ( argument-names )argument-names → argument-name argument-names optargument-name → identifier :

PostfixSelfExpressionApostfixselfexpressionconsistsofanexpressionorthenameofatype,immediatelyfollowedby.self.Ithasthefollowingforms:

expression .self

type .self

Thefirstformevaluatestothevalueoftheexpression.Forexample,x.selfevaluatestox.

Thesecondformevaluatestothevalueofthetype.Usethisformtoaccessatypeasavalue.Forexample,becauseSomeClass.selfevaluatestotheSomeClasstypeitself,youcanpassittoafunctionormethodthatacceptsatype-levelargument.

GRAMMAR OF A POSTF IX SELF EXPRESS ION

postfix-self-expression → postfix-expression . self

SubscriptExpressionAsubscriptexpressionprovidessubscriptaccessusingthegetterandsetterofthecorrespondingsubscriptdeclaration.Ithasthefollowingform:

expression [ indexexpressions ]

Toevaluatethevalueofasubscriptexpression,thesubscriptgetterfortheexpression’stypeiscalledwiththeindexexpressionspassedasthesubscriptparameters.Tosetitsvalue,thesubscriptsetteriscalledinthesameway.

Forinformationaboutsubscriptdeclarations,seeProtocolSubscriptDeclaration.

GRAMMAR OF A SUBSCR IPT EXPRESS ION

subscript-expression → postfix-expression [ function-call-argument-list ]

Forced-ValueExpressionAforced-valueexpressionunwrapsanoptionalvaluethatyouarecertainisnotnil.Ithasthefollowingform:

expression !

Ifthevalueoftheexpressionisnotnil,theoptionalvalueisunwrappedandreturnedwiththecorrespondingnon-optionaltype.Otherwise,aruntimeerrorisraised.

Theunwrappedvalueofaforced-valueexpressioncanbemodified,eitherbymutatingthevalueitself,orbyassigningtooneofthevalue’smembers.Forexample:

1 varx:Int?=0

2 x!+=1

3 //xisnow1

4

5 varsomeDictionary=["a":[1,2,3],"b":[10,20]]

6 someDictionary["a"]![0]=100

7 //someDictionaryisnow["a":[100,2,3],"b":[10,20]]

GRAMMAR OF A FORCED -VALUE EXPRESS ION

forced-value-expression → postfix-expression !

Optional-ChainingExpression

Anoptional-chainingexpressionprovidesasimplifiedsyntaxforusingoptionalvaluesinpostfixexpressions.Ithasthefollowingform:

expression ?

Thepostfix?operatormakesanoptional-chainingexpressionfromanexpressionwithoutchangingtheexpression’svalue.

Optional-chainingexpressionsmustappearwithinapostfixexpression,andtheycausethepostfixexpressiontobeevaluatedinaspecialway.Ifthevalueoftheoptional-chainingexpressionisnil,alloftheotheroperationsinthepostfixexpressionareignoredandtheentirepostfixexpressionevaluatestonil.Ifthevalueoftheoptional-chainingexpressionisnotnil,thevalueoftheoptional-chainingexpressionisunwrappedandusedtoevaluatetherestofthepostfixexpression.Ineithercase,thevalueofthepostfixexpressionisstillofanoptionaltype.

Ifapostfixexpressionthatcontainsanoptional-chainingexpressionisnestedinsideotherpostfixexpressions,onlytheoutermostexpressionreturnsanoptionaltype.Intheexamplebelow,whencisnotnil,itsvalueisunwrappedandusedtoevaluate.property,thevalueofwhichisusedtoevaluate.performAction().Theentireexpressionc?.property.performAction()hasavalueofanoptionaltype.

1 varc:SomeClass?

2 varresult:Bool?=c?.property.performAction()

Thefollowingexampleshowsthebehavioroftheexampleabovewithoutusingoptionalchaining.

1 varresult:Bool?

2 ifletunwrappedC=c{

3 result=unwrappedC.property.performAction()

4 }

Theunwrappedvalueofanoptional-chainingexpressioncanbemodified,either

bymutatingthevalueitself,orbyassigningtooneofthevalue’smembers.Ifthevalueoftheoptional-chainingexpressionisnil,theexpressionontheright-handsideoftheassignmentoperatorisnotevaluated.Forexample:

1 funcsomeFunctionWithSideEffects()->Int{

2 return42//Noactualsideeffects.

3 }

4 varsomeDictionary=["a":[1,2,3],"b":[10,20]]

5

6 someDictionary["nothere"]?[0]=someFunctionWithSideEffects()

7 //someFunctionWithSideEffectsisnotevaluated

8 //someDictionaryisstill["a":[1,2,3],"b":[10,20]]

9

10 someDictionary["a"]?[0]=someFunctionWithSideEffects()

11 //someFunctionWithSideEffectsisevaluatedandreturns42

12 //someDictionaryisnow["a":[42,2,3],"b":[10,20]]

GRAMMAR OF AN OPT IONAL -CHA IN ING EXPRESS ION

optional-chaining-expression → postfix-expression ?

Statements

InSwift,therearethreekindsofstatements:simplestatements,compilercontrolstatements,andcontrolflowstatements.Simplestatementsarethemostcommonandconsistofeitheranexpressionoradeclaration.Compilercontrolstatementsallowtheprogramtochangeaspectsofthecompiler’sbehaviorandincludeaconditionalcompilationblockandalinecontrolstatement.

Controlflowstatementsareusedtocontroltheflowofexecutioninaprogram.ThereareseveraltypesofcontrolflowstatementsinSwift,includingloopstatements,branchstatements,andcontroltransferstatements.Loopstatementsallowablockofcodetobeexecutedrepeatedly,branchstatementsallowacertainblockofcodetobeexecutedonlywhencertainconditionsaremet,andcontroltransferstatementsprovideawaytoaltertheorderinwhichcodeisexecuted.Inaddition,Swiftprovidesadostatementtointroducescope,andcatchandhandleerrors,andadeferstatementforrunningcleanupactionsjustbeforethecurrentscopeexits.

Asemicolon(;)canoptionallyappearafteranystatementandisusedtoseparatemultiplestatementsiftheyappearonthesameline.

GRAMMAR OF A STATEMENT

statement → expression ;optstatement → declaration ;optstatement → loop-statement ;optstatement → branch-statement ;optstatement → labeled-statement ;optstatement → control-transfer-statement ;optstatement → defer-statement ;optstatement → do-statement ;optstatement → compiler-control-statementstatements → statement statements opt

LoopStatements

Loopstatementsallowablockofcodetobeexecutedrepeatedly,dependingontheconditionsspecifiedintheloop.Swifthasthreeloopstatements:afor-instatement,awhilestatement,andarepeat-whilestatement.

ControlflowinaloopstatementcanbechangedbyabreakstatementandacontinuestatementandisdiscussedinBreakStatementandContinueStatementbelow.

GRAMMAR OF A LOOP STATEMENT

loop-statement → for-in-statementloop-statement → while-statementloop-statement → repeat-while-statement

For-InStatementAfor-instatementallowsablockofcodetobeexecutedonceforeachiteminacollection(oranytype)thatconformstotheSequenceprotocol.

Afor-instatementhasthefollowingform:

for item in collection {

statements

}

ThemakeIterator()methodiscalledonthecollectionexpressiontoobtainavalueofaniteratortype—thatis,atypethatconformstotheIteratorProtocolprotocol.Theprogrambeginsexecutingaloopbycallingthenext()methodontheiterator.Ifthevaluereturnedisnotnil,itisassignedtotheitempattern,theprogramexecutesthestatements,andthencontinuesexecutionatthebeginningoftheloop.Otherwise,theprogramdoesnotperformassignmentorexecutethestatements,anditisfinishedexecutingthefor-instatement.

GRAMMAR OF A FOR - IN STATEMENT

for-in-statement → for caseopt pattern in expression where-clause opt code-block

WhileStatementAwhilestatementallowsablockofcodetobeexecutedrepeatedly,aslongasaconditionremainstrue.

Awhilestatementhasthefollowingform:

while condition {

statements

}

Awhilestatementisexecutedasfollows:

1. Theconditionisevaluated.

Iftrue,executioncontinuestostep2.Iffalse,theprogramisfinishedexecutingthewhilestatement.

2. Theprogramexecutesthestatements,andexecutionreturnstostep1.

Becausethevalueoftheconditionisevaluatedbeforethestatementsareexecuted,thestatementsinawhilestatementcanbeexecutedzeroormoretimes.

ThevalueoftheconditionmustbeoftypeBooloratypebridgedtoBool.Theconditioncanalsobeanoptionalbindingdeclaration,asdiscussedinOptionalBinding.

GRAMMAR OF AWH I LE STATEMENT

while-statement → while condition-list code-blockcondition-list → condition | condition , condition-listcondition → expression | availability-condition | case-condition | optional-binding-

conditioncase-condition → case pattern initializeroptional-binding-condition → let pattern initializer | var pattern initializer

Repeat-WhileStatement

Arepeat-whilestatementallowsablockofcodetobeexecutedoneormoretimes,aslongasaconditionremainstrue.

Arepeat-whilestatementhasthefollowingform:

repeat{

statements

}while condition

Arepeat-whilestatementisexecutedasfollows:

1. Theprogramexecutesthestatements,andexecutioncontinuestostep2.

2. Theconditionisevaluated.

Iftrue,executionreturnstostep1.Iffalse,theprogramisfinishedexecutingtherepeat-whilestatement.

Becausethevalueoftheconditionisevaluatedafterthestatementsareexecuted,thestatementsinarepeat-whilestatementareexecutedatleastonce.

ThevalueoftheconditionmustbeoftypeBooloratypebridgedtoBool.Theconditioncanalsobeanoptionalbindingdeclaration,asdiscussedinOptionalBinding.

GRAMMAR OF AREPEAT-WH I LE STATEMENT

repeat-while-statement → repeat code-block while expression

BranchStatements

Branchstatementsallowtheprogramtoexecutecertainpartsofcodedependingonthevalueofoneormoreconditions.Thevaluesoftheconditionsspecifiedinabranchstatementcontrolhowtheprogrambranchesand,therefore,whatblockofcodeisexecuted.Swifthasthreebranchstatements:anifstatement,aguardstatement,andaswitchstatement.

ControlflowinanifstatementoraswitchstatementcanbechangedbyabreakstatementandisdiscussedinBreakStatementbelow.

GRAMMAR OF A BRANCH STATEMENT

branch-statement → if-statementbranch-statement → guard-statementbranch-statement → switch-statement

IfStatementAnifstatementisusedforexecutingcodebasedontheevaluationofoneormoreconditions.

Therearetwobasicformsofanifstatement.Ineachform,theopeningandclosingbracesarerequired.

Thefirstformallowscodetobeexecutedonlywhenaconditionistrueandhasthefollowingform:

if condition {

statements

}

Thesecondformofanifstatementprovidesanadditionalelseclause(introducedbytheelsekeyword)andisusedforexecutingonepartofcodewhentheconditionistrueandanotherpartofcodewhenthesameconditionisfalse.Whenasingleelseclauseispresent,anifstatementhasthefollowingform:

if condition {

statementstoexecuteifconditionistrue

}else{

statementstoexecuteifconditionisfalse

}

Theelseclauseofanifstatementcancontainanotherifstatementtotestmore

thanonecondition.Anifstatementchainedtogetherinthiswayhasthefollowingform:

if condition1 {

statementstoexecuteifcondition1istrue

}elseif condition2 {

statementstoexecuteifcondition2istrue

}else{

statementstoexecuteifbothconditionsarefalse

}

ThevalueofanyconditioninanifstatementmustbeoftypeBooloratypebridgedtoBool.Theconditioncanalsobeanoptionalbindingdeclaration,asdiscussedinOptionalBinding.

GRAMMAR OF AN I F STATEMENT

if-statement → if condition-list code-block else-clause optelse-clause → else code-block | else if-statement

GuardStatementAguardstatementisusedtotransferprogramcontroloutofascopeifoneormoreconditionsaren’tmet.

Aguardstatementhasthefollowingform:

guard condition else{

statements

}

ThevalueofanyconditioninaguardstatementmustbeoftypeBooloratypebridgedtoBool.Theconditioncanalsobeanoptionalbindingdeclaration,asdiscussedinOptionalBinding.

Anyconstantsorvariablesassignedavaluefromanoptionalbindingdeclaration

inaguardstatementconditioncanbeusedfortherestoftheguardstatement’senclosingscope.

Theelseclauseofaguardstatementisrequired,andmusteithercallafunctionwiththeNeverreturntypeortransferprogramcontroloutsidetheguardstatement’senclosingscopeusingoneofthefollowingstatements:

return

break

continue

throw

ControltransferstatementsarediscussedinControlTransferStatementsbelow.FormoreinformationonfunctionswiththeNeverreturntype,seeFunctionsthatNeverReturn.

GRAMMAR OF AGUARD STATEMENT

guard-statement → guard condition-list else code-block

SwitchStatementAswitchstatementallowscertainblocksofcodetobeexecuteddependingonthevalueofacontrolexpression.

Aswitchstatementhasthefollowingform:

switch controlexpression {

case pattern1 :

statements

case pattern2 where condition :

statements

case pattern3 where condition ,

pattern4 where condition :

statements

default:

statements

}

Thecontrolexpressionoftheswitchstatementisevaluatedandthencomparedwiththepatternsspecifiedineachcase.Ifamatchisfound,theprogramexecutesthestatementslistedwithinthescopeofthatcase.Thescopeofeachcasecan’tbeempty.Asaresult,youmustincludeatleastonestatementfollowingthecolon(:)ofeachcaselabel.Useasinglebreakstatementifyoudon’tintendtoexecuteanycodeinthebodyofamatchedcase.

Thevaluesofexpressionsyourcodecanbranchonareveryflexible.Forexample,inadditiontothevaluesofscalartypes,suchasintegersandcharacters,yourcodecanbranchonthevaluesofanytype,includingfloating-pointnumbers,strings,tuples,instancesofcustomclasses,andoptionals.Thevalueofthecontrolexpressioncanevenbematchedtothevalueofacaseinanenumerationandcheckedforinclusioninaspecifiedrangeofvalues.Forexamplesofhowtousethesevarioustypesofvaluesinswitchstatements,seeSwitchinControlFlow.

Aswitchcasecanoptionallycontainawhereclauseaftereachpattern.Awhereclauseisintroducedbythewherekeywordfollowedbyanexpression,andisusedtoprovideanadditionalconditionbeforeapatterninacaseisconsideredmatchedtothecontrolexpression.Ifawhereclauseispresent,thestatementswithintherelevantcaseareexecutedonlyifthevalueofthecontrolexpressionmatchesoneofthepatternsofthecaseandtheexpressionofthewhereclauseevaluatestotrue.Forexample,acontrolexpressionmatchesthecaseintheexamplebelowonlyifitisatuplethatcontainstwoelementsofthesamevalue,suchas(1,1).

caselet(x,y)wherex==y:

Astheaboveexampleshows,patternsinacasecanalsobindconstantsusingtheletkeyword(theycanalsobindvariablesusingthevarkeyword).Theseconstants(orvariables)canthenbereferencedinacorrespondingwhereclause

andthroughouttherestofthecodewithinthescopeofthecase.Ifthecasecontainsmultiplepatternsthatmatchthecontrolexpression,allofthepatternsmustcontainthesameconstantorvariablebindings,andeachboundvariableorconstantmusthavethesametypeinallofthecase’spatterns.

Aswitchstatementcanalsoincludeadefaultcase,introducedbythedefaultkeyword.Thecodewithinadefaultcaseisexecutedonlyifnoothercasesmatchthecontrolexpression.Aswitchstatementcanincludeonlyonedefaultcase,whichmustappearattheendoftheswitchstatement.

Althoughtheactualexecutionorderofpattern-matchingoperations,andinparticulartheevaluationorderofpatternsincases,isunspecified,patternmatchinginaswitchstatementbehavesasiftheevaluationisperformedinsourceorder—thatis,theorderinwhichtheyappearinsourcecode.Asaresult,ifmultiplecasescontainpatternsthatevaluatetothesamevalue,andthuscanmatchthevalueofthecontrolexpression,theprogramexecutesonlythecodewithinthefirstmatchingcaseinsourceorder.

SwitchStatementsMustBeExhaustive

InSwift,everypossiblevalueofthecontrolexpression’stypemustmatchthevalueofatleastonepatternofacase.Whenthissimplyisn’tfeasible(forexample,whenthecontrolexpression’stypeisInt),youcanincludeadefaultcasetosatisfytherequirement.

SwitchingOverFutureEnumerationCases

Anonfrozenenumerationisaspecialkindofenumerationthatmaygainnewenumerationcasesinthefuture—evenafteryoucompileandshipanapp.Switchingoveranonfrozenenumerationrequiresextraconsideration.Whenalibrary’sauthorsmarkanenumerationasnonfrozen,theyreservetherighttoaddnewenumerationcases,andanycodethatinteractswiththatenumerationmustbeabletohandlethosefuturecaseswithoutbeingrecompiled.Codethat’scompiledinlibraryevolutionmode,codeinthestandardlibrary,SwiftoverlaysforAppleframeworks,andCandObjective-Ccodecandeclarenonfrozenenumerations.Forinformationaboutfrozenandnonfrozenenumerations,seefrozen.

Whenswitchingoveranonfrozenenumerationvalue,youalwaysneedtoincludeadefaultcase,evenifeverycaseoftheenumerationalreadyhasacorrespondingswitchcase.Youcanapplythe@unknownattributetothedefaultcase,whichindicatesthatthedefaultcaseshouldmatchonlyenumerationcasesthatareaddedinthefuture.Swiftproducesawarningifthedefaultcasematchesanyenumerationcasethatisknownatcompilertime.Thisfuturewarninginformsyouthatthelibraryauthoraddedanewcasetotheenumerationthatdoesn’thaveacorrespondingswitchcase.

Thefollowingexampleswitchesoverallthreeexistingcasesofthestandardlibrary’sMirror.AncestorRepresentationenumeration.Ifyouaddadditionalcasesinthefuture,thecompilergeneratesawarningtoindicatethatyouneedtoupdatetheswitchstatementtotakethenewcasesintoaccount.

1 letrepresentation:Mirror.AncestorRepresentation=.generated

2 switchrepresentation{

3 case.customized:

4 print("Usethenearestancestor’simplementation.")

5 case.generated:

6 print("Generateadefaultmirrorforallancestor

classes.")

7 case.suppressed:

8 print("Suppresstherepresentationofallancestor

classes.")

9 @unknowndefault:

10 print("Usearepresentationthatwasunknownwhenthis

codewascompiled.")

11 }

12 //Prints"Generateadefaultmirrorforallancestor

classes."

ExecutionDoesNotFallThroughCasesImplicitly

Afterthecodewithinamatchedcasehasfinishedexecuting,theprogramexitsfromtheswitchstatement.Programexecutiondoesnotcontinueor“fallthrough”tothenextcaseordefaultcase.Thatsaid,ifyouwantexecutiontocontinuefromonecasetothenext,explicitlyincludeafallthroughstatement,whichsimplyconsistsofthefallthroughkeyword,inthecasefromwhichyouwantexecutiontocontinue.Formoreinformationaboutthefallthroughstatement,seeFallthroughStatementbelow.

GRAMMAR OF A SW ITCH STATEMENT

switch-statement → switch expression { switch-cases opt }switch-cases → switch-case switch-cases optswitch-case → case-label statementsswitch-case → default-label statementsswitch-case → conditional-switch-casecase-label → attributes opt case case-item-list :case-item-list → pattern where-clause opt | pattern where-clause opt , case-item-listdefault-label → attributes opt default :where-clause → where where-expressionwhere-expression → expressionconditional-switch-case → switch-if-directive-clause switch-elseif-directive-clauses opt

switch-else-directive-clause opt endif-directiveswitch-if-directive-clause → if-directive compilation-condition switch-cases optswitch-elseif-directive-clauses → elseif-directive-clause switch-elseif-directive-clauses optswitch-elseif-directive-clause → elseif-directive compilation-condition switch-cases optswitch-else-directive-clause → else-directive switch-cases opt

LabeledStatement

Youcanprefixaloopstatement,anifstatement,aswitchstatement,oradostatementwithastatementlabel,whichconsistsofthenameofthelabelfollowedimmediatelybyacolon(:).Usestatementlabelswithbreakandcontinuestatementstobeexplicitabouthowyouwanttochangecontrolflowinaloopstatementoraswitchstatement,asdiscussedinBreakStatementandContinueStatementbelow.

Thescopeofalabeledstatementistheentirestatementfollowingthestatementlabel.Youcannestlabeledstatements,butthenameofeachstatementlabelmust

beunique.

Formoreinformationandtoseeexamplesofhowtousestatementlabels,seeLabeledStatementsinControlFlow.

GRAMMAR OF A LABELED STATEMENT

labeled-statement → statement-label loop-statementlabeled-statement → statement-label if-statementlabeled-statement → statement-label switch-statementlabeled-statement → statement-label do-statementstatement-label → label-name :label-name → identifier

ControlTransferStatements

Controltransferstatementscanchangetheorderinwhichcodeinyourprogramisexecutedbyunconditionallytransferringprogramcontrolfromonepieceofcodetoanother.Swifthasfivecontroltransferstatements:abreakstatement,acontinuestatement,afallthroughstatement,areturnstatement,andathrowstatement.

GRAMMAR OF ACONTROLTRANSFER STATEMENT

control-transfer-statement → break-statementcontrol-transfer-statement → continue-statementcontrol-transfer-statement → fallthrough-statementcontrol-transfer-statement → return-statementcontrol-transfer-statement → throw-statement

BreakStatementAbreakstatementendsprogramexecutionofaloop,anifstatement,oraswitchstatement.Abreakstatementcanconsistofonlythebreakkeyword,oritcanconsistofthebreakkeywordfollowedbythenameofastatementlabel,asshownbelow.

break

break labelname

Whenabreakstatementisfollowedbythenameofastatementlabel,itendsprogramexecutionoftheloop,ifstatement,orswitchstatementnamedbythatlabel.

Whenabreakstatementisnotfollowedbythenameofastatementlabel,itendsprogramexecutionoftheswitchstatementortheinnermostenclosingloopstatementinwhichitoccurs.Youcan’tuseanunlabeledbreakstatementtobreakoutofanifstatement.

Inbothcases,programcontrolisthentransferredtothefirstlineofcodefollowingtheenclosinglooporswitchstatement,ifany.

Forexamplesofhowtouseabreakstatement,seeBreakandLabeledStatementsinControlFlow.

GRAMMAR OF A BREAK STATEMENT

break-statement → break label-name opt

ContinueStatementAcontinuestatementendsprogramexecutionofthecurrentiterationofaloopstatementbutdoesnotstopexecutionoftheloopstatement.Acontinuestatementcanconsistofonlythecontinuekeyword,oritcanconsistofthecontinuekeywordfollowedbythenameofastatementlabel,asshownbelow.

continue

continue labelname

Whenacontinuestatementisfollowedbythenameofastatementlabel,itendsprogramexecutionofthecurrentiterationoftheloopstatementnamedbythatlabel.

Whenacontinuestatementisnotfollowedbythenameofastatementlabel,itendsprogramexecutionofthecurrentiterationoftheinnermostenclosingloop

statementinwhichitoccurs.

Inbothcases,programcontrolisthentransferredtotheconditionoftheenclosingloopstatement.

Inaforstatement,theincrementexpressionisstillevaluatedafterthecontinuestatementisexecuted,becausetheincrementexpressionisevaluatedaftertheexecutionoftheloop’sbody.

Forexamplesofhowtouseacontinuestatement,seeContinueandLabeledStatementsinControlFlow.

GRAMMAR OF ACONT INUE STATEMENT

continue-statement → continue label-name opt

FallthroughStatementAfallthroughstatementconsistsofthefallthroughkeywordandoccursonlyinacaseblockofaswitchstatement.Afallthroughstatementcausesprogramexecutiontocontinuefromonecaseinaswitchstatementtothenextcase.Programexecutioncontinuestothenextcaseevenifthepatternsofthecaselabeldonotmatchthevalueoftheswitchstatement’scontrolexpression.

Afallthroughstatementcanappearanywhereinsideaswitchstatement,notjustasthelaststatementofacaseblock,butitcan’tbeusedinthefinalcaseblock.Italsocannottransfercontrolintoacaseblockwhosepatterncontainsvaluebindingpatterns.

Foranexampleofhowtouseafallthroughstatementinaswitchstatement,seeControlTransferStatementsinControlFlow.

GRAMMAR OF A FALLTHROUGH STATEMENT

fallthrough-statement → fallthrough

ReturnStatement

Areturnstatementoccursinthebodyofafunctionormethoddefinitionandcausesprogramexecutiontoreturntothecallingfunctionormethod.Programexecutioncontinuesatthepointimmediatelyfollowingthefunctionormethodcall.

Areturnstatementcanconsistofonlythereturnkeyword,oritcanconsistofthereturnkeywordfollowedbyanexpression,asshownbelow.

return

return expression

Whenareturnstatementisfollowedbyanexpression,thevalueoftheexpressionisreturnedtothecallingfunctionormethod.Ifthevalueoftheexpressiondoesnotmatchthevalueofthereturntypedeclaredinthefunctionormethoddeclaration,theexpression’svalueisconvertedtothereturntypebeforeitisreturnedtothecallingfunctionormethod.

NOTE

AsdescribedinFailableInitializers,aspecialformofthereturnstatement(returnnil)canbeusedinafailableinitializertoindicateinitializationfailure.

Whenareturnstatementisnotfollowedbyanexpression,itcanbeusedonlytoreturnfromafunctionormethodthatdoesnotreturnavalue(thatis,whenthereturntypeofthefunctionormethodisVoidor()).

GRAMMAR OF ARETURN STATEMENT

return-statement → return expression opt

ThrowStatementAthrowstatementoccursinthebodyofathrowingfunctionormethod,orinthebodyofaclosureexpressionwhosetypeismarkedwiththethrowskeyword.

Athrowstatementcausesaprogramtoendexecutionofthecurrentscopeandbeginerrorpropagationtoitsenclosingscope.Theerrorthat’sthrowncontinues

topropagateuntilit’shandledbyacatchclauseofadostatement.

Athrowstatementconsistsofthethrowkeywordfollowedbyanexpression,asshownbelow.

throw expression

ThevalueoftheexpressionmusthaveatypethatconformstotheErrorprotocol.

Foranexampleofhowtouseathrowstatement,seePropagatingErrorsUsingThrowingFunctionsinErrorHandling.

GRAMMAR OF ATHROWSTATEMENT

throw-statement → throw expression

DeferStatement

Adeferstatementisusedforexecutingcodejustbeforetransferringprogramcontroloutsideofthescopethatthedeferstatementappearsin.

Adeferstatementhasthefollowingform:

defer{

statements

}

Thestatementswithinthedeferstatementareexecutednomatterhowprogramcontrolistransferred.Thismeansthatadeferstatementcanbeused,forexample,toperformmanualresourcemanagementsuchasclosingfiledescriptors,andtoperformactionsthatneedtohappenevenifanerroristhrown.

Ifmultipledeferstatementsappearinthesamescope,theordertheyappearisthereverseoftheordertheyareexecuted.Executingthelastdeferstatementina

givenscopefirstmeansthatstatementsinsidethatlastdeferstatementcanrefertoresourcesthatwillbecleanedupbyotherdeferstatements.

1 funcf(){

2 defer{print("Firstdefer")}

3 defer{print("Seconddefer")}

4 print("Endoffunction")

5 }

6 f()

7 //Prints"Endoffunction"

8 //Prints"Seconddefer"

9 //Prints"Firstdefer"

Thestatementsinthedeferstatementcan’ttransferprogramcontroloutsideofthedeferstatement.

GRAMMAR OF ADEFER STATEMENT

defer-statement → defer code-block

DoStatement

Thedostatementisusedtointroduceanewscopeandcanoptionallycontainoneormorecatchclauses,whichcontainpatternsthatmatchagainstdefinederrorconditions.Variablesandconstantsdeclaredinthescopeofadostatementcanbeaccessedonlywithinthatscope.

AdostatementinSwiftissimilartocurlybraces({})inCusedtodelimitacodeblock,anddoesnotincuraperformancecostatruntime.

Adostatementhasthefollowingform:

do{

try expression

statements

}catch pattern1 {

statements

}catch pattern2 where condition {

statements

}

Likeaswitchstatement,thecompilerattemptstoinferwhethercatchclausesareexhaustive.Ifsuchadeterminationcanbemade,theerrorisconsideredhandled.Otherwise,theerrorcanpropagateoutofthecontainingscope,whichmeanstheerrormustbehandledbyanenclosingcatchclauseorthecontainingfunctionmustbedeclaredwiththrows.

Toensurethatanerrorishandled,useacatchclausewithapatternthatmatchesallerrors,suchasawildcardpattern(_).Ifacatchclausedoesnotspecifyapattern,thecatchclausematchesandbindsanyerrortoalocalconstantnamederror.Formoreinformationaboutthepatternsyoucanuseinacatchclause,seePatterns.

Toseeanexampleofhowtouseadostatementwithseveralcatchclauses,seeHandlingErrors.

GRAMMAR OF ADO STATEMENT

do-statement → do code-block catch-clauses optcatch-clauses → catch-clause catch-clauses optcatch-clause → catch pattern opt where-clause opt code-block

CompilerControlStatements

Compilercontrolstatementsallowtheprogramtochangeaspectsofthecompiler’sbehavior.Swifthasthreecompilercontrolstatements:aconditionalcompilationblockalinecontrolstatement,andacompile-timediagnosticstatement.

GRAMMAR OF ACOMP I LER CONTROLSTATEMENT

compiler-control-statement → conditional-compilation-blockcompiler-control-statement → line-control-statementcompiler-control-statement → diagnostic-statement

ConditionalCompilationBlockAconditionalcompilationblockallowscodetobeconditionallycompileddependingonthevalueofoneormorecompilationconditions.

Everyconditionalcompilationblockbeginswiththe#ifcompilationdirectiveandendswiththe#endifcompilationdirective.Asimpleconditionalcompilationblockhasthefollowingform:

#if compilationcondition

statements

#endif

Unliketheconditionofanifstatement,thecompilationconditionisevaluatedatcompiletime.Asaresult,thestatementsarecompiledandexecutedonlyifthecompilationconditionevaluatestotrueatcompiletime.

ThecompilationconditioncanincludethetrueandfalseBooleanliterals,anidentifierusedwiththe-Dcommandlineflag,oranyoftheplatformconditionslistedinthetablebelow.

Platformcondition Validarguments

os() macOS,iOS,watchOS,tvOS,Linux

arch() i386,x86_64,arm,arm64

swift() >=or<followedbyaversionnumber

compiler() >=or<followedbyaversionnumber

canImport() Amodulename

targetEnvironment() simulator,macCatalyst

Theversionnumberfortheswift()andcompiler()platformconditionsconsistsofamajornumber,optionalminornumber,optionalpatchnumber,andsoon,withadot(.)separatingeachpartoftheversionnumber.Theremustnotbewhitespacebetweenthecomparisonoperatorandtheversionnumber.Theversionforcompiler()isthecompilerversion,regardlessoftheSwiftversionsettingpassedtothecompiler.Theversionforswift()isthelanguageversioncurrentlybeingcompiled.Forexample,ifyoucompileyourcodeusingtheSwift5compilerinSwift4.2mode,thecompilerversionis5andthelanguageversionis4.2.Withthosesettings,thefollowingcodeprintsallthreemessages:

1 #ifcompiler(>=5)

2 print("CompiledwiththeSwift5compilerorlater")

3 #endif

4 #ifswift(>=4.2)

5 print("CompiledinSwift4.2modeorlater")

6 #endif

7 #ifcompiler(>=5)&&swift(<5)

8 print("CompiledwiththeSwift5compilerorlaterinaSwift

modeearlierthan5")

9 #endif

10 //Prints"CompiledwiththeSwift5compilerorlater"

11 //Prints"CompiledinSwift4.2modeorlater"

12 //Prints"CompiledwiththeSwift5compilerorlaterina

Swiftmodeearlierthan5"

TheargumentforthecanImport()platformconditionisthenameofamodulethatmaynotbepresentonallplatforms.Thisconditiontestswhetherit’spossibletoimportthemodule,butdoesn’tactuallyimportit.Ifthemoduleispresent,theplatformconditionreturnstrue;otherwise,itreturnsfalse.

ThetargetEnvironment()platformconditionreturnstruewhencodeiscompiledforasimulator;otherwise,itreturnsfalse.

NOTE

Thearch(arm)platformconditiondoesnotreturntrueforARM64devices.Thearch(i386)platformconditionreturnstruewhencodeiscompiledforthe32–bitiOSsimulator.

Youcancombinecompilationconditionsusingthelogicaloperators&&,||,and!anduseparenthesesforgrouping.TheseoperatorshavethesameassociativityandprecedenceasthelogicaloperatorsthatareusedtocombineordinaryBooleanexpressions.

Similartoanifstatement,youcanaddmultipleconditionalbranchestotestfordifferentcompilationconditions.Youcanaddanynumberofadditionalbranchesusing#elseifclauses.Youcanalsoaddafinaladditionalbranchusingan#elseclause.Conditionalcompilationblocksthatcontainmultiplebrancheshavethefollowingform:

#if compilationcondition1

statementstocompileifcompilationcondition1istrue

#elseif compilationcondition2

statementstocompileifcompilationcondition2istrue

#else

statementstocompileifbothcompilationconditionsarefalse

#endif

NOTE

Eachstatementinthebodyofaconditionalcompilationblockisparsedevenifit’snotcompiled.However,thereisanexceptionifthecompilationconditionincludesaswift()platformcondition:Thestatementsareparsedonlyifthecompiler’sversionofSwiftmatcheswhatisspecifiedinthe

platformcondition.Thisexceptionensuresthatanoldercompilerdoesn’tattempttoparsesyntaxintroducedinanewerversionofSwift.

GRAMMAR OF ACOND I T IONALCOMP I LAT ION BLOCK

conditional-compilation-block → if-directive-clause elseif-directive-clauses opt else-directive-clause opt endif-directive

if-directive-clause → if-directive compilation-condition statements optelseif-directive-clauses → elseif-directive-clause elseif-directive-clauses optelseif-directive-clause → elseif-directive compilation-condition statements optelse-directive-clause → else-directive statements optif-directive → #ifelseif-directive → #elseifelse-directive → #elseendif-directive → #endifcompilation-condition → platform-conditioncompilation-condition → identifiercompilation-condition → boolean-literalcompilation-condition → ( compilation-condition )compilation-condition → ! compilation-conditioncompilation-condition → compilation-condition && compilation-conditioncompilation-condition → compilation-condition || compilation-conditionplatform-condition → os ( operating-system )platform-condition → arch ( architecture )platform-condition → swift ( >= swift-version ) | swift ( < swift-version )platform-condition → compiler ( >= swift-version ) | compiler ( < swift-version

)

platform-condition → canImport ( module-name )platform-condition → targetEnvironment ( environment )operating-system → macOS | iOS | watchOS | tvOSarchitecture → i386 | x86_64 | arm | arm64swift-version → decimal-digits swift-version-continuation optswift-version-continuation → . decimal-digits swift-version-continuation optmodule-name → identifierenvironment → simulator

LineControlStatementAlinecontrolstatementisusedtospecifyalinenumberandfilenamethatcanbedifferentfromthelinenumberandfilenameofthesourcecodebeingcompiled.UsealinecontrolstatementtochangethesourcecodelocationusedbySwiftfordiagnosticanddebuggingpurposes.

Alinecontrolstatementhasthefollowingforms:

#sourceLocation(file: filename ,line: linenumber )

#sourceLocation()

Thefirstformofalinecontrolstatementchangesthevaluesofthe#lineand#fileliteralexpressions,beginningwiththelineofcodefollowingthelinecontrolstatement.Thelinenumberchangesthevalueof#lineandisanyintegerliteralgreaterthanzero.Thefilenamechangesthevalueof#fileandisastringliteral.

Thesecondformofalinecontrolstatement,#sourceLocation(),resetsthesourcecodelocationbacktothedefaultlinenumberingandfilename.

GRAMMAR OF A L I NE CONTROLSTATEMENT

line-control-statement → #sourceLocation ( file: file-name , line: line-number )

line-control-statement → #sourceLocation ( )line-number → Adecimalintegergreaterthanzerofile-name → static-string-literal

Compile-TimeDiagnosticStatementAcompile-timediagnosticstatementcausesthecompilertoemitanerrororawarningduringcompilation.Acompile-timediagnosticstatementhasthefollowingforms:

#error(" errormessage ")

#warning(" warningmessage ")

Thefirstformemitstheerrormessageasafatalerrorandterminatesthecompilationprocess.Thesecondformemitsthewarningmessageasanonfatalwarningandallowscompilationtoproceed.Youwritethediagnosticmessageasastaticstringliteral.Staticstringliteralscan’tusefeatureslikestringinterpolationorconcatenation,buttheycanusethemultilinestringliteralsyntax.

GRAMMAR OF ACOMP I LE - T IME D I AGNOST IC STATEMENT

diagnostic-statement → #error ( diagnostic-message )diagnostic-statement → #warning ( diagnostic-message )diagnostic-message → static-string-literal

AvailabilityCondition

Anavailabilityconditionisusedasaconditionofanif,while,andguardstatementtoquerytheavailabilityofAPIsatruntime,basedonspecifiedplatformsarguments.

Anavailabilityconditionhasthefollowingform:

if#available( platformname version , ... ,*){

statementstoexecuteiftheAPIsareavailable

}else{

fallbackstatementstoexecuteiftheAPIsareunavailable

}

Youuseanavailabilityconditiontoexecuteablockofcode,dependingonwhethertheAPIsyouwanttouseareavailableatruntime.ThecompilerusestheinformationfromtheavailabilityconditionwhenitverifiesthattheAPIsinthatblockofcodeareavailable.

Theavailabilityconditiontakesacomma-separatedlistofplatformnamesandversions.UseiOS,macOS,watchOS,andtvOSfortheplatformnames,andincludethecorrespondingversionnumbers.The*argumentisrequiredandspecifiesthatonanyotherplatform,thebodyofthecodeblockguardedbytheavailabilityconditionexecutesontheminimumdeploymenttargetspecifiedbyyourtarget.

UnlikeBooleanconditions,youcan’tcombineavailabilityconditionsusing

logicaloperatorssuchas&&and||.

GRAMMAR OF AN AVA I LAB I L I TY COND I T ION

availability-condition → #available ( availability-arguments )availability-arguments → availability-argument | availability-argument , availability-

argumentsavailability-argument → platform-name platform-versionavailability-argument → *platform-name → iOS | iOSApplicationExtensionplatform-name → macOS | macOSApplicationExtensionplatform-name → watchOSplatform-name → tvOSplatform-version → decimal-digitsplatform-version → decimal-digits . decimal-digitsplatform-version → decimal-digits . decimal-digits . decimal-digits

Declarations

Adeclarationintroducesanewnameorconstructintoyourprogram.Forexample,youusedeclarationstointroducefunctionsandmethods,tointroducevariablesandconstants,andtodefineenumeration,structure,class,andprotocoltypes.Youcanalsouseadeclarationtoextendthebehaviorofanexistingnamedtypeandtoimportsymbolsintoyourprogramthataredeclaredelsewhere.

InSwift,mostdeclarationsarealsodefinitionsinthesensethattheyareimplementedorinitializedatthesametimetheyaredeclared.Thatsaid,becauseprotocolsdon’timplementtheirmembers,mostprotocolmembersaredeclarationsonly.Forconvenienceandbecausethedistinctionisn’tthatimportantinSwift,thetermdeclarationcoversbothdeclarationsanddefinitions.

GRAMMAR OF ADECLARAT ION

declaration → import-declarationdeclaration → constant-declarationdeclaration → variable-declarationdeclaration → typealias-declarationdeclaration → function-declarationdeclaration → enum-declarationdeclaration → struct-declarationdeclaration → class-declarationdeclaration → protocol-declarationdeclaration → initializer-declarationdeclaration → deinitializer-declarationdeclaration → extension-declarationdeclaration → subscript-declarationdeclaration → operator-declarationdeclaration → precedence-group-declarationdeclarations → declaration declarations opt

Top-LevelCode

Thetop-levelcodeinaSwiftsourcefileconsistsofzeroormorestatements,declarations,andexpressions.Bydefault,variables,constants,andothernameddeclarationsthataredeclaredatthetop-levelofasourcefileareaccessibleto

codeineverysourcefilethatispartofthesamemodule.Youcanoverridethisdefaultbehaviorbymarkingthedeclarationwithanaccess-levelmodifier,asdescribedinAccessControlLevels.

GRAMMAR OF ATOP - LEVELDECLARAT ION

top-level-declaration → statements opt

CodeBlocks

Acodeblockisusedbyavarietyofdeclarationsandcontrolstructurestogroupstatementstogether.Ithasthefollowingform:

{

statements

}

Thestatementsinsideacodeblockincludedeclarations,expressions,andotherkindsofstatementsandareexecutedinorderoftheirappearanceinsourcecode.

GRAMMAR OF ACODE BLOCK

code-block → { statements opt }

ImportDeclaration

Animportdeclarationletsyouaccesssymbolsthataredeclaredoutsidethecurrentfile.Thebasicformimportstheentiremodule;itconsistsoftheimportkeywordfollowedbyamodulename:

import module

Providingmoredetaillimitswhichsymbolsareimported—youcanspecifya

specificsubmoduleoraspecificdeclarationwithinamoduleorsubmodule.Whenthisdetailedformisused,onlytheimportedsymbol(andnotthemodulethatdeclaresit)ismadeavailableinthecurrentscope.

import importkind module . symbolname

import module . submodule

GRAMMAR OF AN IMPORT DECLARAT ION

import-declaration → attributes opt import import-kind opt import-pathimport-kind → typealias | struct | class | enum | protocol | let | var |

func

import-path → import-path-identifier | import-path-identifier . import-pathimport-path-identifier → identifier | operator

ConstantDeclaration

Aconstantdeclarationintroducesaconstantnamedvalueintoyourprogram.Constantdeclarationsaredeclaredusingtheletkeywordandhavethefollowingform:

let constantname : type = expression

Aconstantdeclarationdefinesanimmutablebindingbetweentheconstantnameandthevalueoftheinitializerexpression;afterthevalueofaconstantisset,itcannotbechanged.Thatsaid,ifaconstantisinitializedwithaclassobject,theobjectitselfcanchange,butthebindingbetweentheconstantnameandtheobjectitreferstocan’t.

Whenaconstantisdeclaredatglobalscope,itmustbeinitializedwithavalue.Whenaconstantdeclarationoccursinthecontextofafunctionormethod,itcanbeinitializedlater,aslongasitisguaranteedtohaveavaluesetbeforethefirsttimeitsvalueisread.Whenaconstantdeclarationoccursinthecontextofaclassorstructuredeclaration,itisconsideredaconstantproperty.Constantdeclarationsarenotcomputedpropertiesandthereforedonothavegettersorsetters.

Iftheconstantnameofaconstantdeclarationisatuplepattern,thenameofeachiteminthetupleisboundtothecorrespondingvalueintheinitializerexpression.

let(firstNumber,secondNumber)=(10,42)

Inthisexample,firstNumberisanamedconstantforthevalue10,andsecondNumberisanamedconstantforthevalue42.Bothconstantscannowbeusedindependently:

1 print("Thefirstnumberis\(firstNumber).")

2 //Prints"Thefirstnumberis10."

3 print("Thesecondnumberis\(secondNumber).")

4 //Prints"Thesecondnumberis42."

Thetypeannotation(:type)isoptionalinaconstantdeclarationwhenthetypeoftheconstantnamecanbeinferred,asdescribedinTypeInference.

Todeclareaconstanttypeproperty,markthedeclarationwiththestaticdeclarationmodifier.Aconstanttypepropertyofaclassisalwaysimplicitlyfinal;youcan’tmarkitwiththeclassorfinaldeclarationmodifiertoallowordisallowoverridingbysubclasses.TypepropertiesarediscussedinTypeProperties.

Formoreinformationaboutconstantsandforguidanceaboutwhentousethem,seeConstantsandVariablesandStoredProperties.

GRAMMAR OF ACONSTANT DECLARAT ION

constant-declaration → attributes opt declaration-modifiers opt let pattern-initializer-list

pattern-initializer-list → pattern-initializer | pattern-initializer , pattern-initializer-listpattern-initializer → pattern initializer optinitializer → = expression

VariableDeclaration

Avariabledeclarationintroducesavariablenamedvalueintoyourprogramandisdeclaredusingthevarkeyword.

Variabledeclarationshaveseveralformsthatdeclaredifferentkindsofnamed,mutablevalues,includingstoredandcomputedvariablesandproperties,storedvariableandpropertyobservers,andstaticvariableproperties.Theappropriateformtousedependsonthescopeatwhichthevariableisdeclaredandthekindofvariableyouintendtodeclare.

NOTE

Youcanalsodeclarepropertiesinthecontextofaprotocoldeclaration,asdescribedinProtocolPropertyDeclaration.

Youcanoverrideapropertyinasubclassbymarkingthesubclass’spropertydeclarationwiththeoverridedeclarationmodifier,asdescribedinOverriding.

StoredVariablesandStoredVariablePropertiesThefollowingformdeclaresastoredvariableorstoredvariableproperty:

var variablename : type = expression

Youdefinethisformofavariabledeclarationatglobalscope,thelocalscopeofafunction,orinthecontextofaclassorstructuredeclaration.Whenavariabledeclarationofthisformisdeclaredatglobalscopeorthelocalscopeofafunction,itisreferredtoasastoredvariable.Whenitisdeclaredinthecontextofaclassorstructuredeclaration,itisreferredtoasastoredvariableproperty.

Theinitializerexpressioncan’tbepresentinaprotocoldeclaration,butinallothercontexts,theinitializerexpressionisoptional.Thatsaid,ifnoinitializerexpressionispresent,thevariabledeclarationmustincludeanexplicittypeannotation(:type).

Aswithconstantdeclarations,ifthevariablenameisatuplepattern,thenameofeachiteminthetupleisboundtothecorrespondingvalueintheinitializerexpression.

Astheirnamessuggest,thevalueofastoredvariableorastoredvariablepropertyisstoredinmemory.

ComputedVariablesandComputedPropertiesThefollowingformdeclaresacomputedvariableorcomputedproperty:

var variablename : type {

get{

statements

}

set( settername ){

statements

}

}

Youdefinethisformofavariabledeclarationatglobalscope,thelocalscopeofafunction,orinthecontextofaclass,structure,enumeration,orextensiondeclaration.Whenavariabledeclarationofthisformisdeclaredatglobalscopeorthelocalscopeofafunction,itisreferredtoasacomputedvariable.Whenitisdeclaredinthecontextofaclass,structure,orextensiondeclaration,itisreferredtoasacomputedproperty.

Thegetterisusedtoreadthevalue,andthesetterisusedtowritethevalue.Thesetterclauseisoptional,andwhenonlyagetterisneeded,youcanomitbothclausesandsimplyreturntherequestedvaluedirectly,asdescribedinRead-OnlyComputedProperties.Butifyouprovideasetterclause,youmustalsoprovideagetterclause.

Thesetternameandenclosingparenthesesisoptional.Ifyouprovideasettername,itisusedasthenameoftheparametertothesetter.Ifyoudonotprovideasettername,thedefaultparameternametothesetterisnewValue,asdescribedinShorthandSetterDeclaration.

Unlikestorednamedvaluesandstoredvariableproperties,thevalueofa

computednamedvalueoracomputedpropertyisnotstoredinmemory.

Formoreinformationandtoseeexamplesofcomputedproperties,seeComputedProperties.

StoredVariableObserversandPropertyObserversYoucanalsodeclareastoredvariableorpropertywithwillSetanddidSetobservers.Astoredvariableorpropertydeclaredwithobservershasthefollowingform:

var variablename : type = expression {

willSet( settername ){

statements

}

didSet( settername ){

statements

}

}

Youdefinethisformofavariabledeclarationatglobalscope,thelocalscopeofafunction,orinthecontextofaclassorstructuredeclaration.Whenavariabledeclarationofthisformisdeclaredatglobalscopeorthelocalscopeofafunction,theobserversarereferredtoasstoredvariableobservers.Whenitisdeclaredinthecontextofaclassorstructuredeclaration,theobserversarereferredtoaspropertyobservers.

Youcanaddpropertyobserverstoanystoredproperty.Youcanalsoaddpropertyobserverstoanyinheritedproperty(whetherstoredorcomputed)byoverridingthepropertywithinasubclass,asdescribedinOverridingPropertyObservers.

Theinitializerexpressionisoptionalinthecontextofaclassorstructuredeclaration,butrequiredelsewhere.Thetypeannotationisoptionalwhenthetypecanbeinferredfromtheinitializerexpression.

ThewillSetanddidSetobserversprovideawaytoobserve(andtorespondappropriately)whenthevalueofavariableorpropertyisbeingset.Theobserversarenotcalledwhenthevariableorpropertyisfirstinitialized.Instead,theyarecalledonlywhenthevalueissetoutsideofaninitializationcontext.

AwillSetobserveriscalledjustbeforethevalueofthevariableorpropertyisset.ThenewvalueispassedtothewillSetobserverasaconstant,andthereforeitcan’tbechangedintheimplementationofthewillSetclause.ThedidSetobserveriscalledimmediatelyafterthenewvalueisset.IncontrasttothewillSetobserver,theoldvalueofthevariableorpropertyispassedtothedidSetobserverincaseyoustillneedaccesstoit.Thatsaid,ifyouassignavaluetoavariableorpropertywithinitsowndidSetobserverclause,thatnewvaluethatyouassignwillreplacetheonethatwasjustsetandpassedtothewillSetobserver.

ThesetternameandenclosingparenthesesinthewillSetanddidSetclausesareoptional.Ifyouprovidesetternames,theyareusedastheparameternamestothewillSetanddidSetobservers.Ifyoudonotprovidesetternames,thedefaultparameternametothewillSetobserverisnewValueandthedefaultparameternametothedidSetobserverisoldValue.

ThedidSetclauseisoptionalwhenyouprovideawillSetclause.Likewise,thewillSetclauseisoptionalwhenyouprovideadidSetclause.

Formoreinformationandtoseeanexampleofhowtousepropertyobservers,seePropertyObservers.

TypeVariablePropertiesTodeclareatypevariableproperty,markthedeclarationwiththestaticdeclarationmodifier.Classescanmarktypecomputedpropertieswiththeclassdeclarationmodifierinsteadtoallowsubclassestooverridethesuperclass’simplementation.TypepropertiesarediscussedinTypeProperties.

GRAMMAR OF A VAR IABLE DECLARAT ION

variable-declaration → variable-declaration-head pattern-initializer-listvariable-declaration → variable-declaration-head variable-name type-annotation code-

blockvariable-declaration → variable-declaration-head variable-name type-annotation getter-

setter-blockvariable-declaration → variable-declaration-head variable-name type-annotation getter-

setter-keyword-blockvariable-declaration → variable-declaration-head variable-name initializer willSet-didSet-

blockvariable-declaration → variable-declaration-head variable-name type-annotation

initializer opt willSet-didSet-blockvariable-declaration-head → attributes opt declaration-modifiers opt varvariable-name → identifiergetter-setter-block → code-blockgetter-setter-block → { getter-clause setter-clause opt }getter-setter-block → { setter-clause getter-clause }getter-clause → attributes opt mutation-modifier opt get code-blocksetter-clause → attributes opt mutation-modifier opt set setter-name opt code-blocksetter-name → ( identifier )getter-setter-keyword-block → { getter-keyword-clause setter-keyword-clause opt }getter-setter-keyword-block → { setter-keyword-clause getter-keyword-clause }getter-keyword-clause → attributes opt mutation-modifier opt getsetter-keyword-clause → attributes opt mutation-modifier opt setwillSet-didSet-block → { willSet-clause didSet-clause opt }willSet-didSet-block → { didSet-clause willSet-clause opt }willSet-clause → attributes opt willSet setter-name opt code-blockdidSet-clause → attributes opt didSet setter-name opt code-block

TypeAliasDeclaration

Atypealiasdeclarationintroducesanamedaliasofanexistingtypeintoyourprogram.Typealiasdeclarationsaredeclaredusingthetypealiaskeywordandhavethefollowingform:

typealias name = existingtype

Afteratypealiasisdeclared,thealiasednamecanbeusedinsteadoftheexistingtypeeverywhereinyourprogram.Theexistingtypecanbeanamedtypeoracompoundtype.Typealiasesdonotcreatenewtypes;theysimplyallowanametorefertoanexistingtype.

Atypealiasdeclarationcanusegenericparameterstogiveanametoanexistinggenerictype.Thetypealiascanprovideconcretetypesforsomeorallofthegenericparametersoftheexistingtype.Forexample:

1 typealiasStringDictionary<Value>=Dictionary<String,Value>

2

3 //Thefollowingdictionarieshavethesametype.

4 vardictionary1:StringDictionary<Int>=[:]

5 vardictionary2:Dictionary<String,Int>=[:]

Whenatypealiasisdeclaredwithgenericparameters,theconstraintsonthoseparametersmustmatchexactlytheconstraintsontheexistingtype’sgenericparameters.Forexample:

typealiasDictionaryOfInts<Key:Hashable>=Dictionary<Key,

Int>

Becausethetypealiasandtheexistingtypecanbeusedinterchangeably,thetypealiascan’tintroduceadditionalgenericconstraints.

Atypealiascanforwardanexistingtype’sgenericparametersbyomittingallgenericparametersfromthedeclaration.Forexample,theDiccionariotypealiasdeclaredherehasthesamegenericparametersandconstraintsasDictionary.

typealiasDiccionario=Dictionary

Insideaprotocoldeclaration,atypealiascangiveashorterandmoreconvenientnametoatypethatisusedfrequently.Forexample:

1 protocolSequence{

2 associatedtypeIterator:IteratorProtocol

3 typealiasElement=Iterator.Element

4 }

5

6 funcsum<T:Sequence>(_sequence:T)->IntwhereT.Element==

Int{

7 //...

8 }

Withoutthistypealias,thesumfunctionwouldhavetorefertotheassociatedtypeasT.Iterator.ElementinsteadofT.Element.

SeealsoProtocolAssociatedTypeDeclaration.

GRAMMAR OF ATYPE AL I AS DECLARAT ION

typealias-declaration → attributes opt access-level-modifier opt typealias typealias-name generic-parameter-clause opt typealias-assignment

typealias-name → identifiertypealias-assignment → = type

FunctionDeclaration

Afunctiondeclarationintroducesafunctionormethodintoyourprogram.Afunctiondeclaredinthecontextofclass,structure,enumeration,orprotocolisreferredtoasamethod.Functiondeclarationsaredeclaredusingthefunckeywordandhavethefollowingform:

func functionname ( parameters )-> returntype {

statements

}

IfthefunctionhasareturntypeofVoid,thereturntypecanbeomittedasfollows:

func functionname ( parameters ){

statements

}

Thetypeofeachparametermustbeincluded—itcan’tbeinferred.Ifyouwriteinoutinfrontofaparameter’stype,theparametercanbemodifiedinsidethescopeofthefunction.In-outparametersarediscussedindetailinIn-OutParameters,below.

Afunctiondeclarationwhosestatementsincludeonlyasingleexpressionisunderstoodtoreturnthevalueofthatexpression.

Functionscanreturnmultiplevaluesusingatupletypeasthereturntypeofthefunction.

Afunctiondefinitioncanappearinsideanotherfunctiondeclaration.Thiskindoffunctionisknownasanestedfunction.

Anestedfunctionisnonescapingifitcapturesavaluethatisguaranteedtoneverescape—suchasanin-outparameter—orpassedasanonescapingfunctionargument.Otherwise,thenestedfunctionisanescapingfunction.

Foradiscussionofnestedfunctions,seeNestedFunctions.

ParameterNamesFunctionparametersareacomma-separatedlistwhereeachparameterhasoneofseveralforms.Theorderofargumentsinafunctioncallmustmatchtheorderofparametersinthefunction’sdeclaration.Thesimplestentryinaparameterlisthasthefollowingform:

parametername : parametertype

Aparameterhasaname,whichisusedwithinthefunctionbody,aswellasanargumentlabel,whichisusedwhencallingthefunctionormethod.Bydefault,parameternamesarealsousedasargumentlabels.Forexample:

1 funcf(x:Int,y:Int)->Int{returnx+y}

2 f(x:1,y:2)//bothxandyarelabeled

Youcanoverridethedefaultbehaviorforargumentlabelswithoneofthefollowingforms:

argumentlabel parametername : parametertype

_ parametername : parametertype

Anamebeforetheparameternamegivestheparameteranexplicitargumentlabel,whichcanbedifferentfromtheparametername.Thecorrespondingargumentmustusethegivenargumentlabelinfunctionormethodcalls.

Anunderscore(_)beforeaparameternamesuppressestheargumentlabel.Thecorrespondingargumentmusthavenolabelinfunctionormethodcalls.

1 funcrepeatGreeting(_greeting:String,countn:Int){/*

Greetntimes*/}

2 repeatGreeting("Hello,world!",count:2)//countislabeled,

greetingisnot

In-OutParametersIn-outparametersarepassedasfollows:

1. Whenthefunctioniscalled,thevalueoftheargumentiscopied.

2. Inthebodyofthefunction,thecopyismodified.

3. Whenthefunctionreturns,thecopy’svalueisassignedtotheoriginalargument.

Thisbehaviorisknownascopy-incopy-outorcallbyvalueresult.Forexample,whenacomputedpropertyorapropertywithobserversispassedasanin-outparameter,itsgetteriscalledaspartofthefunctioncallanditssetteriscalledaspartofthefunctionreturn.

Asanoptimization,whentheargumentisavaluestoredataphysicaladdressinmemory,thesamememorylocationisusedbothinsideandoutsidethefunction

body.Theoptimizedbehaviorisknownascallbyreference;itsatisfiesalloftherequirementsofthecopy-incopy-outmodelwhileremovingtheoverheadofcopying.Writeyourcodeusingthemodelgivenbycopy-incopy-out,withoutdependingonthecall-by-referenceoptimization,sothatitbehavescorrectlywithorwithouttheoptimization.

Withinafunction,don’taccessavaluethatwaspassedasanin-outargument,eveniftheoriginalvalueisavailableinthecurrentscope.Accessingtheoriginalisasimultaneousaccessofthevalue,whichviolatesSwift’smemoryexclusivityguarantee.Forthesamereason,youcan’tpassthesamevaluetomultiplein-outparameters.

Formoreinformationaboutmemorysafetyandmemoryexclusivity,seeMemorySafety.

Aclosureornestedfunctionthatcapturesanin-outparametermustbenonescaping.Ifyouneedtocaptureanin-outparameterwithoutmutatingitortoobservechangesmadebyothercode,useacapturelisttoexplicitlycapturetheparameterimmutably.

1 funcsomeFunction(a:inoutInt)->()->Int{

2 return{[a]inreturna+1}

3 }

Ifyouneedtocaptureandmutateanin-outparameter,useanexplicitlocalcopy,suchasinmultithreadedcodethatensuresallmutationhasfinishedbeforethefunctionreturns.

1 funcmultithreadedFunction(queue:DispatchQueue,x:inoutInt)

{

2 //Makealocalcopyandmanuallycopyitback.

3 varlocalX=x

4 defer{x=localX}

5

6 //OperateonlocalXasynchronously,thenwaitbefore

returning.

7 queue.async{someMutatingOperation(&localX)}

8 queue.sync{}

9 }

Formorediscussionandexamplesofin-outparameters,seeIn-OutParameters.

SpecialKindsofParametersParameterscanbeignored,takeavariablenumberofvalues,andprovidedefaultvaluesusingthefollowingforms:

_: parametertype

parametername : parametertype ...

parametername : parametertype = defaultargumentvalue

Anunderscore(_)parameterisexplicitlyignoredandcan’tbeaccessedwithinthebodyofthefunction.

Aparameterwithabasetypenamefollowedimmediatelybythreedots(...)isunderstoodasavariadicparameter.Afunctioncanhaveatmostonevariadicparameter.Avariadicparameteristreatedasanarraythatcontainselementsofthebasetypename.Forexample,thevariadicparameterInt...istreatedas[Int].Foranexamplethatusesavariadicparameter,seeVariadicParameters.

Aparameterwithanequalssign(=)andanexpressionafteritstypeisunderstoodtohaveadefaultvalueofthegivenexpression.Thegivenexpressionisevaluatedwhenthefunctioniscalled.Iftheparameterisomittedwhencallingthefunction,thedefaultvalueisusedinstead.

1 funcf(x:Int=42)->Int{returnx}

2 f()//Valid,usesdefaultvalue

3 f(x:7)//Valid,usesthevalueprovided

4 f(7)//Invalid,missingargumentlabel

SpecialKindsofMethodsMethodsonanenumerationorastructurethatmodifyselfmustbemarkedwiththemutatingdeclarationmodifier.

Methodsthatoverrideasuperclassmethodmustbemarkedwiththeoverridedeclarationmodifier.It’sacompile-timeerrortooverrideamethodwithouttheoverridemodifierortousetheoverridemodifieronamethodthatdoesn’toverrideasuperclassmethod.

Methodsassociatedwithatyperatherthananinstanceofatypemustbemarkedwiththestaticdeclarationmodifierforenumerationsandstructures,orwitheitherthestaticorclassdeclarationmodifierforclasses.Aclasstypemethodmarkedwiththeclassdeclarationmodifiercanbeoverriddenbyasubclassimplementation;aclasstypemethodmarkedwithclassfinalorstaticcan’tbeoverridden.

ThrowingFunctionsandMethodsFunctionsandmethodsthatcanthrowanerrormustbemarkedwiththethrowskeyword.Thesefunctionsandmethodsareknownasthrowingfunctionsandthrowingmethods.Theyhavethefollowingform:

func functionname ( parameters )throws-> returntype {

statements

}

Callstoathrowingfunctionormethodmustbewrappedinatryortry!expression(thatis,inthescopeofatryortry!operator).

Thethrowskeywordispartofafunction’stype,andnonthrowingfunctionsaresubtypesofthrowingfunctions.Asaresult,youcanuseanonthrowingfunctioninthesameplacesasathrowingone.

Youcan’toverloadafunctionbasedonlyonwhetherthefunctioncanthrowanerror.Thatsaid,youcanoverloadafunctionbasedonwhetherafunctionparametercanthrowanerror.

Athrowingmethodcan’toverrideanonthrowingmethod,andathrowingmethodcan’tsatisfyaprotocolrequirementforanonthrowingmethod.Thatsaid,anonthrowingmethodcanoverrideathrowingmethod,andanonthrowingmethodcansatisfyaprotocolrequirementforathrowingmethod.

RethrowingFunctionsandMethodsAfunctionormethodcanbedeclaredwiththerethrowskeywordtoindicatethatitthrowsanerroronlyifoneofitsfunctionparametersthrowsanerror.Thesefunctionsandmethodsareknownasrethrowingfunctionsandrethrowingmethods.Rethrowingfunctionsandmethodsmusthaveatleastonethrowingfunctionparameter.

1 funcsomeFunction(callback:()throws->Void)rethrows{

2 trycallback()

3 }

Arethrowingfunctionormethodcancontainathrowstatementonlyinsideacatchclause.Thisletsyoucallthethrowingfunctioninsideado-catchblockandhandleerrorsinthecatchclausebythrowingadifferenterror.Inaddition,thecatchclausemusthandleonlyerrorsthrownbyoneoftherethrowingfunction’sthrowingparameters.Forexample,thefollowingisinvalidbecausethecatchclausewouldhandletheerrorthrownbyalwaysThrows().

1 funcalwaysThrows()throws{

2 throwSomeError.error

3 }

4 funcsomeFunction(callback:()throws->Void)rethrows{

5 do{

6 trycallback()

7 tryalwaysThrows()//Invalid,alwaysThrows()isn'ta

throwingparameter

8 }catch{

9 throwAnotherError.error

10 }

11 }

Athrowingmethodcan’toverridearethrowingmethod,andathrowingmethodcan’tsatisfyaprotocolrequirementforarethrowingmethod.Thatsaid,arethrowingmethodcanoverrideathrowingmethod,andarethrowingmethodcansatisfyaprotocolrequirementforathrowingmethod.

FunctionsthatNeverReturnSwiftdefinesaNevertype,whichindicatesthatafunctionormethoddoesn’treturntoitscaller.FunctionsandmethodswiththeNeverreturntypearecallednonreturning.Nonreturningfunctionsandmethodseithercauseanirrecoverableerrororbeginasequenceofworkthatcontinuesindefinitely.Thismeansthatcodethatwouldotherwiserunimmediatelyafterthecallisneverexecuted.Throwingandrethrowingfunctionscantransferprogramcontroltoanappropriatecatchblock,evenwhentheyarenonreturning.

Anonreturningfunctionormethodcanbecalledtoconcludetheelseclauseofaguardstatement,asdiscussedinGuardStatement.

Youcanoverrideanonreturningmethod,butthenewmethodmustpreserveitsreturntypeandnonreturningbehavior.

GRAMMAR OF A FUNCT ION DECLARAT ION

function-declaration → function-head function-name generic-parameter-clause optfunction-signature generic-where-clause opt function-body opt

function-head → attributes opt declaration-modifiers opt funcfunction-name → identifier | operatorfunction-signature → parameter-clause throwsopt function-result optfunction-signature → parameter-clause rethrows function-result optfunction-result → -> attributes opt typefunction-body → code-blockparameter-clause → ( ) | ( parameter-list )parameter-list → parameter | parameter , parameter-listparameter → external-parameter-name opt local-parameter-name type-annotation

default-argument-clause optparameter → external-parameter-name opt local-parameter-name type-annotationparameter → external-parameter-name opt local-parameter-name type-annotation ...external-parameter-name → identifierlocal-parameter-name → identifierdefault-argument-clause → = expression

EnumerationDeclaration

Anenumerationdeclarationintroducesanamedenumerationtypeintoyourprogram.

Enumerationdeclarationshavetwobasicformsandaredeclaredusingtheenumkeyword.Thebodyofanenumerationdeclaredusingeitherformcontainszeroormorevalues—calledenumerationcases—andanynumberofdeclarations,includingcomputedproperties,instancemethods,typemethods,initializers,typealiases,andevenotherenumeration,structure,andclassdeclarations.Enumerationdeclarationscan’tcontaindeinitializerorprotocoldeclarations.

Enumerationtypescanadoptanynumberofprotocols,butcan’tinheritfromclasses,structures,orotherenumerations.

Unlikeclassesandstructures,enumerationtypesdonothaveanimplicitlyprovideddefaultinitializer;allinitializersmustbedeclaredexplicitly.Initializerscandelegatetootherinitializersintheenumeration,buttheinitializationprocessiscompleteonlyafteraninitializerassignsoneoftheenumerationcasestoself.

Likestructuresbutunlikeclasses,enumerationsarevaluetypes;instancesofanenumerationarecopiedwhenassignedtovariablesorconstants,orwhenpassedasargumentstoafunctioncall.Forinformationaboutvaluetypes,seeStructuresandEnumerationsAreValueTypes.

Youcanextendthebehaviorofanenumerationtypewithanextensiondeclaration,asdiscussedinExtensionDeclaration.

EnumerationswithCasesofAnyTypeThefollowingformdeclaresanenumerationtypethatcontainsenumerationcasesofanytype:

enum enumerationname : adoptedprotocols {

case enumerationcase1

case enumerationcase2 ( associatedvaluetypes )

}

Enumerationsdeclaredinthisformaresometimescalleddiscriminatedunionsinotherprogramminglanguages.

Inthisform,eachcaseblockconsistsofthecasekeywordfollowedbyoneormoreenumerationcases,separatedbycommas.Thenameofeachcasemustbeunique.Eachcasecanalsospecifythatitstoresvaluesofagiventype.Thesetypesarespecifiedintheassociatedvaluetypestuple,immediatelyfollowingthenameofthecase.

Enumerationcasesthatstoreassociatedvaluescanbeusedasfunctionsthatcreateinstancesoftheenumerationwiththespecifiedassociatedvalues.Andjustlikefunctions,youcangetareferencetoanenumerationcaseandapplyitlaterinyourcode.

1 enumNumber{

2 caseinteger(Int)

3 casereal(Double)

4 }

5 letf=Number.integer

6 //fisafunctionoftype(Int)->Number

7

8 //ApplyftocreateanarrayofNumberinstanceswithinteger

values

9 letevenInts:[Number]=[0,2,4,6].map(f)

Formoreinformationandtoseeexamplesofcaseswithassociatedvaluetypes,seeAssociatedValues.

EnumerationswithIndirection

Enumerationscanhavearecursivestructure,thatis,theycanhavecaseswithassociatedvaluesthatareinstancesoftheenumerationtypeitself.However,instancesofenumerationtypeshavevaluesemantics,whichmeanstheyhaveafixedlayoutinmemory.Tosupportrecursion,thecompilermustinsertalayerofindirection.

Toenableindirectionforaparticularenumerationcase,markitwiththeindirectdeclarationmodifier.Anindirectcasemusthaveanassociatedvalue.

1 enumTree<T>{

2 caseempty

3 indirectcasenode(value:T,left:Tree,right:Tree)

4 }

Toenableindirectionforallthecasesofanenumerationthathaveanassociatedvalue,marktheentireenumerationwiththeindirectmodifier—thisisconvenientwhentheenumerationcontainsmanycasesthatwouldeachneedtobemarkedwiththeindirectmodifier.

Anenumerationthatismarkedwiththeindirectmodifiercancontainamixtureofcasesthathaveassociatedvaluesandcasesthosethatdon’t.Thatsaid,itcan’tcontainanycasesthatarealsomarkedwiththeindirectmodifier.

EnumerationswithCasesofaRaw-ValueTypeThefollowingformdeclaresanenumerationtypethatcontainsenumerationcasesofthesamebasictype:

enum enumerationname : raw-valuetype , adoptedprotocols {

case enumerationcase1 = rawvalue1

case enumerationcase2 = rawvalue2

}

Inthisform,eachcaseblockconsistsofthecasekeyword,followedbyoneormoreenumerationcases,separatedbycommas.Unlikethecasesinthefirstform,eachcasehasanunderlyingvalue,calledarawvalue,ofthesamebasictype.Thetypeofthesevaluesisspecifiedintheraw-valuetypeandmustrepresentaninteger,floating-pointnumber,string,orsinglecharacter.Inparticular,theraw-valuetypemustconformtotheEquatableprotocolandoneofthefollowingprotocols:ExpressibleByIntegerLiteralforintegerliterals,ExpressibleByFloatLiteralforfloating-pointliterals,ExpressibleByStringLiteralforstringliteralsthatcontainanynumberofcharacters,andExpressibleByUnicodeScalarLiteralorExpressibleByExtendedGraphemeClusterLiteralforstringliteralsthatcontainonlyasinglecharacter.Eachcasemusthaveauniquenameandbeassignedauniquerawvalue.

Iftheraw-valuetypeisspecifiedasIntandyoudon’tassignavaluetothecasesexplicitly,theyareimplicitlyassignedthevalues0,1,2,andsoon.EachunassignedcaseoftypeIntisimplicitlyassignedarawvaluethatisautomaticallyincrementedfromtherawvalueofthepreviouscase.

1 enumExampleEnum:Int{

2 casea,b,c=5,d

3 }

Intheaboveexample,therawvalueofExampleEnum.ais0andthevalueofExampleEnum.bis1.AndbecausethevalueofExampleEnum.cisexplicitlysetto5,thevalueofExampleEnum.disautomaticallyincrementedfrom5andistherefore6.

Iftheraw-valuetypeisspecifiedasStringandyoudon’tassignvaluestothecasesexplicitly,eachunassignedcaseisimplicitlyassignedastringwiththesametextasthenameofthatcase.

1 enumGamePlayMode:String{

2 casecooperative,individual,competitive

3 }

Intheaboveexample,therawvalueofGamePlayMode.cooperativeis"cooperative",therawvalueofGamePlayMode.individualis"individual",andtherawvalueofGamePlayMode.competitiveis"competitive".

Enumerationsthathavecasesofaraw-valuetypeimplicitlyconformtotheRawRepresentableprotocol,definedintheSwiftstandardlibrary.Asaresult,theyhavearawValuepropertyandafailableinitializerwiththesignatureinit?(rawValue:RawValue).YoucanusetherawValuepropertytoaccesstherawvalueofanenumerationcase,asinExampleEnum.b.rawValue.Youcanalsousearawvaluetofindacorrespondingcase,ifthereisone,bycallingtheenumeration’sfailableinitializer,asinExampleEnum(rawValue:5),whichreturnsanoptionalcase.Formoreinformationandtoseeexamplesofcaseswithraw-valuetypes,seeRawValues.

AccessingEnumerationCasesToreferencethecaseofanenumerationtype,usedot(.)syntax,asinEnumerationType.enumerationCase.Whentheenumerationtypecanbeinferredfromcontext,youcanomitit(thedotisstillrequired),asdescribedinEnumerationSyntaxandImplicitMemberExpression.

Tocheckthevaluesofenumerationcases,useaswitchstatement,asshowninMatchingEnumerationValueswithaSwitchStatement.Theenumerationtypeispattern-matchedagainsttheenumerationcasepatternsinthecaseblocksoftheswitchstatement,asdescribedinEnumerationCasePattern.

GRAMMAR OF AN ENUMERAT ION DECLARAT ION

enum-declaration → attributes opt access-level-modifier opt union-style-enumenum-declaration → attributes opt access-level-modifier opt raw-value-style-enumunion-style-enum → indirectopt enum enum-name generic-parameter-clause opt

type-inheritance-clause opt generic-where-clause opt { union-style-enum-members

opt }union-style-enum-members → union-style-enum-member union-style-enum-members optunion-style-enum-member → declaration | union-style-enum-case-clause | compiler-

control-statementunion-style-enum-case-clause → attributes opt indirectopt case union-style-enum-

case-listunion-style-enum-case-list → union-style-enum-case | union-style-enum-case , union-

style-enum-case-listunion-style-enum-case → enum-case-name tuple-type optenum-name → identifierenum-case-name → identifierraw-value-style-enum → enum enum-name generic-parameter-clause opt type-

inheritance-clause generic-where-clause opt { raw-value-style-enum-members }raw-value-style-enum-members → raw-value-style-enum-member raw-value-style-enum-

members optraw-value-style-enum-member → declaration | raw-value-style-enum-case-clause |

compiler-control-statementraw-value-style-enum-case-clause → attributes opt case raw-value-style-enum-case-

listraw-value-style-enum-case-list → raw-value-style-enum-case | raw-value-style-enum-

case , raw-value-style-enum-case-listraw-value-style-enum-case → enum-case-name raw-value-assignment optraw-value-assignment → = raw-value-literalraw-value-literal → numeric-literal | static-string-literal | boolean-literal

StructureDeclaration

Astructuredeclarationintroducesanamedstructuretypeintoyourprogram.Structuredeclarationsaredeclaredusingthestructkeywordandhavethefollowingform:

struct structurename : adoptedprotocols {

declarations

}

Thebodyofastructurecontainszeroormoredeclarations.Thesedeclarationscanincludebothstoredandcomputedproperties,typeproperties,instancemethods,typemethods,initializers,subscripts,typealiases,andevenotherstructure,class,andenumerationdeclarations.Structuredeclarationscan’tcontaindeinitializerorprotocoldeclarations.Foradiscussionandseveralexamplesofstructuresthatincludevariouskindsofdeclarations,seeStructures

andClasses.

Structuretypescanadoptanynumberofprotocols,butcan’tinheritfromclasses,enumerations,orotherstructures.

Therearethreewaystocreateaninstanceofapreviouslydeclaredstructure:

Calloneoftheinitializersdeclaredwithinthestructure,asdescribedinInitializers.

Ifnoinitializersaredeclared,callthestructure’smemberwiseinitializer,asdescribedinMemberwiseInitializersforStructureTypes.

Ifnoinitializersaredeclared,andallpropertiesofthestructuredeclarationweregiveninitialvalues,callthestructure’sdefaultinitializer,asdescribedinDefaultInitializers.

Theprocessofinitializingastructure’sdeclaredpropertiesisdescribedinInitialization.

Propertiesofastructureinstancecanbeaccessedusingdot(.)syntax,asdescribedinAccessingProperties.

Structuresarevaluetypes;instancesofastructurearecopiedwhenassignedtovariablesorconstants,orwhenpassedasargumentstoafunctioncall.Forinformationaboutvaluetypes,seeStructuresandEnumerationsAreValueTypes.

Youcanextendthebehaviorofastructuretypewithanextensiondeclaration,asdiscussedinExtensionDeclaration.

GRAMMAR OF A STRUCTURE DECLARAT ION

struct-declaration → attributes opt access-level-modifier opt struct struct-namegeneric-parameter-clause opt type-inheritance-clause opt generic-where-clause optstruct-body

struct-name → identifierstruct-body → { struct-members opt }struct-members → struct-member struct-members optstruct-member → declaration | compiler-control-statement

ClassDeclaration

Aclassdeclarationintroducesanamedclasstypeintoyourprogram.Classdeclarationsaredeclaredusingtheclasskeywordandhavethefollowingform:

class classname : superclass , adoptedprotocols {

declarations

}

Thebodyofaclasscontainszeroormoredeclarations.Thesedeclarationscanincludebothstoredandcomputedproperties,instancemethods,typemethods,initializers,asingledeinitializer,subscripts,typealiases,andevenotherclass,structure,andenumerationdeclarations.Classdeclarationscan’tcontainprotocoldeclarations.Foradiscussionandseveralexamplesofclassesthatincludevariouskindsofdeclarations,seeStructuresandClasses.

Aclasstypecaninheritfromonlyoneparentclass,itssuperclass,butcanadoptanynumberofprotocols.Thesuperclassappearsfirstaftertheclassnameandcolon,followedbyanyadoptedprotocols.Genericclassescaninheritfromothergenericandnongenericclasses,butanongenericclasscaninheritonlyfromothernongenericclasses.Whenyouwritethenameofagenericsuperclassclassafterthecolon,youmustincludethefullnameofthatgenericclass,includingitsgenericparameterclause.

AsdiscussedinInitializerDeclaration,classescanhavedesignatedandconvenienceinitializers.Thedesignatedinitializerofaclassmustinitializealloftheclass’sdeclaredpropertiesanditmustdosobeforecallinganyofitssuperclass’sdesignatedinitializers.

Aclasscanoverrideproperties,methods,subscripts,andinitializersofitssuperclass.Overriddenproperties,methods,subscripts,anddesignatedinitializersmustbemarkedwiththeoverridedeclarationmodifier.

Torequirethatsubclassesimplementasuperclass’sinitializer,markthesuperclass’sinitializerwiththerequireddeclarationmodifier.Thesubclass’simplementationofthatinitializermustalsobemarkedwiththerequireddeclarationmodifier.

Althoughpropertiesandmethodsdeclaredinthesuperclassareinheritedbythecurrentclass,designatedinitializersdeclaredinthesuperclassareonlyinheritedwhenthesubclassmeetstheconditionsdescribedinAutomaticInitializerInheritance.Swiftclassesdonotinheritfromauniversalbaseclass.

Therearetwowaystocreateaninstanceofapreviouslydeclaredclass:

Calloneoftheinitializersdeclaredwithintheclass,asdescribedinInitializers.

Ifnoinitializersaredeclared,andallpropertiesoftheclassdeclarationweregiveninitialvalues,calltheclass’sdefaultinitializer,asdescribedinDefaultInitializers.

Accesspropertiesofaclassinstancewithdot(.)syntax,asdescribedinAccessingProperties.

Classesarereferencetypes;instancesofaclassarereferredto,ratherthancopied,whenassignedtovariablesorconstants,orwhenpassedasargumentstoafunctioncall.Forinformationaboutreferencetypes,seeStructuresandEnumerationsAreValueTypes.

Youcanextendthebehaviorofaclasstypewithanextensiondeclaration,asdiscussedinExtensionDeclaration.

GRAMMAR OF ACLASS DECLARAT ION

class-declaration → attributes opt access-level-modifier opt finalopt class class-name generic-parameter-clause opt type-inheritance-clause opt generic-where-clause

opt class-bodyclass-declaration → attributes opt final access-level-modifier opt class class-

name generic-parameter-clause opt type-inheritance-clause opt generic-where-clause

opt class-bodyclass-name → identifierclass-body → { class-members opt }class-members → class-member class-members optclass-member → declaration | compiler-control-statement

ProtocolDeclaration

Aprotocoldeclarationintroducesanamedprotocoltypeintoyourprogram.Protocoldeclarationsaredeclaredatglobalscopeusingtheprotocolkeywordandhavethefollowingform:

protocol protocolname : inheritedprotocols {

protocolmemberdeclarations

}

Thebodyofaprotocolcontainszeroormoreprotocolmemberdeclarations,whichdescribetheconformancerequirementsthatanytypeadoptingtheprotocolmustfulfill.Inparticular,aprotocolcandeclarethatconformingtypesmustimplementcertainproperties,methods,initializers,andsubscripts.Protocolscanalsodeclarespecialkindsoftypealiases,calledassociatedtypes,thatcanspecifyrelationshipsamongthevariousdeclarationsoftheprotocol.Protocoldeclarationscan’tcontainclass,structure,enumeration,orotherprotocoldeclarations.Theprotocolmemberdeclarationsarediscussedindetailbelow.

Protocoltypescaninheritfromanynumberofotherprotocols.Whenaprotocoltypeinheritsfromotherprotocols,thesetofrequirementsfromthoseotherprotocolsareaggregated,andanytypethatinheritsfromthecurrentprotocolmustconformtoallthoserequirements.Foranexampleofhowtouseprotocolinheritance,seeProtocolInheritance.

NOTE

Youcanalsoaggregatetheconformancerequirementsofmultipleprotocolsusingprotocolcompositiontypes,asdescribedinProtocolCompositionTypeandProtocolComposition.

Youcanaddprotocolconformancetoapreviouslydeclaredtypebyadoptingtheprotocolinanextensiondeclarationofthattype.Intheextension,youmustimplementalloftheadoptedprotocol’srequirements.Ifthetypealreadyimplementsalloftherequirements,youcanleavethebodyoftheextensiondeclarationempty.

Bydefault,typesthatconformtoaprotocolmustimplementallproperties,

methods,andsubscriptsdeclaredintheprotocol.Thatsaid,youcanmarktheseprotocolmemberdeclarationswiththeoptionaldeclarationmodifiertospecifythattheirimplementationbyaconformingtypeisoptional.Theoptionalmodifiercanbeappliedonlytomembersthataremarkedwiththeobjcattribute,andonlytomembersofprotocolsthataremarkedwiththeobjcattribute.Asaresult,onlyclasstypescanadoptandconformtoaprotocolthatcontainsoptionalmemberrequirements.Formoreinformationabouthowtousetheoptionaldeclarationmodifierandforguidanceabouthowtoaccessoptionalprotocolmembers—forexample,whenyou’renotsurewhetheraconformingtypeimplementsthem—seeOptionalProtocolRequirements.

Torestricttheadoptionofaprotocoltoclasstypesonly,includetheAnyObjectprotocolintheinheritedprotocolslistafterthecolon.Forexample,thefollowingprotocolcanbeadoptedonlybyclasstypes:

1 protocolSomeProtocol:AnyObject{

2 /*Protocolmembersgohere*/

3 }

Anyprotocolthatinheritsfromaprotocolthat’smarkedwiththeAnyObjectrequirementcanlikewisebeadoptedonlybyclasstypes.

NOTE

Ifaprotocolismarkedwiththeobjcattribute,theAnyObjectrequirementisimplicitlyappliedtothatprotocol;there’snoneedtomarktheprotocolwiththeAnyObjectrequirementexplicitly.

Protocolsarenamedtypes,andthustheycanappearinallthesameplacesinyourcodeasothernamedtypes,asdiscussedinProtocolsasTypes.However,youcan’tconstructaninstanceofaprotocol,becauseprotocolsdonotactuallyprovidetheimplementationsfortherequirementstheyspecify.

Youcanuseprotocolstodeclarewhichmethodsadelegateofaclassorstructureshouldimplement,asdescribedinDelegation.

GRAMMAR OF A PROTOCOLDECLARAT ION

protocol-declaration → attributes opt access-level-modifier opt protocol protocol-

name type-inheritance-clause opt generic-where-clause opt protocol-bodyprotocol-name → identifierprotocol-body → { protocol-members opt }protocol-members → protocol-member protocol-members optprotocol-member → protocol-member-declaration | compiler-control-statementprotocol-member-declaration → protocol-property-declarationprotocol-member-declaration → protocol-method-declarationprotocol-member-declaration → protocol-initializer-declarationprotocol-member-declaration → protocol-subscript-declarationprotocol-member-declaration → protocol-associated-type-declarationprotocol-member-declaration → typealias-declaration

ProtocolPropertyDeclarationProtocolsdeclarethatconformingtypesmustimplementapropertybyincludingaprotocolpropertydeclarationinthebodyoftheprotocoldeclaration.Protocolpropertydeclarationshaveaspecialformofavariabledeclaration:

var propertyname : type {getset}

Aswithotherprotocolmemberdeclarations,thesepropertydeclarationsdeclareonlythegetterandsetterrequirementsfortypesthatconformtotheprotocol.Asaresult,youdon’timplementthegetterorsetterdirectlyintheprotocolinwhichitisdeclared.

Thegetterandsetterrequirementscanbesatisfiedbyaconformingtypeinavarietyofways.Ifapropertydeclarationincludesboththegetandsetkeywords,aconformingtypecanimplementitwithastoredvariablepropertyoracomputedpropertythatisbothreadableandwriteable(thatis,onethatimplementsbothagetterandasetter).However,thatpropertydeclarationcan’tbeimplementedasaconstantpropertyoraread-onlycomputedproperty.Ifapropertydeclarationincludesonlythegetkeyword,itcanbeimplementedasanykindofproperty.Forexamplesofconformingtypesthatimplementthepropertyrequirementsofaprotocol,seePropertyRequirements.

Todeclareatypepropertyrequirementinaprotocoldeclaration,markthepropertydeclarationwiththestatickeyword.Structuresandenumerationsthatconformtotheprotocoldeclarethepropertywiththestatickeyword,and

classesthatconformtotheprotocoldeclarethepropertywitheitherthestaticorclasskeyword.Extensionsthataddprotocolconformancetoastructure,enumeration,orclassusethesamekeywordasthetypetheyextenduses.Extensionsthatprovideadefaultimplementationforatypepropertyrequirementusethestatickeyword.

SeealsoVariableDeclaration.

GRAMMAR OF A PROTOCOLPROPERTY DECLARAT ION

protocol-property-declaration → variable-declaration-head variable-name type-annotationgetter-setter-keyword-block

ProtocolMethodDeclarationProtocolsdeclarethatconformingtypesmustimplementamethodbyincludingaprotocolmethoddeclarationinthebodyoftheprotocoldeclaration.Protocolmethoddeclarationshavethesameformasfunctiondeclarations,withtwoexceptions:Theydon’tincludeafunctionbody,andyoucan’tprovideanydefaultparametervaluesaspartofthefunctiondeclaration.Forexamplesofconformingtypesthatimplementthemethodrequirementsofaprotocol,seeMethodRequirements.

Todeclareaclassorstaticmethodrequirementinaprotocoldeclaration,markthemethoddeclarationwiththestaticdeclarationmodifier.Structuresandenumerationsthatconformtotheprotocoldeclarethemethodwiththestatickeyword,andclassesthatconformtotheprotocoldeclarethemethodwitheitherthestaticorclasskeyword.Extensionsthataddprotocolconformancetoastructure,enumeration,orclassusethesamekeywordasthetypetheyextenduses.Extensionsthatprovideadefaultimplementationforatypemethodrequirementusethestatickeyword.

SeealsoFunctionDeclaration.

GRAMMAR OF A PROTOCOLMETHOD DECLARAT ION

protocol-method-declaration → function-head function-name generic-parameter-clauseopt function-signature generic-where-clause opt

ProtocolInitializerDeclarationProtocolsdeclarethatconformingtypesmustimplementaninitializerbyincludingaprotocolinitializerdeclarationinthebodyoftheprotocoldeclaration.Protocolinitializerdeclarationshavethesameformasinitializerdeclarations,excepttheydon’tincludetheinitializer’sbody.

Aconformingtypecansatisfyanonfailableprotocolinitializerrequirementbyimplementinganonfailableinitializeroraninit!failableinitializer.Aconformingtypecansatisfyafailableprotocolinitializerrequirementbyimplementinganykindofinitializer.

Whenaclassimplementsaninitializertosatisfyaprotocol’sinitializerrequirement,theinitializermustbemarkedwiththerequireddeclarationmodifieriftheclassisnotalreadymarkedwiththefinaldeclarationmodifier.

SeealsoInitializerDeclaration.

GRAMMAR OF A PROTOCOL I N I T I A L I ZER DECLARAT ION

protocol-initializer-declaration → initializer-head generic-parameter-clause optparameter-clause throwsopt generic-where-clause opt

protocol-initializer-declaration → initializer-head generic-parameter-clause optparameter-clause rethrows generic-where-clause opt

ProtocolSubscriptDeclarationProtocolsdeclarethatconformingtypesmustimplementasubscriptbyincludingaprotocolsubscriptdeclarationinthebodyoftheprotocoldeclaration.Protocolsubscriptdeclarationshaveaspecialformofasubscriptdeclaration:

subscript( parameters )-> returntype {getset}

Subscriptdeclarationsonlydeclaretheminimumgetterandsetterimplementationrequirementsfortypesthatconformtotheprotocol.Ifthesubscriptdeclarationincludesboththegetandsetkeywords,aconformingtypemustimplementbothagetterandasetterclause.Ifthesubscriptdeclarationincludesonlythegetkeyword,aconformingtypemustimplementatleasta

getterclauseandoptionallycanimplementasetterclause.

Todeclareastaticsubscriptrequirementinaprotocoldeclaration,markthesubscriptdeclarationwiththestaticdeclarationmodifier.Structuresandenumerationsthatconformtotheprotocoldeclarethesubscriptwiththestatickeyword,andclassesthatconformtotheprotocoldeclarethesubscriptwitheitherthestaticorclasskeyword.Extensionsthataddprotocolconformancetoastructure,enumeration,orclassusethesamekeywordasthetypetheyextenduses.Extensionsthatprovideadefaultimplementationforastaticsubscriptrequirementusethestatickeyword.

SeealsoSubscriptDeclaration.

GRAMMAR OF A PROTOCOLSUBSCR IPT DECLARAT ION

protocol-subscript-declaration → subscript-head subscript-result generic-where-clauseopt getter-setter-keyword-block

ProtocolAssociatedTypeDeclarationProtocolsdeclareassociatedtypesusingtheassociatedtypekeyword.Anassociatedtypeprovidesanaliasforatypethatisusedaspartofaprotocol’sdeclaration.Associatedtypesaresimilartotypeparametersingenericparameterclauses,butthey’reassociatedwithSelfintheprotocolinwhichthey’redeclared.Inthatcontext,Selfreferstotheeventualtypethatconformstotheprotocol.Formoreinformationandexamples,seeAssociatedTypes.

Youuseagenericwhereclauseinaprotocoldeclarationtoaddconstraintstoanassociatedtypesinheritedfromanotherprotocol,withoutredeclaringtheassociatedtypes.Forexample,thedeclarationsofSubProtocolbelowareequivalent:

1 protocolSomeProtocol{

2 associatedtypeSomeType

3 }

4

5 protocolSubProtocolA:SomeProtocol{

6 //Thissyntaxproducesawarning.

7 associatedtypeSomeType:Equatable

8 }

9

10 //Thissyntaxispreferred.

11 protocolSubProtocolB:SomeProtocolwhereSomeType:Equatable

{}

SeealsoTypeAliasDeclaration.

GRAMMAR OF A PROTOCOLASSOC IATED TYPE DECLARAT ION

protocol-associated-type-declaration → attributes opt access-level-modifier optassociatedtype typealias-name type-inheritance-clause opt typealias-assignment

opt generic-where-clause opt

InitializerDeclaration

Aninitializerdeclarationintroducesaninitializerforaclass,structure,orenumerationintoyourprogram.Initializerdeclarationsaredeclaredusingtheinitkeywordandhavetwobasicforms.

Structure,enumeration,andclasstypescanhaveanynumberofinitializers,buttherulesandassociatedbehaviorforclassinitializersaredifferent.Unlikestructuresandenumerations,classeshavetwokindsofinitializers:designatedinitializersandconvenienceinitializers,asdescribedinInitialization.

Thefollowingformdeclaresinitializersforstructures,enumerations,anddesignatedinitializersofclasses:

init( parameters ){

statements

}

Adesignatedinitializerofaclassinitializesalloftheclass’spropertiesdirectly.Itcan’tcallanyotherinitializersofthesameclass,andiftheclasshasasuperclass,itmustcalloneofthesuperclass’sdesignatedinitializers.Iftheclassinheritsanypropertiesfromitssuperclass,oneofthesuperclass’sdesignatedinitializersmustbecalledbeforeanyofthesepropertiescanbesetormodifiedinthecurrentclass.

Designatedinitializerscanbedeclaredinthecontextofaclassdeclarationonlyandthereforecan’tbeaddedtoaclassusinganextensiondeclaration.

Initializersinstructuresandenumerationscancallotherdeclaredinitializerstodelegatepartoralloftheinitializationprocess.

Todeclareconvenienceinitializersforaclass,marktheinitializerdeclarationwiththeconveniencedeclarationmodifier.

convenienceinit( parameters ){

statements

}

Convenienceinitializerscandelegatetheinitializationprocesstoanotherconvenienceinitializerortooneoftheclass’sdesignatedinitializers.Thatsaid,theinitializationprocessesmustendwithacalltoadesignatedinitializerthatultimatelyinitializestheclass’sproperties.Convenienceinitializerscan’tcallasuperclass’sinitializers.

Youcanmarkdesignatedandconvenienceinitializerswiththerequireddeclarationmodifiertorequirethateverysubclassimplementtheinitializer.Asubclass’simplementationofthatinitializermustalsobemarkedwiththerequireddeclarationmodifier.

Bydefault,initializersdeclaredinasuperclassarenotinheritedbysubclasses.Thatsaid,ifasubclassinitializesallofitsstoredpropertieswithdefaultvaluesanddoesn’tdefineanyinitializersofitsown,itinheritsallofthesuperclass’sinitializers.Ifthesubclassoverridesallofthesuperclass’sdesignatedinitializers,itinheritsthesuperclass’sconvenienceinitializers.

Aswithmethods,properties,andsubscripts,youneedtomarkoverridden

designatedinitializerswiththeoverridedeclarationmodifier.

NOTE

Ifyoumarkaninitializerwiththerequireddeclarationmodifier,youdon’talsomarktheinitializerwiththeoverridemodifierwhenyouoverridetherequiredinitializerinasubclass.

Justlikefunctionsandmethods,initializerscanthroworrethrowerrors.Andjustlikefunctionsandmethods,youusethethrowsorrethrowskeywordafteraninitializer’sparameterstoindicatetheappropriatebehavior.

Toseeexamplesofinitializersinvarioustypedeclarations,seeInitialization.

FailableInitializersAfailableinitializerisatypeofinitializerthatproducesanoptionalinstanceoranimplicitlyunwrappedoptionalinstanceofthetypetheinitializerisdeclaredon.Asaresult,afailableinitializercanreturnniltoindicatethatinitializationfailed.

Todeclareafailableinitializerthatproducesanoptionalinstance,appendaquestionmarktotheinitkeywordintheinitializerdeclaration(init?).Todeclareafailableinitializerthatproducesanimplicitlyunwrappedoptionalinstance,appendanexclamationmarkinstead(init!).Theexamplebelowshowsaninit?failableinitializerthatproducesanoptionalinstanceofastructure.

1 structSomeStruct{

2 letproperty:String

3 //producesanoptionalinstanceof'SomeStruct'

4 init?(input:String){

5 ifinput.isEmpty{

6 //discard'self'andreturn'nil'

7 returnnil

8 }

9 property=input

10 }

11 }

Youcallaninit?failableinitializerinthesamewaythatyoucallanonfailableinitializer,exceptthatyoumustdealwiththeoptionalityoftheresult.

1 ifletactualInstance=SomeStruct(input:"Hello"){

2 //dosomethingwiththeinstanceof'SomeStruct'

3 }else{

4 //initializationof'SomeStruct'failedandthe

initializerreturned'nil'

5 }

Afailableinitializercanreturnnilatanypointintheimplementationoftheinitializer’sbody.

Afailableinitializercandelegatetoanykindofinitializer.Anonfailableinitializercandelegatetoanothernonfailableinitializerortoaninit!failableinitializer.Anonfailableinitializercandelegatetoaninit?failableinitializerbyforce-unwrappingtheresultofthesuperclass’sinitializer—forexample,bywritingsuper.init()!.

Initializationfailurepropagatesthroughinitializerdelegation.Specifically,ifafailableinitializerdelegatestoaninitializerthatfailsandreturnsnil,thentheinitializerthatdelegatedalsofailsandimplicitlyreturnsnil.Ifanonfailableinitializerdelegatestoaninit!failableinitializerthatfailsandreturnsnil,thenaruntimeerrorisraised(asifyouusedthe!operatortounwrapanoptionalthathasanilvalue).

Afailabledesignatedinitializercanbeoverriddeninasubclassbyanykindofdesignatedinitializer.Anonfailabledesignatedinitializercanbeoverriddeninasubclassbyanonfailabledesignatedinitializeronly.

Formoreinformationandtoseeexamplesoffailableinitializers,seeFailable

Initializers.

GRAMMAR OF AN I N I T I A L I ZER DECLARAT ION

initializer-declaration → initializer-head generic-parameter-clause opt parameter-clausethrowsopt generic-where-clause opt initializer-body

initializer-declaration → initializer-head generic-parameter-clause opt parameter-clauserethrows generic-where-clause opt initializer-body

initializer-head → attributes opt declaration-modifiers opt initinitializer-head → attributes opt declaration-modifiers opt init ?initializer-head → attributes opt declaration-modifiers opt init !initializer-body → code-block

DeinitializerDeclaration

Adeinitializerdeclarationdeclaresadeinitializerforaclasstype.Deinitializerstakenoparametersandhavethefollowingform:

deinit{

statements

}

Adeinitializeriscalledautomaticallywhentherearenolongeranyreferencestoaclassobject,justbeforetheclassobjectisdeallocated.Adeinitializercanbedeclaredonlyinthebodyofaclassdeclaration—butnotinanextensionofaclass—andeachclasscanhaveatmostone.

Asubclassinheritsitssuperclass’sdeinitializer,whichisimplicitlycalledjustbeforethesubclassobjectisdeallocated.Thesubclassobjectisnotdeallocateduntilalldeinitializersinitsinheritancechainhavefinishedexecuting.

Deinitializersarenotcalleddirectly.

Foranexampleofhowtouseadeinitializerinaclassdeclaration,seeDeinitialization.

GRAMMAR OF ADE IN I T I A L I ZER DECLARAT ION

deinitializer-declaration → attributes opt deinit code-block

ExtensionDeclaration

Anextensiondeclarationallowsyoutoextendthebehaviorofexistingtypes.Extensiondeclarationsaredeclaredusingtheextensionkeywordandhavethefollowingform:

extension typename where requirements {

declarations

}

Thebodyofanextensiondeclarationcontainszeroormoredeclarations.Thesedeclarationscanincludecomputedproperties,computedtypeproperties,instancemethods,typemethods,initializers,subscriptdeclarations,andevenclass,structure,andenumerationdeclarations.Extensiondeclarationscan’tcontaindeinitializerorprotocoldeclarations,storedproperties,propertyobservers,orotherextensiondeclarations.Declarationsinaprotocolextensioncan’tbemarkedfinal.Foradiscussionandseveralexamplesofextensionsthatincludevariouskindsofdeclarations,seeExtensions.

Ifthetypenameisaclass,structure,orenumerationtype,theextensionextendsthattype.Ifthetypenameisaprotocoltype,theextensionextendsalltypesthatconformtothatprotocol.

Extensiondeclarationsthatextendagenerictypeoraprotocolwithassociatedtypescanincluderequirements.Ifaninstanceoftheextendedtypeorofatypethatconformstotheextendedprotocolsatisfiestherequirements,theinstancegainsthebehaviorspecifiedinthedeclaration.

Extensiondeclarationscancontaininitializerdeclarations.Thatsaid,ifthetypeyou’reextendingisdefinedinanothermodule,aninitializerdeclarationmustdelegatetoaninitializeralreadydefinedinthatmoduletoensuremembersofthattypeareproperlyinitialized.

Properties,methods,andinitializersofanexistingtypecan’tbeoverriddeninanextensionofthattype.

Extensiondeclarationscanaddprotocolconformancetoanexistingclass,structure,orenumerationtypebyspecifyingadoptedprotocols:

extension typename : adoptedprotocols where requirements

{

declarations

}

Extensiondeclarationscan’taddclassinheritancetoanexistingclass,andthereforeyoucanspecifyonlyalistofprotocolsafterthetypenameandcolon.

ConditionalConformanceYoucanextendagenerictypetoconditionallyconformtoaprotocol,sothatinstancesofthetypeconformtotheprotocolonlywhencertainrequirementsaremet.Youaddconditionalconformancetoaprotocolbyincludingrequirementsinanextensiondeclaration.

OverriddenRequirementsAren’tUsedinSomeGenericContexts

Insomegenericcontexts,typesthatgetbehaviorfromconditionalconformancetoaprotocoldon’talwaysusethespecializedimplementationsofthatprotocol’srequirements.Toillustratethisbehavior,thefollowingexampledefinestwoprotocolsandagenerictypethatconditionallyconformstobothprotocols.

1 protocolLoggable{

2 funclog()

3 }

4 extensionLoggable{

5 funclog(){

6 print(self)

7 }

8 }

9

10 protocolTitledLoggable:Loggable{

11 staticvarlogTitle:String{get}

12 }

13 extensionTitledLoggable{

14 funclog(){

15 print("\(Self.logTitle):\(self)")

16 }

17 }

18

19 structPair<T>:CustomStringConvertible{

20 letfirst:T

21 letsecond:T

22 vardescription:String{

23 return"(\(first),\(second))"

24 }

25 }

26

27 extensionPair:LoggablewhereT:Loggable{}

28 extensionPair:TitledLoggablewhereT:TitledLoggable{

29 staticvarlogTitle:String{

30 return"Pairof'\(T.logTitle)'"

31 }

32 }

33

34 extensionString:TitledLoggable{

35 staticvarlogTitle:String{

36 return"String"

37 }

38 }

ThePairstructureconformstoLoggableandTitledLoggablewheneveritsgenerictypeconformstoLoggableorTitledLoggable,respectively.Intheexamplebelow,oneAndTwoisaninstanceofPair<String>,whichconformstoTitledLoggablebecauseStringconformstoTitledLoggable.Whenthelog()methodiscalledononeAndTwodirectly,thespecializedversioncontainingthetitlestringisused.

1 letoneAndTwo=Pair(first:"one",second:"two")

2 oneAndTwo.log()

3 //Prints"Pairof'String':(one,two)"

However,whenoneAndTwoisusedinagenericcontextorasaninstanceoftheLoggableprotocol,thespecializedversionisn’tused.Swiftpickswhichimplementationoflog()tocallbyconsultingonlytheminimumrequirementsthatPairneedstoconformtoLoggable.Forthisreason,thedefaultimplementationprovidedbytheLoggableprotocolisusedinstead.

1 funcdoSomething<T:Loggable>(withx:T){

2 x.log()

3 }

4 doSomething(with:oneAndTwo)

5 //Prints"(one,two)"

Whenlog()iscalledontheinstancethat’spassedtodoSomething(_:),thecustomizedtitleisomittedfromtheloggedstring.

ProtocolConformanceMustNotBeRedundantAconcretetypecanconformtoaparticularprotocolonlyonce.Swiftmarksredundantprotocolconformancesasanerror.You’relikelytoencounterthiskindoferrorintwokindsofsituations.Thefirstsituationiswhenyouexplicitly

conformtothesameprotocolmultipletimes,butwithdifferentrequirements.Thesecondsituationiswhenyouimplicitlyinheritfromthesameprotocolmultipletimes.Thesesituationsarediscussedinthesectionsbelow.

ResolvingExplicitRedundancy

Multipleextensionsonaconcretetypecan’taddconformancetothesameprotocol,eveniftheextensions’requirementsaremutuallyexclusive.Thisrestrictionisdemonstratedintheexamplebelow.TwoextensiondeclarationsattempttoaddconditionalconformancetotheSerializableprotocol,oneforforarrayswithIntelements,andoneforarrayswithStringelements.

1 protocolSerializable{

2 funcserialize()->Any

3 }

4

5 extensionArray:SerializablewhereElement==Int{

6 funcserialize()->Any{

7 //implementation

8 }

9 }

10 extensionArray:SerializablewhereElement==String{

11 funcserialize()->Any{

12 //implementation

13 }

14 }

15 //Error:redundantconformanceof'Array<Element>'to

protocol'Serializable'

Ifyouneedtoaddconditionalconformancebasedonmultipleconcretetypes,createanewprotocolthateachtypecanconformtoandusethatprotocolastherequirementwhendeclaringconditionalconformance.

1 protocolSerializableInArray{}

2 extensionInt:SerializableInArray{}

3 extensionString:SerializableInArray{}

4

5 extensionArray:SerializablewhereElement:

SerializableInArray{

6 funcserialize()->Any{

7 //implementation

8 }

9 }

ResolvingImplicitRedundancy

Whenaconcretetypeconditionallyconformstoaprotocol,thattypeimplicitlyconformstoanyparentprotocolswiththesamerequirements.

Ifyouneedatypetoconditionallyconformtotwoprotocolsthatinheritfromasingleparent,explicitlydeclareconformancetotheparentprotocol.Thisavoidsimplicitlyconformingtotheparentprotocoltwicewithdifferentrequirements.

ThefollowingexampleexplicitlydeclarestheconditionalconformanceofArraytoLoggabletoavoidaconflictwhendeclaringitsconditionalconformancetobothTitledLoggableandthenewMarkedLoggableprotocol.

1 protocolMarkedLoggable:Loggable{

2 funcmarkAndLog()

3 }

4

5 extensionMarkedLoggable{

6 funcmarkAndLog(){

7 print("----------")

8 log()

9 }

10 }

11

12 extensionArray:LoggablewhereElement:Loggable{}

13 extensionArray:TitledLoggablewhereElement:TitledLoggable

{

14 staticvarlogTitle:String{

15 return"Arrayof'\(Element.logTitle)'"

16 }

17 }

18 extensionArray:MarkedLoggablewhereElement:MarkedLoggable

{}

WithouttheextensiontoexplicitlydeclareconditionalconformancetoLoggable,theotherArrayextensionswouldimplicitlycreatethesedeclarations,resultinginanerror:

1 extensionArray:LoggablewhereElement:TitledLoggable{}

2 extensionArray:LoggablewhereElement:MarkedLoggable{}

3 //Error:redundantconformanceof'Array<Element>'toprotocol

'Loggable'

GRAMMAR OF AN EXTENS ION DECLARAT ION

extension-declaration → attributes opt access-level-modifier opt extension type-identifier type-inheritance-clause opt generic-where-clause opt extension-body

extension-body → { extension-members opt }extension-members → extension-member extension-members optextension-member → declaration | compiler-control-statement

SubscriptDeclaration

Asubscriptdeclarationallowsyoutoaddsubscriptingsupportforobjectsofaparticulartypeandaretypicallyusedtoprovideaconvenientsyntaxforaccessingtheelementsinacollection,list,orsequence.Subscriptdeclarationsaredeclaredusingthesubscriptkeywordandhavethefollowingform:

subscript( parameters )-> returntype {

get{

statements

}

set( settername ){

statements

}

}

Subscriptdeclarationscanappearonlyinthecontextofaclass,structure,enumeration,extension,orprotocoldeclaration.

Theparametersspecifyoneormoreindexesusedtoaccesselementsofthecorrespondingtypeinasubscriptexpression(forexample,theiintheexpressionobject[i]).Althoughtheindexesusedtoaccesstheelementscanbeofanytype,eachparametermustincludeatypeannotationtospecifythetypeofeachindex.Thereturntypespecifiesthetypeoftheelementbeingaccessed.

Aswithcomputedproperties,subscriptdeclarationssupportreadingandwritingthevalueoftheaccessedelements.Thegetterisusedtoreadthevalue,andthesetterisusedtowritethevalue.Thesetterclauseisoptional,andwhenonlyagetterisneeded,youcanomitbothclausesandsimplyreturntherequestedvaluedirectly.Thatsaid,ifyouprovideasetterclause,youmustalsoprovideagetterclause.

Thesetternameandenclosingparenthesesareoptional.Ifyouprovideasettername,itisusedasthenameoftheparametertothesetter.Ifyoudonotprovideasettername,thedefaultparameternametothesetterisvalue.Thetypeoftheparametertothesetteristhesameasthereturntype.

Youcanoverloadasubscriptdeclarationinthetypeinwhichitisdeclared,as

longastheparametersorthereturntypedifferfromtheoneyou’reoverloading.Youcanalsooverrideasubscriptdeclarationinheritedfromasuperclass.Whenyoudoso,youmustmarktheoverriddensubscriptdeclarationwiththeoverridedeclarationmodifier.

Subscriptparametersfollowthesamerulesasfunctionparameters,withtwoexceptions.Bydefault,theparametersusedinsubscriptingdon’thaveargumentlabels,unlikefunctions,methods,andinitializers.However,youcanprovideexplicitargumentlabelsusingthesamesyntaxthatfunctions,methods,andinitializersuse.Inaddition,subscriptscan’thavein-outparameters.

Youcanalsodeclaresubscriptsinthecontextofaprotocoldeclaration,asdescribedinProtocolSubscriptDeclaration.

Formoreinformationaboutsubscriptingandtoseeexamplesofsubscriptdeclarations,seeSubscripts.

TypeSubscriptDeclarationsTodeclareasubscriptthat’sexposedbythetype,ratherthanbyinstancesofthetype,markthesubscriptdeclarationwiththestaticdeclarationmodifier.Classescanmarktypecomputedpropertieswiththeclassdeclarationmodifierinsteadtoallowsubclassestooverridethesuperclass’simplementation.Inaclassdeclaration,thestatickeywordhasthesameeffectasmarkingthedeclarationwithboththeclassandfinaldeclarationmodifiers.

GRAMMAR OF A SUBSCR IPT DECLARAT ION

subscript-declaration → subscript-head subscript-result generic-where-clause opt code-block

subscript-declaration → subscript-head subscript-result generic-where-clause opt getter-setter-block

subscript-declaration → subscript-head subscript-result generic-where-clause opt getter-setter-keyword-block

subscript-head → attributes opt declaration-modifiers opt subscript generic-parameter-clause opt parameter-clause

subscript-result → -> attributes opt type

OperatorDeclaration

Anoperatordeclarationintroducesanewinfix,prefix,orpostfixoperatorintoyourprogramandisdeclaredusingtheoperatorkeyword.

Youcandeclareoperatorsofthreedifferentfixities:infix,prefix,andpostfix.Thefixityofanoperatorspecifiestherelativepositionofanoperatortoitsoperands.

Therearethreebasicformsofanoperatordeclaration,oneforeachfixity.Thefixityoftheoperatorisspecifiedbymarkingtheoperatordeclarationwiththeinfix,prefix,orpostfixdeclarationmodifierbeforetheoperatorkeyword.Ineachform,thenameoftheoperatorcancontainonlytheoperatorcharactersdefinedinOperators.

Thefollowingformdeclaresanewinfixoperator:

infixoperator operatorname : precedencegroup

Aninfixoperatorisabinaryoperatorthatiswrittenbetweenitstwooperands,suchasthefamiliaradditionoperator(+)intheexpression1+2.

Infixoperatorscanoptionallyspecifyaprecedencegroup.Ifyouomittheprecedencegroupforanoperator,Swiftusesthedefaultprecedencegroup,DefaultPrecedence,whichspecifiesaprecedencejusthigherthanTernaryPrecedence.Formoreinformation,seePrecedenceGroupDeclaration.

Thefollowingformdeclaresanewprefixoperator:

prefixoperator operatorname

Aprefixoperatorisaunaryoperatorthatiswrittenimmediatelybeforeitsoperand,suchastheprefixlogicalNOToperator(!)intheexpression!a.

Prefixoperatorsdeclarationsdon’tspecifyaprecedencelevel.Prefixoperatorsarenonassociative.

Thefollowingformdeclaresanewpostfixoperator:

postfixoperator operatorname

Apostfixoperatorisaunaryoperatorthatiswrittenimmediatelyafteritsoperand,suchasthepostfixforced-unwrapoperator(!)intheexpressiona!.

Aswithprefixoperators,postfixoperatordeclarationsdon’tspecifyaprecedencelevel.Postfixoperatorsarenonassociative.

Afterdeclaringanewoperator,youimplementitbydeclaringastaticmethodthathasthesamenameastheoperator.Thestaticmethodisamemberofoneofthetypeswhosevaluestheoperatortakesasanargument—forexample,anoperatorthatmultipliesaDoublebyanIntisimplementedasastaticmethodoneithertheDoubleorIntstructure.Ifyou’reimplementingaprefixorpostfixoperator,youmustalsomarkthatmethoddeclarationwiththecorrespondingprefixorpostfixdeclarationmodifier.Toseeanexampleofhowtocreateandimplementanewoperator,seeCustomOperators.

GRAMMAR OF AN OPERATOR DECLARAT ION

operator-declaration → prefix-operator-declaration | postfix-operator-declaration | infix-operator-declaration

prefix-operator-declaration → prefix operator operatorpostfix-operator-declaration → postfix operator operatorinfix-operator-declaration → infix operator operator infix-operator-group optinfix-operator-group → : precedence-group-name

PrecedenceGroupDeclaration

Aprecedencegroupdeclarationintroducesanewgroupingforinfixoperatorprecedenceintoyourprogram.Theprecedenceofanoperatorspecifieshowtightlytheoperatorbindstoitsoperands,intheabsenceofgroupingparentheses.

Aprecedencegroupdeclarationhasthefollowingform:

precedencegroup precedencegroupname {

higherThan: lowergroupnames

lowerThan: highergroupnames

associativity: associativity

assignment: assignment

}

Thelowergroupnamesandhighergroupnameslistsspecifythenewprecedencegroup’srelationtoexistingprecedencegroups.ThelowerThanprecedencegroupattributemayonlybeusedtorefertoprecedencegroupsdeclaredoutsideofthecurrentmodule.Whentwooperatorscompetewitheachotherfortheiroperands,suchasintheexpression2+3*5,theoperatorwiththehigherrelativeprecedencebindsmoretightlytoitsoperands.

NOTE

Precedencegroupsrelatedtoeachotherusinglowergroupnamesandhighergroupnamesmustfitintoasinglerelationalhierarchy,buttheydon’thavetoformalinearhierarchy.Thismeansitispossibletohaveprecedencegroupswithundefinedrelativeprecedence.Operatorsfromthoseprecedencegroupscan’tbeusednexttoeachotherwithoutgroupingparentheses.

Swiftdefinesnumerousprecedencegroupstogoalongwiththeoperatorsprovidedbythestandardlibrary.Forexample,theaddition(+)andsubtraction(-)operatorsbelongtotheAdditionPrecedencegroup,andthemultiplication(*)anddivision(/)operatorsbelongtotheMultiplicationPrecedencegroup.ForacompletelistofprecedencegroupsprovidedbytheSwiftstandardlibrary,seeOperatorDeclarations.

Theassociativityofanoperatorspecifieshowasequenceofoperatorswiththesameprecedencelevelaregroupedtogetherintheabsenceofgroupingparentheses.Youspecifytheassociativityofanoperatorbywritingoneofthecontext-sensitivekeywordsleft,right,ornone—ifyouromittheassociativity,thedefaultisnone.Operatorsthatareleft-associativegroupleft-to-right.Forexample,thesubtractionoperator(-)isleft-associative,sotheexpression4-5-6isgroupedas(4-5)-6andevaluatesto-7.Operatorsthatareright-associativegroupright-to-left,andoperatorsthatarespecifiedwithanassociativityofnonedon’tassociateatall.Nonassociativeoperatorsofthesameprecedencelevelcan’tappearadjacenttoeachtoother.Forexample,the<operatorhasanassociativityofnone,whichmeans1<2<3isnotavalidexpression.

Theassignmentofaprecedencegroupspecifiestheprecedenceofanoperatorwhenusedinanoperationthatincludesoptionalchaining.Whensettotrue,anoperatorinthecorrespondingprecedencegroupusesthesamegroupingrulesduringoptionalchainingastheassignmentoperatorsfromthestandardlibrary.Otherwise,whensettofalseoromitted,operatorsintheprecedencegroupfollowsthesameoptionalchainingrulesasoperatorsthatdon’tperformassignment.

GRAMMAR OF A PRECEDENCE GROUP DECLARAT ION

precedence-group-declaration → precedencegroup precedence-group-name {precedence-group-attributes opt }

precedence-group-attributes → precedence-group-attribute precedence-group-attributesopt

precedence-group-attribute → precedence-group-relationprecedence-group-attribute → precedence-group-assignmentprecedence-group-attribute → precedence-group-associativityprecedence-group-relation → higherThan : precedence-group-namesprecedence-group-relation → lowerThan : precedence-group-namesprecedence-group-assignment → assignment : boolean-literalprecedence-group-associativity → associativity : leftprecedence-group-associativity → associativity : rightprecedence-group-associativity → associativity : noneprecedence-group-names → precedence-group-name | precedence-group-name ,

precedence-group-namesprecedence-group-name → identifier

DeclarationModifiers

Declarationmodifiersarekeywordsorcontext-sensitivekeywordsthatmodifythebehaviorormeaningofadeclaration.Youspecifyadeclarationmodifierbywritingtheappropriatekeywordorcontext-sensitivekeywordbetweenadeclaration’sattributes(ifany)andthekeywordthatintroducesthedeclaration.

class

Applythismodifiertoamemberofaclasstoindicatethatthememberisamemberoftheclassitself,ratherthanamemberofinstancesoftheclass.Membersofasuperclassthathavethismodifieranddon’thavethefinalmodifiercanbeoverriddenbysubclasses.

dynamic

ApplythismodifiertoanymemberofaclassthatcanberepresentedbyObjective-C.Whenyoumarkamemberdeclarationwiththedynamicmodifier,accesstothatmemberisalwaysdynamicallydispatchedusingtheObjective-Cruntime.Accesstothatmemberisneverinlinedordevirtualizedbythecompiler.

BecausedeclarationsmarkedwiththedynamicmodifieraredispatchedusingtheObjective-Cruntime,theymustbemarkedwiththeobjcattribute.

final

Applythismodifiertoaclassortoaproperty,method,orsubscriptmemberofaclass.It’sappliedtoaclasstoindicatethattheclasscan’tbesubclassed.It’sappliedtoaproperty,method,orsubscriptofaclasstoindicatethataclassmembercan’tbeoverriddeninanysubclass.Foranexampleofhowtousethefinalattribute,seePreventingOverrides.

lazy

Applythismodifiertoastoredvariablepropertyofaclassorstructuretoindicatethattheproperty’sinitialvalueiscalculatedandstoredatmostonce,whenthepropertyisfirstaccessed.Foranexampleofhowtousethelazymodifier,seeLazyStoredProperties.

optional

Applythismodifiertoaprotocol’sproperty,method,orsubscriptmemberstoindicatethataconformingtypeisn’trequiredtoimplementthosemembers.

Youcanapplytheoptionalmodifieronlytoprotocolsthataremarkedwiththeobjcattribute.Asaresult,onlyclasstypescanadoptandconformtoaprotocolthatcontainsoptionalmemberrequirements.Formoreinformationabouthowtousetheoptionalmodifierandforguidanceabouthowtoaccessoptionalprotocolmembers—forexample,whenyou’renotsurewhetheraconformingtypeimplementsthem—seeOptionalProtocolRequirements.

required

Applythismodifiertoadesignatedorconvenienceinitializerofaclasstoindicatethateverysubclassmustimplementthatinitializer.Thesubclass’simplementationofthatinitializermustalsobemarkedwiththerequiredmodifier.

static

Applythismodifiertoamemberofastructure,class,enumeration,orprotocoltoindicatethatthememberisamemberofthetype,ratherthanamemberofinstancesofthattype.Inthescopeofaclassdeclaration,writingthestaticmodifieronamemberdeclarationhasthesameeffectaswritingtheclassandfinalmodifiersonthatmemberdeclaration.However,constanttypepropertiesofaclassareanexception:statichasitsnormal,nonclassmeaningtherebecauseyoucan’twriteclassorfinalonthosedeclarations.

unowned

Applythismodifiertoastoredvariable,constant,orstoredpropertytoindicatethatthevariableorpropertyhasanunownedreferencetotheobjectstoredasitsvalue.Ifyoutrytoaccessthevariableorpropertyaftertheobjecthasbeendeallocated,aruntimeerrorisraised.Likeaweakreference,thetypeofthepropertyorvaluemustbeaclasstype;unlikeaweakreference,thetypeisnon-optional.Foranexampleandmoreinformationabouttheunownedmodifier,seeUnownedReferences.

unowned(safe)

Anexplicitspellingofunowned.

unowned(unsafe)

Applythismodifiertoastoredvariable,constant,orstoredpropertytoindicatethatthevariableorpropertyhasanunownedreferencetotheobjectstoredasitsvalue.Ifyoutrytoaccessthevariableorpropertyaftertheobjecthasbeendeallocated,you’llaccessthememoryatthelocationwheretheobjectusedtobe,whichisamemory-unsafeoperation.Likeaweakreference,thetypeofthepropertyorvaluemustbeaclasstype;unlikea

weakreference,thetypeisnon-optional.Foranexampleandmoreinformationabouttheunownedmodifier,seeUnownedReferences.

weak

Applythismodifiertoastoredvariableorstoredvariablepropertytoindicatethatthevariableorpropertyhasaweakreferencetotheobjectstoredasitsvalue.Thetypeofthevariableorpropertymustbeanoptionalclasstype.Ifyouaccessthevariableorpropertyaftertheobjecthasbeendeallocated,itsvalueisnil.Foranexampleandmoreinformationabouttheweakmodifier,seeWeakReferences.

AccessControlLevelsSwiftprovidesfivelevelsofaccesscontrol:open,public,internal,fileprivate,andprivate.Youcanmarkadeclarationwithoneoftheaccess-levelmodifiersbelowtospecifythedeclaration’saccesslevel.AccesscontrolisdiscussedindetailinAccessControl.

open

Applythismodifiertoadeclarationtoindicatethedeclarationcanbeaccessedandsubclassedbycodeinthesamemoduleasthedeclaration.Declarationsmarkedwiththeopenaccess-levelmodifiercanalsobeaccessedandsubclassedbycodeinamodulethatimportsthemodulethatcontainsthatdeclaration.

public

Applythismodifiertoadeclarationtoindicatethedeclarationcanbeaccessedandsubclassedbycodeinthesamemoduleasthedeclaration.Declarationsmarkedwiththepublicaccess-levelmodifiercanalsobeaccessed(butnotsubclassed)bycodeinamodulethatimportsthemodulethatcontainsthatdeclaration.

internal

Applythismodifiertoadeclarationtoindicatethedeclarationcanbe

accessedonlybycodeinthesamemoduleasthedeclaration.Bydefault,mostdeclarationsareimplicitlymarkedwiththeinternalaccess-levelmodifier.

fileprivate

Applythismodifiertoadeclarationtoindicatethedeclarationcanbeaccessedonlybycodeinthesamesourcefileasthedeclaration.

private

Applythismodifiertoadeclarationtoindicatethedeclarationcanbeaccessedonlybycodewithinthedeclaration’simmediateenclosingscope.

Forthepurposeofaccesscontrol,extensionstothesametypethatareinthesamefileshareanaccess-controlscope.Ifthetypetheyextendisalsointhesamefile,theysharethetype’saccess-controlscope.Privatemembersdeclaredinthetype’sdeclarationcanbeaccessedfromextensions,andprivatemembersdeclaredinoneextensioncanbeaccessedfromotherextensionsandfromthetype’sdeclaration.

Eachaccess-levelmodifieraboveoptionallyacceptsasingleargument,whichconsistsofthesetkeywordenclosedinparentheses(forexample,private(set)).Usethisformofanaccess-levelmodifierwhenyouwanttospecifyanaccesslevelforthesetterofavariableorsubscriptthat’slessthanorequaltotheaccesslevelofthevariableorsubscriptitself,asdiscussedinGettersandSetters.

GRAMMAR OF ADECLARAT ION MOD IF I ER

declaration-modifier → class | convenience | dynamic | final | infix | lazy |optional | override | postfix | prefix | required | static | unowned |unowned ( safe ) | unowned ( unsafe ) | weak

declaration-modifier → access-level-modifierdeclaration-modifier → mutation-modifierdeclaration-modifiers → declaration-modifier declaration-modifiers optaccess-level-modifier → private | private ( set )access-level-modifier → fileprivate | fileprivate ( set )access-level-modifier → internal | internal ( set )access-level-modifier → public | public ( set )access-level-modifier → open | open ( set )mutation-modifier → mutating | nonmutating

Attributes

TherearetwokindsofattributesinSwift—thosethatapplytodeclarationsandthosethatapplytotypes.Anattributeprovidesadditionalinformationaboutthedeclarationortype.Forexample,thediscardableResultattributeonafunctiondeclarationindicatesthat,althoughthefunctionreturnsavalue,thecompilershouldn’tgenerateawarningifthereturnvalueisunused.

Youspecifyanattributebywritingthe@symbolfollowedbytheattribute’snameandanyargumentsthattheattributeaccepts:

@ attributename

@ attributename ( attributearguments )

Somedeclarationattributesacceptargumentsthatspecifymoreinformationabouttheattributeandhowitappliestoaparticulardeclaration.Theseattributeargumentsareenclosedinparentheses,andtheirformatisdefinedbytheattributetheybelongto.

DeclarationAttributes

Youcanapplyadeclarationattributetodeclarationsonly.

availableApplythisattributetoindicateadeclaration’slifecyclerelativetocertainSwiftlanguageversionsorcertainplatformsandoperatingsystemversions.

Theavailableattributealwaysappearswithalistoftwoormorecomma-separatedattributearguments.Theseargumentsbeginwithoneofthefollowingplatformorlanguagenames:

iOS

iOSApplicationExtension

macOS

macOSApplicationExtension

watchOS

watchOSApplicationExtension

tvOS

tvOSApplicationExtension

swift

Youcanalsouseanasterisk(*)toindicatetheavailabilityofthedeclarationonalloftheplatformnameslistedabove.AnavailableattributethatspecifiesavailabilityusingaSwiftversionnumbercan’tusetheasterisk.

Theremainingargumentscanappearinanyorderandspecifyadditionalinformationaboutthedeclaration’slifecycle,includingimportantmilestones.

Theunavailableargumentindicatesthatthedeclarationisn’tavailableonthespecifiedplatform.Thisargumentcan’tbeusedwhenspecifyingSwiftversionavailability.

Theintroducedargumentindicatesthefirstversionofthespecifiedplatformorlanguageinwhichthedeclarationwasintroduced.Ithasthefollowingform:

introduced: versionnumber

Theversionnumberconsistsofonetothreepositiveintegers,separatedbyperiods.

Thedeprecatedargumentindicatesthefirstversionofthespecified

platformorlanguageinwhichthedeclarationwasdeprecated.Ithasthefollowingform:

deprecated: versionnumber

Theoptionalversionnumberconsistsofonetothreepositiveintegers,separatedbyperiods.Omittingtheversionnumberindicatesthatthedeclarationiscurrentlydeprecated,withoutgivinganyinformationaboutwhenthedeprecationoccurred.Ifyouomittheversionnumber,omitthecolon(:)aswell.

Theobsoletedargumentindicatesthefirstversionofthespecifiedplatformorlanguageinwhichthedeclarationwasobsoleted.Whenadeclarationisobsoleted,it’sremovedfromthespecifiedplatformorlanguageandcannolongerbeused.Ithasthefollowingform:

obsoleted: versionnumber

Theversionnumberconsistsofonetothreepositiveintegers,separatedbyperiods.

Themessageargumentprovidesatextualmessagethatthecompilerdisplayswhenemittingawarningorerrorabouttheuseofadeprecatedorobsoleteddeclaration.Ithasthefollowingform:

message: message

Themessageconsistsofastringliteral.

Therenamedargumentprovidesatextualmessagethatindicatesthenewnameforadeclarationthat’sbeenrenamed.Thecompilerdisplaysthenewnamewhenemittinganerrorabouttheuseofarenameddeclaration.Ithasthefollowingform:

renamed: newname

Thenewnameconsistsofastringliteral.

Youcanapplytheavailableattributewiththerenamedandunavailableargumentstoatypealiasdeclaration,asshownbelow,toindicatethatthenameofadeclarationchangedbetweenreleasesofaframeworkorlibrary.Thiscombinationresultsinacompile-timeerrorthatthedeclarationhasbeenrenamed.

1 //Firstrelease

2 protocolMyProtocol{

3 //protocoldefinition

4 }

1 //SubsequentreleaserenamesMyProtocol

2 protocolMyRenamedProtocol{

3 //protocoldefinition

4 }

5

6 @available(*,unavailable,renamed:"MyRenamedProtocol")

7 typealiasMyProtocol=MyRenamedProtocol

Youcanapplymultipleavailableattributesonasingledeclarationtospecifythedeclaration’savailabilityondifferentplatformsanddifferentversionsofSwift.Thedeclarationthattheavailableattributeappliestoisignorediftheattributespecifiesaplatformorlanguageversionthatdoesn’tmatchthecurrenttarget.Ifyouusemultipleavailableattributes,theeffectiveavailabilityisthecombinationoftheplatformandSwiftavailabilities.

Ifanavailableattributeonlyspecifiesanintroducedargumentinadditiontoaplatformorlanguagenameargument,youcanusethefollowingshorthandsyntaxinstead:

@available( platformname versionnumber ,*)

@available(swift versionnumber )

Theshorthandsyntaxforavailableattributesconciselyexpressesavailability

formultipleplatforms.Althoughthetwoformsarefunctionallyequivalent,theshorthandformispreferredwheneverpossible.

1 @available(iOS10.0,macOS10.12,*)

2 classMyClass{

3 //classdefinition

4 }

AnavailableattributethatspecifiesavailabilityusingaSwiftversionnumbercan’tadditionallyspecifyadeclaration’splatformavailability.Instead,useseparateavailableattributestospecifyaSwiftversionavailabilityandoneormoreplatformavailabilities.

1 @available(swift3.0.2)

2 @available(macOS10.12,*)

3 structMyStruct{

4 //structdefinition

5 }

discardableResultApplythisattributetoafunctionormethoddeclarationtosuppressthecompilerwarningwhenthefunctionormethodthatreturnsavalueiscalledwithoutusingitsresult.

dynamicCallableApplythisattributetoaclass,structure,enumeration,orprotocoltotreatinstancesofthetypeascallablefunctions.ThetypemustimplementeitheradynamicallyCall(withArguments:)method,adynamicallyCall(withKeywordArguments:)method,orboth.

Youcancallaninstanceofadynamicallycallabletypeasifit’safunctionthat

takesanynumberofarguments.

1 @dynamicCallable

2 structTelephoneExchange{

3 funcdynamicallyCall(withArgumentsphoneNumber:[Int]){

4 ifphoneNumber==[4,1,1]{

5 print("GetSwifthelponforums.swift.org")

6 }else{

7 print("Unrecognizednumber")

8 }

9 }

10 }

11

12 letdial=TelephoneExchange()

13

14 //Useadynamicmethodcall.

15 dial(4,1,1)

16 //Prints"GetSwifthelponforums.swift.org"

17

18 dial(8,6,7,5,3,0,9)

19 //Prints"Unrecognizednumber"

20

21 //Calltheunderlyingmethoddirectly.

22 dial.dynamicallyCall(withArguments:[4,1,1])

ThedeclarationofthedynamicallyCall(withArguments:)methodmusthaveasingleparameterthatconformstotheExpressibleByArrayLiteralprotocol—like[Int]intheexampleabove.Thereturntypecanbeanytype.

YoucanincludelabelsinadynamicmethodcallifyouimplementthedynamicallyCall(withKeywordArguments:)method.

1 @dynamicCallable

2 structRepeater{

3 funcdynamicallyCall(withKeywordArgumentspairs:

KeyValuePairs<String,Int>)->String{

4 returnpairs

5 .map{label,countin

6 repeatElement(label,count:

count).joined(separator:"")

7 }

8 .joined(separator:"\n")

9 }

10 }

11

12 letrepeatLabels=Repeater()

13 print(repeatLabels(a:1,b:2,c:3,b:2,a:1))

14 //a

15 //bb

16 //ccc

17 //bb

18 //a

ThedeclarationofthedynamicallyCall(withKeywordArguments:)methodmusthaveasingleparameterthatconformstotheExpressibleByDictionaryLiteralprotocol,andthereturntypecanbeanytype.Theparameter’sKeymustbeExpressibleByStringLiteral.ThepreviousexampleusesKeyValuePairsastheparametertypesothatcallerscanincludeduplicateparameterlabels—aandbappearmultipletimesinthecalltorepeat.

IfyouimplementbothdynamicallyCallmethods,dynamicallyCall(withKeywordArguments:)iscalledwhenthemethodcallincludeskeywordarguments.Inallothercases,dynamicallyCall(withArguments:)iscalled.

YoucanonlycalladynamicallycallableinstancewithargumentsandareturnvaluethatmatchthetypesyouspecifyinoneofyourdynamicallyCallmethodimplementations.Thecallinthefollowingexampledoesn’tcompilebecausethereisn’tanimplementationofdynamicallyCall(withArguments:)thattakesKeyValuePairs<String,String>.

repeatLabels(a:"four")//Error

dynamicMemberLookupApplythisattributetoaclass,structure,enumeration,orprotocoltoenablememberstobelookedupbynameatruntime.Thetypemustimplementasubscript(dynamicMemberLookup:)subscript.

Inanexplicitmemberexpression,ifthereisn’tacorrespondingdeclarationforthenamedmember,theexpressionisunderstoodasacalltothetype’ssubscript(dynamicMemberLookup:)subscript,passinginformationaboutthememberastheargument.Thesubscriptcanacceptaparameterthat’seitherakeypathoramembername;ifyouimplementbothsubscripts,thesubscriptthattakeskeypathargumentisused.

Animplementationofsubscript(dynamicMemberLookup:)canacceptkeypathsusinganargumentoftypeKeyPath,WritableKeyPath,orReferenceWritableKeyPath.ItcanacceptmembernamesusinganargumentofatypethatconformstotheExpressibleByStringLiteralprotocol—inmostcases,String.Thesubscript’sreturntypecanbeanytype.

Dynamicmemberlookupbymembernamecanbeusedtocreateawrappertypearounddatathatcan’tbetypecheckedatcompiletime,suchaswhenbridgingdatafromotherlanguagesintoSwift.Forexample:

1 @dynamicMemberLookup

2 structDynamicStruct{

3 letdictionary=["someDynamicMember":325,

4 "someOtherMember":787]

5 subscript(dynamicMembermember:String)->Int{

6 returndictionary[member]??1054

7 }

8 }

9 lets=DynamicStruct()

10

11 //Usedynamicmemberlookup.

12 letdynamic=s.someDynamicMember

13 print(dynamic)

14 //Prints"325"

15

16 //Calltheunderlyingsubscriptdirectly.

17 letequivalent=s[dynamicMember:"someDynamicMember"]

18 print(dynamic==equivalent)

19 //Prints"true"

Dynamicmemberlookupbykeypathcanbeusedtoimplementawrappertypeinawaythatsupportscompile-timetypechecking.Forexample:

1 structPoint{varx,y:Int}

2

3 @dynamicMemberLookup

4 structPassthroughWrapper<Value>{

5 varvalue:Value

6 subscript<T>(dynamicMembermember:KeyPath<Value,T>)->T

{

7 get{returnvalue[keyPath:member]}

8 }

9 }

10

11 letpoint=Point(x:381,y:431)

12 letwrapper=PassthroughWrapper(value:point)

13 print(wrapper.x)

frozenApplythisattributetoastructureorenumerationdeclarationtorestrictthekindsofchangesyoucanmaketothetype.Thisattributeisallowedonlywhencompilinginlibraryevolutionmode.Futureversionsofthelibrarycan’tchangethedeclarationbyadding,removing,orreorderinganenumeration’scasesorastructure’sstoredinstanceproperties.Thesechangesareallowedonnonfrozentypes,buttheybreakABIcompatibilityforfrozentypes.

NOTE

Whenthecompilerisn’tinlibraryevolutionmode,allstructuresandenumerationsareimplicitlyfrozen,andyoucan’tusethisattribute.

Inlibraryevolutionmode,codethatinteractswithmembersofnonfrozenstructuresandenumerationsiscompiledinawaythatallowsittocontinueworkingwithoutrecompilingevenifafutureversionofthelibraryadds,removes,orreorderssomeofthattype’smembers.Thecompilermakesthispossibleusingtechniqueslikelookingupinformationatruntimeandaddingalayerofindirection.Markingastructureorenumerationasfrozengivesupthisflexibilitytogainperformance:Futureversionsofthelibrarycanmakeonlylimitedchangestothetype,butthecompilercanmakeadditionaloptimizationsincodethatinteractswiththetype’smembers.

Frozentypes,thetypesofthestoredpropertiesoffrozenstructures,andtheassociatedvaluesoffrozenenumerationcasesmustbepublicormarkedwiththeusableFromInlineattribute.Thepropertiesofafrozenstructurecan’thavepropertyobservers,andexpressionsthatprovidetheinitialvalueforstoredinstancepropertiesmustfollowthesamerestrictionsasinlinablefunctions,asdiscussedininlinable.

Toenablelibraryevolutionmodeonthecommandline,passthe-enable-

library-evolutionoptiontotheSwiftcompiler.ToenableitinXcode,setthe“BuildLibrariesforDistribution”buildsetting(BUILD_LIBRARY_FOR_DISTRIBUTION)toYes,asdescribedinXcodeHelp.

Aswitchstatementoverafrozenenumerationdoesn’trequireadefaultcase,asdiscussedinSwitchingOverFutureEnumerationCases.Includingadefaultor@unknowndefaultcasewhenswitchingoverafrozenenumerationproducesawarningbecausethatcodeisneverexecuted.

GKInspectableApplythisattributetoexposeacustomGameplayKitcomponentpropertytotheSpriteKiteditorUI.Applyingthisattributealsoimpliestheobjcattribute.

inlinableApplythisattributetoafunction,method,computedproperty,subscript,convenienceinitializer,ordeinitializerdeclarationtoexposethatdeclaration’simplementationaspartofthemodule’spublicinterface.Thecompilerisallowedtoreplacecallstoaninlinablesymbolwithacopyofthesymbol’simplementationatthecallsite.

Inlinablecodecaninteractwithpublicsymbolsdeclaredinanymodule,anditcaninteractwithinternalsymbolsdeclaredinthesamemodulethataremarkedwiththeusableFromInlineattribute.Inlinablecodecan’tinteractwithprivateorfileprivatesymbols.

Thisattributecan’tbeappliedtodeclarationsthatarenestedinsidefunctionsortofileprivateorprivatedeclarations.Functionsandclosuresthataredefinedinsideaninlinablefunctionareimplicitlyinlinable,eventhoughtheycan’tbemarkedwiththisattribute.

nonobjcApplythisattributetoamethod,property,subscript,orinitializerdeclarationto

suppressanimplicitobjcattribute.ThenonobjcattributetellsthecompilertomakethedeclarationunavailableinObjective-Ccode,eventhoughit’spossibletorepresentitinObjective-C.

Applyingthisattributetoanextensionhasthesameeffectasapplyingittoeverymemberofthatextensionthatisn’texplicitlymarkedwiththeobjcattribute.

Youusethenonobjcattributetoresolvecircularityforbridgingmethodsinaclassmarkedwiththeobjcattribute,andtoallowoverloadingofmethodsandinitializersinaclassmarkedwiththeobjcattribute.

Amethodmarkedwiththenonobjcattributecan’toverrideamethodmarkedwiththeobjcattribute.However,amethodmarkedwiththeobjcattributecanoverrideamethodmarkedwiththenonobjcattribute.Similarly,amethodmarkedwiththenonobjcattributecan’tsatisfyaprotocolrequirementforamethodmarkedwiththeobjcattribute.

NSApplicationMainApplythisattributetoaclasstoindicatethatit’stheapplicationdelegate.UsingthisattributeisequivalenttocallingtheNSApplicationMain(_:_:)function.

Ifyoudon’tusethisattribute,supplyamain.swiftfilewithcodeatthetoplevelthatcallstheNSApplicationMain(_:_:)functionasfollows:

1 importAppKit

2 NSApplicationMain(CommandLine.argc,CommandLine.unsafeArgv)

NSCopyingApplythisattributetoastoredvariablepropertyofaclass.Thisattributecausestheproperty’ssettertobesynthesizedwithacopyoftheproperty’svalue—returnedbythecopyWithZone(_:)method—insteadofthevalueofthepropertyitself.ThetypeofthepropertymustconformtotheNSCopyingprotocol.

TheNSCopyingattributebehavesinawaysimilartotheObjective-Ccopypropertyattribute.

NSManagedApplythisattributetoaninstancemethodorstoredvariablepropertyofaclassthatinheritsfromNSManagedObjecttoindicatethatCoreDatadynamicallyprovidesitsimplementationatruntime,basedontheassociatedentitydescription.ForapropertymarkedwiththeNSManagedattribute,CoreDataalsoprovidesthestorageatruntime.Applyingthisattributealsoimpliestheobjcattribute.

objcApplythisattributetoanydeclarationthatcanberepresentedinObjective-C—forexample,nonnestedclasses,protocols,nongenericenumerations(constrainedtointegerraw-valuetypes),propertiesandmethods(includinggettersandsetters)ofclasses,protocolsandoptionalmembersofaprotocol,initializers,andsubscripts.TheobjcattributetellsthecompilerthatadeclarationisavailabletouseinObjective-Ccode.

Applyingthisattributetoanextensionhasthesameeffectasapplyingittoeverymemberofthatextensionthatisn’texplicitlymarkedwiththenonobjcattribute.

ThecompilerimplicitlyaddstheobjcattributetosubclassesofanyclassdefinedinObjective-C.However,thesubclassmustnotbegeneric,andmustnotinheritfromanygenericclasses.Youcanexplicitlyaddtheobjcattributetoasubclassthatmeetsthesecriteria,tospecifyitsObjective-Cnameasdiscussedbelow.Protocolsthataremarkedwiththeobjcattributecan’tinheritfromprotocolsthataren’tmarkedwiththisattribute.

Theobjcattributeisalsoimplicitlyaddedinthefollowingcases:

Thedeclarationisanoverrideinasubclass,andthesuperclass’sdeclarationhastheobjcattribute.

Thedeclarationsatisfiesarequirementfromaprotocolthathastheobjcattribute.

ThedeclarationhastheIBAction,IBSegueAction,IBOutlet,IBDesignable,IBInspectable,NSManaged,orGKInspectableattribute.

Ifyouapplytheobjcattributetoanenumeration,eachenumerationcaseisexposedtoObjective-Ccodeastheconcatenationoftheenumerationnameandthecasename.Thefirstletterofthecasenameiscapitalized.Forexample,acasenamedvenusinaSwiftPlanetenumerationisexposedtoObjective-CcodeasacasenamedPlanetVenus.

Theobjcattributeoptionallyacceptsasingleattributeargument,whichconsistsofanidentifier.TheidentifierspecifiesthenametobeexposedtoObjective-Cfortheentitythattheobjcattributeappliesto.Youcanusethisargumenttonameclasses,enumerations,enumerationcases,protocols,methods,getters,setters,andinitializers.IfyouspecifytheObjective-Cnameforaclass,protocol,orenumeration,includeathree-letterprefixonthename,asdescribedinConventionsinProgrammingwithObjective-C.TheexamplebelowexposesthegetterfortheenabledpropertyoftheExampleClasstoObjective-CcodeasisEnabledratherthanjustasthenameofthepropertyitself.

1 classExampleClass:NSObject{

2 @objcvarenabled:Bool{

3 @objc(isEnabled)get{

4 //Returntheappropriatevalue

5 }

6 }

7 }

objcMembersApplythisattributetoaclassdeclaration,toimplicitlyapplytheobjcattributetoallObjective-Ccompatiblemembersoftheclass,itsextensions,itssubclasses,andalloftheextensionsofitssubclasses.

Mostcodeshouldusetheobjcattributeinstead,toexposeonlythedeclarationsthatareneeded.Ifyouneedtoexposemanydeclarations,youcangrouptheminanextensionthathastheobjcattribute.TheobjcMembersattributeisaconvenienceforlibrariesthatmakeheavyuseoftheintrospectionfacilitiesoftheObjective-Cruntime.Applyingtheobjcattributewhenitisn’tneededcanincreaseyourbinarysizeandadverselyaffectperformance.

propertyWrapperApplythisattributetoaclass,structure,orenumerationdeclarationtousethattypeasapropertywrapper.Whenyouapplythisattributetoatype,youcreateacustomattributewiththesamenameasthetype.Applythatnewattributetoapropertyofaclass,structure,orenumerationtowrapaccesstothepropertythroughaninstanceofthewrappertype.Localandglobalvariablescan’tusepropertywrappers.

ThewrappermustdefineawrappedValueinstanceproperty.Thewrappedvalueofthepropertyisthevaluethatthegetterandsetterforthispropertyexpose.Inmostcases,wrappedValueisacomputedvalue,butitcanbeastoredvalueinstead.Thewrapperisresponsiblefordefiningandmanaginganyunderlyingstorageneededbyitswrappedvalue.Thecompilersynthesizesstoragefortheinstanceofthewrappertypebyprefixingthenameofthewrappedpropertywithanunderscore(_)—forexample,thewrapperforsomePropertyisstoredas_someProperty.Thesynthesizedstorageforthewrapperhasanaccesscontrollevelofprivate.

ApropertythathasapropertywrappercanincludewillSetanddidSetblocks,butitcan’toverridethecompiler-synthesizedgetorsetblocks.

Swiftprovidestwoformsofsyntacticsugarforinitializationofapropertywrapper.Youcanuseassignmentsyntaxinthedefinitionofawrappedvaluetopasstheexpressionontheright-handsideoftheassignmentastheargumenttothewrappedValueparameterofthepropertywrapper’sinitializer.Youcanalsoprovideargumentstotheattributewhenyouapplyittoaproperty,andthoseargumentsarepassedtothepropertywrapper’sinitializer.Forexample,inthecodebelow,SomeStructcallseachoftheinitializersthatSomeWrapperdefines.

1 @propertyWrapper

2 structSomeWrapper{

3 varwrappedValue:Int

4 varsomeValue:Double

5 init(){

6 self.wrappedValue=100

7 self.someValue=12.3

8 }

9 init(wrappedValue:Int){

10 self.wrappedValue=wrappedValue

11 self.someValue=45.6

12 }

13 init(wrappedValuevalue:Int,custom:Double){

14 self.wrappedValue=value

15 self.someValue=custom

16 }

17 }

18

19 structSomeStruct{

20 //Usesinit()

21 @SomeWrappervara:Int

22

23 //Usesinit(wrappedValue:)

24 @SomeWrappervarb=10

25

26 //Bothuseinit(wrappedValue:custom:)

27 @SomeWrapper(custom:98.7)varc=30

28 @SomeWrapper(wrappedValue:30,custom:98.7)vard

29 }

Theprojectedvalueforawrappedpropertyisasecondvaluethatapropertywrappercanusetoexposeadditionalfunctionality.Theauthorofapropertywrappertypeisresponsiblefordeterminingthemeaningofitsprojectedvalueanddefiningtheinterfacethattheprojectedvalueexposes.Toprojectavaluefromapropertywrapper,defineaprojectedValueinstancepropertyonthewrappertype.Thecompilersynthesizesanidentifierfortheprojectedvaluebyprefixingthenameofthewrappedpropertywithadollarsign($)—forexample,theprojectedvalueforsomePropertyis$someProperty.Theprojectedvaluehasthesameaccesscontrollevelastheoriginalwrappedproperty.

1 @propertyWrapper

2 structWrapperWithProjection{

3 varwrappedValue:Int

4 varprojectedValue:SomeProjection{

5 returnSomeProjection(wrapper:self)

6 }

7 }

8 structSomeProjection{

9 varwrapper:WrapperWithProjection

10 }

11

12 structSomeStruct{

13 @WrapperWithProjectionvarx=123

14 }

15 lets=SomeStruct()

16 s.x//Intvalue

17 s.$x//SomeProjectionvalue

18 s.$x.wrapper//WrapperWithProjectionvalue

requires_stored_property_initsApplythisattributetoaclassdeclarationtorequireallstoredpropertieswithin

theclasstoprovidedefaultvaluesaspartoftheirdefinitions.ThisattributeisinferredforanyclassthatinheritsfromNSManagedObject.

testableApplythisattributetoanimportdeclarationtoimportthatmodulewithchangestoitsaccesscontrolthatsimplifytestingthemodule’scode.Entitiesintheimportedmodulethataremarkedwiththeinternalaccess-levelmodifierareimportedasiftheyweredeclaredwiththepublicaccess-levelmodifier.Classesandclassmembersthataremarkedwiththeinternalorpublicaccess-levelmodifierareimportedasiftheyweredeclaredwiththeopenaccess-levelmodifier.Theimportedmodulemustbecompiledwithtestingenabled.

UIApplicationMainApplythisattributetoaclasstoindicatethatit’stheapplicationdelegate.UsingthisattributeisequivalenttocallingtheUIApplicationMainfunctionandpassingthisclass’snameasthenameofthedelegateclass.

Ifyoudon’tusethisattribute,supplyamain.swiftfilewithcodeatthetoplevelthatcallstheUIApplicationMain(_:_:_:_:)function.Forexample,ifyourappusesacustomsubclassofUIApplicationasitsprincipalclass,calltheUIApplicationMain(_:_:_:_:)functioninsteadofusingthisattribute.

usableFromInlineApplythisattributetoafunction,method,computedproperty,subscript,initializer,ordeinitializerdeclarationtoallowthatsymboltobeusedininlinablecodethat’sdefinedinthesamemoduleasthedeclaration.Thedeclarationmusthavetheinternalaccesslevelmodifier.AstructureorclassmarkedusableFromInlinecanuseonlytypesthatarepublicorusableFromInlineforitsproperties.AnenumerationmarkedusableFromInlinecanuseonlytypesthatarepublicorusableFromInlinefortherawvaluesandassociatedvaluesofitscases.

Likethepublicaccesslevelmodifier,thisattributeexposesthedeclarationas

partofthemodule’spublicinterface.Unlikepublic,thecompilerdoesn’tallowdeclarationsmarkedwithusableFromInlinetobereferencedbynameincodeoutsidethemodule,eventhoughthedeclaration’ssymbolisexported.However,codeoutsidethemodulemightstillbeabletointeractwiththedeclaration’ssymbolbyusingruntimebehavior.

Declarationsmarkedwiththeinlinableattributeareimplicitlyusablefrominlinablecode.AlthougheitherinlinableorusableFromInlinecanbeappliedtointernaldeclarations,applyingbothattributesisanerror.

warn_unqualified_accessApplythisattributetoatop-levelfunction,instancemethod,orclassorstaticmethodtotriggerwarningswhenthatfunctionormethodisusedwithoutaprecedingqualifier,suchasamodulename,typename,orinstancevariableorconstant.Usethisattributetohelpdiscourageambiguitybetweenfunctionswiththesamenamethatareaccessiblefromthesamescope.

Forexample,theSwiftstandardlibraryincludesbothatop-levelmin(_:_:)functionandamin()methodforsequenceswithcomparableelements.Thesequencemethodisdeclaredwiththewarn_unqualified_accessattributetohelpreduceconfusionwhenattemptingtouseoneortheotherfromwithinaSequenceextension.

DeclarationAttributesUsedbyInterfaceBuilderInterfaceBuilderattributesaredeclarationattributesusedbyInterfaceBuildertosynchronizewithXcode.SwiftprovidesthefollowingInterfaceBuilderattributes:IBAction,IBSegueAction,IBOutlet,IBDesignable,andIBInspectable.TheseattributesareconceptuallythesameastheirObjective-Ccounterparts.

YouapplytheIBOutletandIBInspectableattributestopropertydeclarationsofaclass.YouapplytheIBActionandIBSegueActionattributetomethoddeclarationsofaclassandtheIBDesignableattributetoclassdeclarations.

ApplyingtheIBAction,IBSegueAction,IBOutlet,IBDesignable,or

IBInspectableattributealsoimpliestheobjcattribute.

TypeAttributes

Youcanapplytypeattributestotypesonly.

autoclosureApplythisattributetodelaytheevaluationofanexpressionbyautomaticallywrappingthatexpressioninaclosurewithnoarguments.Youapplyittoaparameter’stypeinamethodorfunctiondeclaration,foraparameterwhosetypeisafunctiontypethattakesnoargumentsandthatreturnsavalueofthetypeoftheexpression.Foranexampleofhowtousetheautoclosureattribute,seeAutoclosuresandFunctionType.

conventionApplythisattributetothetypeofafunctiontoindicateitscallingconventions.

Theconventionattributealwaysappearswithoneofthefollowingarguments:

TheswiftargumentindicatesaSwiftfunctionreference.ThisisthestandardcallingconventionforfunctionvaluesinSwift.

TheblockargumentindicatesanObjective-Ccompatibleblockreference.Thefunctionvalueisrepresentedasareferencetotheblockobject,whichisanid-compatibleObjective-Cobjectthatembedsitsinvocationfunctionwithintheobject.TheinvocationfunctionusestheCcallingconvention.

ThecargumentindicatesaCfunctionreference.ThefunctionvaluecarriesnocontextandusestheCcallingconvention.

Withafewexceptions,afunctionofanycallingconventioncanbeusedwhenafunctionanyothercallingconventionisneeded.Anongenericglobalfunction,a

localfunctionthatdoesn’tcaptureanylocalvariablesoraclosurethatdoesn’tcaptureanylocalvariablescanbeconvertedtotheCcallingconvention.OtherSwiftfunctionscan’tbeconvertedtotheCcallingconvention.AfunctionwiththeObjective-Cblockcallingconventioncan’tbeconvertedtotheCcallingconvention.

escapingApplythisattributetoaparameter’stypeinamethodorfunctiondeclarationtoindicatethattheparameter’svaluecanbestoredforlaterexecution.Thismeansthatthevalueisallowedtooutlivethelifetimeofthecall.Functiontypeparameterswiththeescapingtypeattributerequireexplicituseofself.forpropertiesormethods.Foranexampleofhowtousetheescapingattribute,seeEscapingClosures.

SwitchCaseAttributes

Youcanapplyswitchcaseattributestoswitchcasesonly.

unknownApplythisattributetoaswitchcasetoindicatethatitisn’texpectedtobematchedbyanycaseoftheenumerationthat’sknownatthetimethecodeiscompiled.Foranexampleofhowtousetheunknownattribute,seeSwitchingOverFutureEnumerationCases.

GRAMMAR OF AN ATTR IBUTE

attribute → @ attribute-name attribute-argument-clause optattribute-name → identifierattribute-argument-clause → ( balanced-tokens opt )attributes → attribute attributes optbalanced-tokens → balanced-token balanced-tokens optbalanced-token → ( balanced-tokens opt )balanced-token → [ balanced-tokens opt ]

balanced-token → { balanced-tokens opt }balanced-token → Anyidentifier,keyword,literal,oroperatorbalanced-token → Anypunctuationexcept ( , ) , [ , ] , { ,or }

Patterns

Apatternrepresentsthestructureofasinglevalueoracompositevalue.Forexample,thestructureofatuple(1,2)isacomma-separatedlistoftwoelements.Becausepatternsrepresentthestructureofavalueratherthananyoneparticularvalue,youcanmatchthemwithavarietyofvalues.Forinstance,thepattern(x,y)matchesthetuple(1,2)andanyothertwo-elementtuple.Inadditiontomatchingapatternwithavalue,youcanextractpartorallofacompositevalueandbindeachparttoaconstantorvariablename.

InSwift,therearetwobasickindsofpatterns:thosethatsuccessfullymatchanykindofvalue,andthosethatmayfailtomatchaspecifiedvalueatruntime.

Thefirstkindofpatternisusedfordestructuringvaluesinsimplevariable,constant,andoptionalbindings.Theseincludewildcardpatterns,identifierpatterns,andanyvaluebindingortuplepatternscontainingthem.Youcanspecifyatypeannotationforthesepatternstoconstrainthemtomatchonlyvaluesofacertaintype.

Thesecondkindofpatternisusedforfullpatternmatching,wherethevaluesyou’retryingtomatchagainstmaynotbethereatruntime.Theseincludeenumerationcasepatterns,optionalpatterns,expressionpatterns,andtype-castingpatterns.Youusethesepatternsinacaselabelofaswitchstatement,acatchclauseofadostatement,orinthecaseconditionofanif,while,guard,orfor-instatement.

GRAMMAR OF A PATTERN

pattern → wildcard-pattern type-annotation optpattern → identifier-pattern type-annotation optpattern → value-binding-patternpattern → tuple-pattern type-annotation optpattern → enum-case-patternpattern → optional-patternpattern → type-casting-patternpattern → expression-pattern

WildcardPattern

Awildcardpatternmatchesandignoresanyvalueandconsistsofanunderscore(_).Useawildcardpatternwhenyoudon’tcareaboutthevaluesbeingmatchedagainst.Forexample,thefollowingcodeiteratesthroughtheclosedrange1...3,ignoringthecurrentvalueoftherangeoneachiterationoftheloop:

1 for_in1...3{

2 //Dosomethingthreetimes.

3 }

GRAMMAR OF AW I LDCARD PATTERN

wildcard-pattern → _

IdentifierPattern

Anidentifierpatternmatchesanyvalueandbindsthematchedvaluetoavariableorconstantname.Forexample,inthefollowingconstantdeclaration,someValueisanidentifierpatternthatmatchesthevalue42oftypeInt:

letsomeValue=42

Whenthematchsucceeds,thevalue42isbound(assigned)totheconstantnamesomeValue.

Whenthepatternontheleft-handsideofavariableorconstantdeclarationisanidentifierpattern,theidentifierpatternisimplicitlyasubpatternofavalue-bindingpattern.

GRAMMAR OF AN I DENT I F I ER PATTERN

identifier-pattern → identifier

Value-BindingPattern

Avalue-bindingpatternbindsmatchedvaluestovariableorconstantnames.Value-bindingpatternsthatbindamatchedvaluetothenameofaconstantbeginwiththeletkeyword;thosethatbindtothenameofvariablebeginwiththevarkeyword.

Identifierspatternswithinavalue-bindingpatternbindnewnamedvariablesorconstantstotheirmatchingvalues.Forexample,youcandecomposetheelementsofatupleandbindthevalueofeachelementtoacorrespondingidentifierpattern.

1 letpoint=(3,2)

2 switchpoint{

3 //Bindxandytotheelementsofpoint.

4 caselet(x,y):

5 print("Thepointisat(\(x),\(y)).")

6 }

7 //Prints"Thepointisat(3,2)."

Intheexampleabove,letdistributestoeachidentifierpatterninthetuplepattern(x,y).Becauseofthisbehavior,theswitchcasescaselet(x,y):andcase(letx,lety):matchthesamevalues.

GRAMMAR OF A VALUE -B IND ING PATTERN

value-binding-pattern → var pattern | let pattern

TuplePattern

Atuplepatternisacomma-separatedlistofzeroormorepatterns,enclosedinparentheses.Tuplepatternsmatchvaluesofcorrespondingtupletypes.

Youcanconstrainatuplepatterntomatchcertainkindsoftupletypesbyusing

typeannotations.Forexample,thetuplepattern(x,y):(Int,Int)intheconstantdeclarationlet(x,y):(Int,Int)=(1,2)matchesonlytupletypesinwhichbothelementsareoftypeInt.

Whenatuplepatternisusedasthepatterninafor-instatementorinavariableorconstantdeclaration,itcancontainonlywildcardpatterns,identifierpatterns,optionalpatterns,orothertuplepatternsthatcontainthose.Forexample,thefollowingcodeisn’tvalidbecausetheelement0inthetuplepattern(x,0)isanexpressionpattern:

1 letpoints=[(0,0),(1,0),(1,1),(2,0),(2,1)]

2 //Thiscodeisn'tvalid.

3 for(x,0)inpoints{

4 /*...*/

5 }

Theparenthesesaroundatuplepatternthatcontainsasingleelementhavenoeffect.Thepatternmatchesvaluesofthatsingleelement’stype.Forexample,thefollowingareequivalent:

1 leta=2//a:Int=2

2 let(a)=2//a:Int=2

3 let(a):Int=2//a:Int=2

GRAMMAR OF ATUPLE PATTERN

tuple-pattern → ( tuple-pattern-element-list opt )tuple-pattern-element-list → tuple-pattern-element | tuple-pattern-element , tuple-

pattern-element-listtuple-pattern-element → pattern | identifier : pattern

EnumerationCasePattern

Anenumerationcasepatternmatchesacaseofanexistingenumerationtype.

Enumerationcasepatternsappearinswitchstatementcaselabelsandinthecaseconditionsofif,while,guard,andfor-instatements.

Iftheenumerationcaseyou’retryingtomatchhasanyassociatedvalues,thecorrespondingenumerationcasepatternmustspecifyatuplepatternthatcontainsoneelementforeachassociatedvalue.Foranexamplethatusesaswitchstatementtomatchenumerationcasescontainingassociatedvalues,seeAssociatedValues.

Anenumerationcasepatternalsomatchesvaluesofthatcasewrappedinanoptional.Thissimplifiedsyntaxletsyouomitanoptionalpattern.Notethat,becauseOptionalisimplementedasanenumeration,.noneand.somecanappearinthesameswitchasthecasesoftheenumerationtype.

1 enumSomeEnum{caseleft,right}

2 letx:SomeEnum?=.left

3 switchx{

4 case.left:

5 print("Turnleft")

6 case.right:

7 print("Turnright")

8 casenil:

9 print("Keepgoingstraight")

10 }

11 //Prints"Turnleft"

GRAMMAR OF AN ENUMERAT ION CASE PATTERN

enum-case-pattern → type-identifier opt . enum-case-name tuple-pattern opt

OptionalPattern

Anoptionalpatternmatchesvalueswrappedinasome(Wrapped)caseofan

Optional<Wrapped>enumeration.Optionalpatternsconsistofanidentifierpatternfollowedimmediatelybyaquestionmarkandappearinthesameplacesasenumerationcasepatterns.

BecauseoptionalpatternsaresyntacticsugarforOptionalenumerationcasepatterns,thefollowingareequivalent:

1 letsomeOptional:Int?=42

2 //Matchusinganenumerationcasepattern.

3 ifcase.some(letx)=someOptional{

4 print(x)

5 }

6

7 //Matchusinganoptionalpattern.

8 ifcaseletx?=someOptional{

9 print(x)

10 }

Theoptionalpatternprovidesaconvenientwaytoiterateoveranarrayofoptionalvaluesinafor-instatement,executingthebodyofthelooponlyfornon-nilelements.

1 letarrayOfOptionalInts:[Int?]=[nil,2,3,nil,5]

2 //Matchonlynon-nilvalues.

3 forcaseletnumber?inarrayOfOptionalInts{

4 print("Founda\(number)")

5 }

6 //Founda2

7 //Founda3

8 //Founda5

GRAMMAR OF AN OPT IONALPATTERN

optional-pattern → identifier-pattern ?

Type-CastingPatterns

Therearetwotype-castingpatterns,theispatternandtheaspattern.Theispatternappearsonlyinswitchstatementcaselabels.Theisandaspatternshavethefollowingform:

is type

pattern as type

Theispatternmatchesavalueifthetypeofthatvalueatruntimeisthesameasthetypespecifiedintheright-handsideoftheispattern—orasubclassofthattype.Theispatternbehavesliketheisoperatorinthattheybothperformatypecastbutdiscardthereturnedtype.

Theaspatternmatchesavalueifthetypeofthatvalueatruntimeisthesameasthetypespecifiedintheright-handsideoftheaspattern—orasubclassofthattype.Ifthematchsucceeds,thetypeofthematchedvalueiscasttothepatternspecifiedintheright-handsideoftheaspattern.

Foranexamplethatusesaswitchstatementtomatchvalueswithisandaspatterns,seeTypeCastingforAnyandAnyObject.

GRAMMAR OF ATYPE CAST ING PATTERN

type-casting-pattern → is-pattern | as-patternis-pattern → is typeas-pattern → pattern as type

ExpressionPattern

Anexpressionpatternrepresentsthevalueofanexpression.Expressionpatternsappearonlyinswitchstatementcaselabels.

TheexpressionrepresentedbytheexpressionpatterniscomparedwiththevalueofaninputexpressionusingtheSwiftstandardlibrary~=operator.Thematchessucceedsifthe~=operatorreturnstrue.Bydefault,the~=operatorcomparestwovaluesofthesametypeusingthe==operator.Itcanalsomatchavaluewitharangeofvalues,bycheckingwhetherthevalueiscontainedwithintherange,asthefollowingexampleshows.

1 letpoint=(1,2)

2 switchpoint{

3 case(0,0):

4 print("(0,0)isattheorigin.")

5 case(-2...2,-2...2):

6 print("(\(point.0),\(point.1))isneartheorigin.")

7 default:

8 print("Thepointisat(\(point.0),\(point.1)).")

9 }

10 //Prints"(1,2)isneartheorigin."

Youcanoverloadthe~=operatortoprovidecustomexpressionmatchingbehavior.Forexample,youcanrewritetheaboveexampletocomparethepointexpressionwithastringrepresentationsofpoints.

1 //Overloadthe~=operatortomatchastringwithaninteger.

2 func~=(pattern:String,value:Int)->Bool{

3 returnpattern=="\(value)"

4 }

5 switchpoint{

6 case("0","0"):

7 print("(0,0)isattheorigin.")

8 default:

9 print("Thepointisat(\(point.0),\(point.1)).")

10 }

11 //Prints"Thepointisat(1,2)."

GRAMMAR OF AN EXPRESS ION PATTERN

expression-pattern → expression

GenericParametersandArguments

Thischapterdescribesparametersandargumentsforgenerictypes,functions,andinitializers.Whenyoudeclareagenerictype,function,subscript,orinitializer,youspecifythetypeparametersthatthegenerictype,function,orinitializercanworkwith.Thesetypeparametersactasplaceholdersthatarereplacedbyactualconcretetypeargumentswhenaninstanceofagenerictypeiscreatedoragenericfunctionorinitializeriscalled.

ForanoverviewofgenericsinSwift,seeGenerics.

GenericParameterClause

Agenericparameterclausespecifiesthetypeparametersofagenerictypeorfunction,alongwithanyassociatedconstraintsandrequirementsonthoseparameters.Agenericparameterclauseisenclosedinanglebrackets(<>)andhasthefollowingform:

< genericparameterlist >

Thegenericparameterlistisacomma-separatedlistofgenericparameters,eachofwhichhasthefollowingform:

typeparameter : constraint

Agenericparameterconsistsofatypeparameterfollowedbyanoptionalconstraint.Atypeparameterissimplythenameofaplaceholdertype(forexample,T,U,V,Key,Value,andsoon).Youhaveaccesstothetypeparameters(andanyoftheirassociatedtypes)intherestofthetype,function,orinitializerdeclaration,includinginthesignatureofthefunctionorinitializer.

Theconstraintspecifiesthatatypeparameterinheritsfromaspecificclassorconformstoaprotocolorprotocolcomposition.Forexample,inthegeneric

functionbelow,thegenericparameterT:ComparableindicatesthatanytypeargumentsubstitutedforthetypeparameterTmustconformtotheComparableprotocol.

1 funcsimpleMax<T:Comparable>(_x:T,_y:T)->T{

2 ifx<y{

3 returny

4 }

5 returnx

6 }

BecauseIntandDouble,forexample,bothconformtotheComparableprotocol,thisfunctionacceptsargumentsofeithertype.Incontrastwithgenerictypes,youdon’tspecifyagenericargumentclausewhenyouuseagenericfunctionorinitializer.Thetypeargumentsareinsteadinferredfromthetypeoftheargumentspassedtothefunctionorinitializer.

1 simpleMax(17,42)//TisinferredtobeInt

2 simpleMax(3.14159,2.71828)//TisinferredtobeDouble

GenericWhereClausesYoucanspecifyadditionalrequirementsontypeparametersandtheirassociatedtypesbyincludingagenericwhereclauserightbeforetheopeningcurlybraceofatypeorfunction’sbody.Agenericwhereclauseconsistsofthewherekeyword,followedbyacomma-separatedlistofoneormorerequirements.

where requirements

Therequirementsinagenericwhereclausespecifythatatypeparameterinheritsfromaclassorconformstoaprotocolorprotocolcomposition.Althoughthegenericwhereclauseprovidessyntacticsugarforexpressingsimpleconstraintsontypeparameters(forexample,<T:Comparable>isequivalentto<T>whereT:Comparableandsoon),youcanuseittoprovidemorecomplexconstraintson

typeparametersandtheirassociatedtypes.Forexample,youcanconstraintheassociatedtypesoftypeparameterstoconformtoprotocols.Forexample,<S:Sequence>whereS.Iterator.Element:EquatablespecifiesthatSconformstotheSequenceprotocolandthattheassociatedtypeS.Iterator.ElementconformstotheEquatableprotocol.Thisconstraintensuresthateachelementofthesequenceisequatable.

Youcanalsospecifytherequirementthattwotypesbeidentical,usingthe==operator.Forexample,<S1:Sequence,S2:Sequence>whereS1.Iterator.Element==S2.Iterator.ElementexpressestheconstraintsthatS1andS2conformtotheSequenceprotocolandthattheelementsofbothsequencesmustbeofthesametype.

Anytypeargumentsubstitutedforatypeparametermustmeetalltheconstraintsandrequirementsplacedonthetypeparameter.

Youcanoverloadagenericfunctionorinitializerbyprovidingdifferentconstraints,requirements,orbothonthetypeparameters.Whenyoucallanoverloadedgenericfunctionorinitializer,thecompilerusestheseconstraintstoresolvewhichoverloadedfunctionorinitializertoinvoke.

Formoreinformationaboutgenericwhereclausesandtoseeanexampleofoneinagenericfunctiondeclaration,seeGenericWhereClauses.

GRAMMAR OF AGENER IC PARAMETER CLAUSE

generic-parameter-clause → < generic-parameter-list >generic-parameter-list → generic-parameter | generic-parameter , generic-parameter-

listgeneric-parameter → type-namegeneric-parameter → type-name : type-identifiergeneric-parameter → type-name : protocol-composition-typegeneric-where-clause → where requirement-listrequirement-list → requirement | requirement , requirement-listrequirement → conformance-requirement | same-type-requirementconformance-requirement → type-identifier : type-identifierconformance-requirement → type-identifier : protocol-composition-typesame-type-requirement → type-identifier == type

GenericArgumentClause

Agenericargumentclausespecifiesthetypeargumentsofagenerictype.Agenericargumentclauseisenclosedinanglebrackets(<>)andhasthefollowingform:

< genericargumentlist >

Thegenericargumentlistisacomma-separatedlistoftypearguments.Atypeargumentisthenameofanactualconcretetypethatreplacesacorrespondingtypeparameterinthegenericparameterclauseofagenerictype.Theresultisaspecializedversionofthatgenerictype.TheexamplebelowshowsasimplifiedversionoftheSwiftstandardlibrary’sgenericdictionarytype.

1 structDictionary<Key:Hashable,Value>:Collection,

ExpressibleByDictionaryLiteral{

2 /*...*/

3 }

ThespecializedversionofthegenericDictionarytype,Dictionary<String,Int>isformedbyreplacingthegenericparametersKey:HashableandValuewiththeconcretetypeargumentsStringandInt.Eachtypeargumentmustsatisfyalltheconstraintsofthegenericparameteritreplaces,includinganyadditionalrequirementsspecifiedinagenericwhereclause.Intheexampleabove,theKeytypeparameterisconstrainedtoconformtotheHashableprotocolandthereforeStringmustalsoconformtotheHashableprotocol.

Youcanalsoreplaceatypeparameterwithatypeargumentthatisitselfaspecializedversionofagenerictype(provideditsatisfiestheappropriateconstraintsandrequirements).Forexample,youcanreplacethetypeparameterElementinArray<Element>withaspecializedversionofanarray,Array<Int>,toformanarraywhoseelementsarethemselvesarraysofintegers.

letarrayOfArrays:Array<Array<Int>>=[[1,2,3],[4,5,6],

[7,8,9]]

AsmentionedinGenericParameterClause,youdon’tuseagenericargumentclausetospecifythetypeargumentsofagenericfunctionorinitializer.

GRAMMAR OF AGENER IC ARGUMENT CLAUSE

generic-argument-clause → < generic-argument-list >generic-argument-list → generic-argument | generic-argument , generic-argument-listgeneric-argument → type

SummaryoftheGrammar

LexicalStructure

GRAMMAR OF WH I TESPACE

whitespace → whitespace-item whitespace optwhitespace-item → line-breakwhitespace-item → commentwhitespace-item → multiline-commentwhitespace-item → U+0000,U+0009,U+000B,U+000C,orU+0020line-break → U+000Aline-break → U+000Dline-break → U+000DfollowedbyU+000Acomment → // comment-text line-breakmultiline-comment → /* multiline-comment-text */comment-text → comment-text-item comment-text optcomment-text-item → AnyUnicodescalarvalueexceptU+000AorU+000Dmultiline-comment-text → multiline-comment-text-item multiline-comment-text optmultiline-comment-text-item → multiline-commentmultiline-comment-text-item → comment-text-itemmultiline-comment-text-item → AnyUnicodescalarvalueexcept /* or */

GRAMMAR OF AN I DENT I F I ER

identifier → identifier-head identifier-characters optidentifier → ` identifier-head identifier-characters opt `identifier → implicit-parameter-nameidentifier → property-wrapper-projectionidentifier-list → identifier | identifier , identifier-listidentifier-head → Upper-orlowercaseletterAthroughZidentifier-head → _identifier-head → U+00A8,U+00AA,U+00AD,U+00AF,U+00B2–U+00B5,orU+00B7–

U+00BAidentifier-head → U+00BC–U+00BE,U+00C0–U+00D6,U+00D8–U+00F6,orU+00F8–

U+00FFidentifier-head → U+0100–U+02FF,U+0370–U+167F,U+1681–U+180D,orU+180F–

U+1DBFidentifier-head → U+1E00–U+1FFFidentifier-head → U+200B–U+200D,U+202A–U+202E,U+203F–U+2040,U+2054,or

U+2060–U+206Fidentifier-head → U+2070–U+20CF,U+2100–U+218F,U+2460–U+24FF,orU+2776–

U+2793identifier-head → U+2C00–U+2DFForU+2E80–U+2FFFidentifier-head → U+3004–U+3007,U+3021–U+302F,U+3031–U+303F,orU+3040–

U+D7FFidentifier-head → U+F900–U+FD3D,U+FD40–U+FDCF,U+FDF0–U+FE1F,orU+FE30–

U+FE44identifier-head → U+FE47–U+FFFDidentifier-head → U+10000–U+1FFFD,U+20000–U+2FFFD,U+30000–U+3FFFD,or

U+40000–U+4FFFDidentifier-head → U+50000–U+5FFFD,U+60000–U+6FFFD,U+70000–U+7FFFD,or

U+80000–U+8FFFDidentifier-head → U+90000–U+9FFFD,U+A0000–U+AFFFD,U+B0000–U+BFFFD,or

U+C0000–U+CFFFDidentifier-head → U+D0000–U+DFFFDorU+E0000–U+EFFFDidentifier-character → Digit0through9identifier-character → U+0300–U+036F,U+1DC0–U+1DFF,U+20D0–U+20FF,or

U+FE20–U+FE2Fidentifier-character → identifier-headidentifier-characters → identifier-character identifier-characters optimplicit-parameter-name → $ decimal-digitsproperty-wrapper-projection → $ identifier-characters

GRAMMAR OF A L I TERAL

literal → numeric-literal | string-literal | boolean-literal | nil-literalnumeric-literal → -opt integer-literal | -opt floating-point-literalboolean-literal → true | falsenil-literal → nil

GRAMMAR OF AN I NTEGER L I TERAL

integer-literal → binary-literalinteger-literal → octal-literalinteger-literal → decimal-literalinteger-literal → hexadecimal-literalbinary-literal → 0b binary-digit binary-literal-characters optbinary-digit → Digit0or1binary-literal-character → binary-digit | _binary-literal-characters → binary-literal-character binary-literal-characters optoctal-literal → 0o octal-digit octal-literal-characters optoctal-digit → Digit0through7octal-literal-character → octal-digit | _octal-literal-characters → octal-literal-character octal-literal-characters optdecimal-literal → decimal-digit decimal-literal-characters optdecimal-digit → Digit0through9decimal-digits → decimal-digit decimal-digits optdecimal-literal-character → decimal-digit | _

decimal-literal-characters → decimal-literal-character decimal-literal-characters opthexadecimal-literal → 0x hexadecimal-digit hexadecimal-literal-characters opthexadecimal-digit → Digit0through9,athroughf,orAthroughFhexadecimal-literal-character → hexadecimal-digit | _hexadecimal-literal-characters → hexadecimal-literal-character hexadecimal-literal-

characters opt

GRAMMAR OF A F LOAT ING -PO INT L I TERAL

floating-point-literal → decimal-literal decimal-fraction opt decimal-exponent optfloating-point-literal → hexadecimal-literal hexadecimal-fraction opt hexadecimal-

exponentdecimal-fraction → . decimal-literaldecimal-exponent → floating-point-e sign opt decimal-literalhexadecimal-fraction → . hexadecimal-digit hexadecimal-literal-characters opthexadecimal-exponent → floating-point-p sign opt decimal-literalfloating-point-e → e | Efloating-point-p → p | Psign → + | -

GRAMMAR OF A STR ING L I TERAL

string-literal → static-string-literal | interpolated-string-literalstring-literal-opening-delimiter → extended-string-literal-delimiter opt "string-literal-closing-delimiter → " extended-string-literal-delimiter optstatic-string-literal → string-literal-opening-delimiter quoted-text opt string-literal-closing-

delimiterstatic-string-literal → multiline-string-literal-opening-delimiter multiline-quoted-text opt

multiline-string-literal-closing-delimitermultiline-string-literal-opening-delimiter → extended-string-literal-delimiter """multiline-string-literal-closing-delimiter → """ extended-string-literal-delimiterextended-string-literal-delimiter → # extended-string-literal-delimiter optquoted-text → quoted-text-item quoted-text optquoted-text-item → escaped-characterquoted-text-item → AnyUnicodescalarvalueexcept " , \ ,U+000A,orU+000Dmultiline-quoted-text → multiline-quoted-text-item multiline-quoted-text optmultiline-quoted-text-item → escaped-charactermultiline-quoted-text-item → AnyUnicodescalarvalueexcept \multiline-quoted-text-item → escaped-newlineinterpolated-string-literal → string-literal-opening-delimiter interpolated-text opt string-

literal-closing-delimiterinterpolated-string-literal → multiline-string-literal-opening-delimiter interpolated-text opt

multiline-string-literal-closing-delimiterinterpolated-text → interpolated-text-item interpolated-text optinterpolated-text-item → \( expression ) | quoted-text-item

multiline-interpolated-text → multiline-interpolated-text-item multiline-interpolated-text optmultiline-interpolated-text-item → \( expression ) |multiline-quoted-text-itemescape-sequence → \ extended-string-literal-delimiterescaped-character → escape-sequence 0 | escape-sequence \ | escape-sequence t |

escape-sequence n | escape-sequence r | escape-sequence " | escape-sequence'

escaped-character → escape-sequence u { unicode-scalar-digits }unicode-scalar-digits → Betweenoneandeighthexadecimaldigitsescaped-newline → escape-sequence whitespace opt line-break

GRAMMAR OF OPERATORS

operator → operator-head operator-characters optoperator → dot-operator-head dot-operator-charactersoperator-head → / | = | - | + | ! | * | % | < | > | & | | | ^ | ~ | ?operator-head → U+00A1–U+00A7operator-head → U+00A9orU+00ABoperator-head → U+00ACorU+00AEoperator-head → U+00B0–U+00B1operator-head → U+00B6,U+00BB,U+00BF,U+00D7,orU+00F7operator-head → U+2016–U+2017operator-head → U+2020–U+2027operator-head → U+2030–U+203Eoperator-head → U+2041–U+2053operator-head → U+2055–U+205Eoperator-head → U+2190–U+23FFoperator-head → U+2500–U+2775operator-head → U+2794–U+2BFFoperator-head → U+2E00–U+2E7Foperator-head → U+3001–U+3003operator-head → U+3008–U+3020operator-head → U+3030operator-character → operator-headoperator-character → U+0300–U+036Foperator-character → U+1DC0–U+1DFFoperator-character → U+20D0–U+20FFoperator-character → U+FE00–U+FE0Foperator-character → U+FE20–U+FE2Foperator-character → U+E0100–U+E01EFoperator-characters → operator-character operator-characters optdot-operator-head → .dot-operator-character → . | operator-characterdot-operator-characters → dot-operator-character dot-operator-characters optbinary-operator → operatorprefix-operator → operatorpostfix-operator → operator

Types

GRAMMAR OF ATYPE

type → function-typetype → array-typetype → dictionary-typetype → type-identifiertype → tuple-typetype → optional-typetype → implicitly-unwrapped-optional-typetype → protocol-composition-typetype → opaque-typetype → metatype-typetype → self-typetype → Anytype → ( type )

GRAMMAR OF ATYPE ANNOTAT ION

type-annotation → : attributes opt inoutopt type

GRAMMAR OF ATYPE I DENT I F I ER

type-identifier → type-name generic-argument-clause opt | type-name generic-argument-clause opt . type-identifier

type-name → identifier

GRAMMAR OF ATUPLE TYPE

tuple-type → ( ) | ( tuple-type-element , tuple-type-element-list )tuple-type-element-list → tuple-type-element | tuple-type-element , tuple-type-element-

listtuple-type-element → element-name type-annotation | typeelement-name → identifier

GRAMMAR OF A FUNCT ION TYPE

function-type → attributes opt function-type-argument-clause throwsopt -> typefunction-type-argument-clause → ( )function-type-argument-clause → ( function-type-argument-list ...opt )function-type-argument-list → function-type-argument | function-type-argument ,

function-type-argument-listfunction-type-argument → attributes opt inoutopt type | argument-label type-

annotationargument-label → identifier

GRAMMAR OF AN ARRAY TYPE

array-type → [ type ]

GRAMMAR OF AD ICT IONARY TYPE

dictionary-type → [ type : type ]

GRAMMAR OF AN OPT IONALTYPE

optional-type → type ?

GRAMMAR OF AN IMPL IC I T LY UNWRAPPED OPT IONALTYPE

implicitly-unwrapped-optional-type → type !

GRAMMAR OF A PROTOCOLCOMPOS I T ION TYPE

protocol-composition-type → type-identifier & protocol-composition-continuationprotocol-composition-continuation → type-identifier | protocol-composition-type

GRAMMAR OF AN OPAQUE TYPE

opaque-type → some type

GRAMMAR OF AMETATYPE TYPE

metatype-type → type . Type | type . Protocol

GRAMMAR OF A SELF TYPE

self-type → Self

GRAMMAR OF ATYPE I NHER I TANCE CLAUSE

type-inheritance-clause → : type-inheritance-listtype-inheritance-list → type-identifier | type-identifier , type-inheritance-list

Expressions

GRAMMAR OF AN EXPRESS ION

expression → try-operator opt prefix-expression binary-expressions optexpression-list → expression | expression , expression-list

GRAMMAR OF A PREF IX EXPRESS ION

prefix-expression → prefix-operator opt postfix-expressionprefix-expression → in-out-expressionin-out-expression → & identifier

GRAMMAR OF ATRY EXPRESS ION

try-operator → try | try ? | try !

GRAMMAR OF A B INARY EXPRESS ION

binary-expression → binary-operator prefix-expressionbinary-expression → assignment-operator try-operator opt prefix-expressionbinary-expression → conditional-operator try-operator opt prefix-expressionbinary-expression → type-casting-operatorbinary-expressions → binary-expression binary-expressions opt

GRAMMAR OF AN ASS IGNMENT OPERATOR

assignment-operator → =

GRAMMAR OF ACOND I T IONALOPERATOR

conditional-operator → ? expression :

GRAMMAR OF ATYPE -CAST ING OPERATOR

type-casting-operator → is typetype-casting-operator → as typetype-casting-operator → as ? typetype-casting-operator → as ! type

GRAMMAR OF A PR IMARY EXPRESS ION

primary-expression → identifier generic-argument-clause optprimary-expression → literal-expressionprimary-expression → self-expressionprimary-expression → superclass-expressionprimary-expression → closure-expressionprimary-expression → parenthesized-expressionprimary-expression → tuple-expressionprimary-expression → implicit-member-expressionprimary-expression → wildcard-expressionprimary-expression → key-path-expressionprimary-expression → selector-expressionprimary-expression → key-path-string-expression

GRAMMAR OF A L I TERALEXPRESS ION

literal-expression → literalliteral-expression → array-literal | dictionary-literal | playground-literalliteral-expression → #file | #line | #column | #function | #dsohandlearray-literal → [ array-literal-items opt ]array-literal-items → array-literal-item ,opt | array-literal-item , array-literal-itemsarray-literal-item → expressiondictionary-literal → [ dictionary-literal-items ] | [ : ]dictionary-literal-items → dictionary-literal-item ,opt | dictionary-literal-item , dictionary-

literal-itemsdictionary-literal-item → expression : expressionplayground-literal → #colorLiteral ( red : expression , green : expression

, blue : expression , alpha : expression )playground-literal → #fileLiteral ( resourceName : expression )playground-literal → #imageLiteral ( resourceName : expression )

GRAMMAR OF A SELF EXPRESS ION

self-expression → self | self-method-expression | self-subscript-expression | self-initializer-expression

self-method-expression → self . identifierself-subscript-expression → self [ function-call-argument-list ]self-initializer-expression → self . init

GRAMMAR OF A SUPERCLASS EXPRESS ION

superclass-expression → superclass-method-expression | superclass-subscript-expression | superclass-initializer-expression

superclass-method-expression → super . identifiersuperclass-subscript-expression → super [ function-call-argument-list ]superclass-initializer-expression → super . init

GRAMMAR OF ACLOSURE EXPRESS ION

closure-expression → { closure-signature opt statements opt }closure-signature → capture-list opt closure-parameter-clause throwsopt function-

result opt inclosure-signature → capture-list inclosure-parameter-clause → ( ) | ( closure-parameter-list ) | identifier-listclosure-parameter-list → closure-parameter | closure-parameter , closure-parameter-

listclosure-parameter → closure-parameter-name type-annotation optclosure-parameter → closure-parameter-name type-annotation ...closure-parameter-name → identifiercapture-list → [ capture-list-items ]capture-list-items → capture-list-item | capture-list-item , capture-list-items

capture-list-item → capture-specifier opt expressioncapture-specifier → weak | unowned | unowned(safe) | unowned(unsafe)

GRAMMAR OF A IMPL IC I T MEMBER EXPRESS ION

implicit-member-expression → . identifier

GRAMMAR OF A PARENTHES I ZED EXPRESS ION

parenthesized-expression → ( expression )

GRAMMAR OF ATUPLE EXPRESS ION

tuple-expression → ( ) | ( tuple-element , tuple-element-list )tuple-element-list → tuple-element | tuple-element , tuple-element-listtuple-element → expression | identifier : expression

GRAMMAR OF AW I LDCARD EXPRESS ION

wildcard-expression → _

GRAMMAR OF A KEY-PATH EXPRESS ION

key-path-expression → \ type opt . key-path-componentskey-path-components → key-path-component | key-path-component . key-path-

componentskey-path-component → identifier key-path-postfixes opt | key-path-postfixeskey-path-postfixes → key-path-postfix key-path-postfixes optkey-path-postfix → ? | ! | self | [ function-call-argument-list ]

GRAMMAR OF A SELECTOR EXPRESS ION

selector-expression → #selector ( expression )selector-expression → #selector ( getter: expression )selector-expression → #selector ( setter: expression )

GRAMMAR OF A KEY-PATH STR ING EXPRESS ION

key-path-string-expression → #keyPath ( expression )

GRAMMAR OF A POSTF IX EXPRESS ION

postfix-expression → primary-expressionpostfix-expression → postfix-expression postfix-operatorpostfix-expression → function-call-expressionpostfix-expression → initializer-expressionpostfix-expression → explicit-member-expression

postfix-expression → postfix-self-expressionpostfix-expression → subscript-expressionpostfix-expression → forced-value-expressionpostfix-expression → optional-chaining-expression

GRAMMAR OF A FUNCT ION CALL EXPRESS ION

function-call-expression → postfix-expression function-call-argument-clausefunction-call-expression → postfix-expression function-call-argument-clause opt trailing-

closurefunction-call-argument-clause → ( ) | ( function-call-argument-list )function-call-argument-list → function-call-argument | function-call-argument , function-

call-argument-listfunction-call-argument → expression | identifier : expressionfunction-call-argument → operator | identifier : operatortrailing-closure → closure-expression

GRAMMAR OF AN I N I T I A L I ZER EXPRESS ION

initializer-expression → postfix-expression . initinitializer-expression → postfix-expression . init ( argument-names )

GRAMMAR OF AN EXPL IC I T MEMBER EXPRESS ION

explicit-member-expression → postfix-expression . decimal-digitsexplicit-member-expression → postfix-expression . identifier generic-argument-clause

optexplicit-member-expression → postfix-expression . identifier ( argument-names )argument-names → argument-name argument-names optargument-name → identifier :

GRAMMAR OF A POSTF IX SELF EXPRESS ION

postfix-self-expression → postfix-expression . self

GRAMMAR OF A SUBSCR IPT EXPRESS ION

subscript-expression → postfix-expression [ function-call-argument-list ]

GRAMMAR OF A FORCED -VALUE EXPRESS ION

forced-value-expression → postfix-expression !

GRAMMAR OF AN OPT IONAL -CHA IN ING EXPRESS ION

optional-chaining-expression → postfix-expression ?

Statements

GRAMMAR OF A STATEMENT

statement → expression ;optstatement → declaration ;optstatement → loop-statement ;optstatement → branch-statement ;optstatement → labeled-statement ;optstatement → control-transfer-statement ;optstatement → defer-statement ;optstatement → do-statement ;optstatement → compiler-control-statementstatements → statement statements opt

GRAMMAR OF A LOOP STATEMENT

loop-statement → for-in-statementloop-statement → while-statementloop-statement → repeat-while-statement

GRAMMAR OF A FOR - IN STATEMENT

for-in-statement → for caseopt pattern in expression where-clause opt code-block

GRAMMAR OF AWH I LE STATEMENT

while-statement → while condition-list code-blockcondition-list → condition | condition , condition-listcondition → expression | availability-condition | case-condition | optional-binding-

conditioncase-condition → case pattern initializeroptional-binding-condition → let pattern initializer | var pattern initializer

GRAMMAR OF AREPEAT-WH I LE STATEMENT

repeat-while-statement → repeat code-block while expression

GRAMMAR OF A BRANCH STATEMENT

branch-statement → if-statementbranch-statement → guard-statementbranch-statement → switch-statement

GRAMMAR OF AN I F STATEMENT

if-statement → if condition-list code-block else-clause optelse-clause → else code-block | else if-statement

GRAMMAR OF AGUARD STATEMENT

guard-statement → guard condition-list else code-block

GRAMMAR OF A SW ITCH STATEMENT

switch-statement → switch expression { switch-cases opt }switch-cases → switch-case switch-cases optswitch-case → case-label statementsswitch-case → default-label statementsswitch-case → conditional-switch-casecase-label → attributes opt case case-item-list :case-item-list → pattern where-clause opt | pattern where-clause opt , case-item-listdefault-label → attributes opt default :where-clause → where where-expressionwhere-expression → expressionconditional-switch-case → switch-if-directive-clause switch-elseif-directive-clauses opt

switch-else-directive-clause opt endif-directiveswitch-if-directive-clause → if-directive compilation-condition switch-cases optswitch-elseif-directive-clauses → elseif-directive-clause switch-elseif-directive-clauses optswitch-elseif-directive-clause → elseif-directive compilation-condition switch-cases optswitch-else-directive-clause → else-directive switch-cases opt

GRAMMAR OF A LABELED STATEMENT

labeled-statement → statement-label loop-statementlabeled-statement → statement-label if-statementlabeled-statement → statement-label switch-statementlabeled-statement → statement-label do-statementstatement-label → label-name :label-name → identifier

GRAMMAR OF ACONTROLTRANSFER STATEMENT

control-transfer-statement → break-statementcontrol-transfer-statement → continue-statementcontrol-transfer-statement → fallthrough-statementcontrol-transfer-statement → return-statementcontrol-transfer-statement → throw-statement

GRAMMAR OF A BREAK STATEMENT

break-statement → break label-name opt

GRAMMAR OF ACONT INUE STATEMENT

continue-statement → continue label-name opt

GRAMMAR OF A FALLTHROUGH STATEMENT

fallthrough-statement → fallthrough

GRAMMAR OF ARETURN STATEMENT

return-statement → return expression opt

GRAMMAR OF ATHROWSTATEMENT

throw-statement → throw expression

GRAMMAR OF ADEFER STATEMENT

defer-statement → defer code-block

GRAMMAR OF ADO STATEMENT

do-statement → do code-block catch-clauses optcatch-clauses → catch-clause catch-clauses optcatch-clause → catch pattern opt where-clause opt code-block

GRAMMAR OF ACOMP I LER CONTROLSTATEMENT

compiler-control-statement → conditional-compilation-blockcompiler-control-statement → line-control-statementcompiler-control-statement → diagnostic-statement

GRAMMAR OF ACOND I T IONALCOMP I LAT ION BLOCK

conditional-compilation-block → if-directive-clause elseif-directive-clauses opt else-directive-clause opt endif-directive

if-directive-clause → if-directive compilation-condition statements optelseif-directive-clauses → elseif-directive-clause elseif-directive-clauses optelseif-directive-clause → elseif-directive compilation-condition statements optelse-directive-clause → else-directive statements optif-directive → #ifelseif-directive → #elseifelse-directive → #elseendif-directive → #endifcompilation-condition → platform-condition

compilation-condition → identifiercompilation-condition → boolean-literalcompilation-condition → ( compilation-condition )compilation-condition → ! compilation-conditioncompilation-condition → compilation-condition && compilation-conditioncompilation-condition → compilation-condition || compilation-conditionplatform-condition → os ( operating-system )platform-condition → arch ( architecture )platform-condition → swift ( >= swift-version ) | swift ( < swift-version )platform-condition → compiler ( >= swift-version ) | compiler ( < swift-version

)

platform-condition → canImport ( module-name )platform-condition → targetEnvironment ( environment )operating-system → macOS | iOS | watchOS | tvOSarchitecture → i386 | x86_64 | arm | arm64swift-version → decimal-digits swift-version-continuation optswift-version-continuation → . decimal-digits swift-version-continuation optmodule-name → identifierenvironment → simulator

GRAMMAR OF A L I NE CONTROLSTATEMENT

line-control-statement → #sourceLocation ( file: file-name , line: line-number )

line-control-statement → #sourceLocation ( )line-number → Adecimalintegergreaterthanzerofile-name → static-string-literal

GRAMMAR OF ACOMP I LE - T IME D I AGNOST IC STATEMENT

diagnostic-statement → #error ( diagnostic-message )diagnostic-statement → #warning ( diagnostic-message )diagnostic-message → static-string-literal

GRAMMAR OF AN AVA I LAB I L I TY COND I T ION

availability-condition → #available ( availability-arguments )availability-arguments → availability-argument | availability-argument , availability-

argumentsavailability-argument → platform-name platform-versionavailability-argument → *platform-name → iOS | iOSApplicationExtensionplatform-name → macOS | macOSApplicationExtensionplatform-name → watchOSplatform-name → tvOSplatform-version → decimal-digitsplatform-version → decimal-digits . decimal-digits

platform-version → decimal-digits . decimal-digits . decimal-digits

Declarations

GRAMMAR OF ADECLARAT ION

declaration → import-declarationdeclaration → constant-declarationdeclaration → variable-declarationdeclaration → typealias-declarationdeclaration → function-declarationdeclaration → enum-declarationdeclaration → struct-declarationdeclaration → class-declarationdeclaration → protocol-declarationdeclaration → initializer-declarationdeclaration → deinitializer-declarationdeclaration → extension-declarationdeclaration → subscript-declarationdeclaration → operator-declarationdeclaration → precedence-group-declarationdeclarations → declaration declarations opt

GRAMMAR OF ATOP - LEVELDECLARAT ION

top-level-declaration → statements opt

GRAMMAR OF ACODE BLOCK

code-block → { statements opt }

GRAMMAR OF AN IMPORT DECLARAT ION

import-declaration → attributes opt import import-kind opt import-pathimport-kind → typealias | struct | class | enum | protocol | let | var |

func

import-path → import-path-identifier | import-path-identifier . import-pathimport-path-identifier → identifier | operator

GRAMMAR OF ACONSTANT DECLARAT ION

constant-declaration → attributes opt declaration-modifiers opt let pattern-initializer-list

pattern-initializer-list → pattern-initializer | pattern-initializer , pattern-initializer-list

pattern-initializer → pattern initializer optinitializer → = expression

GRAMMAR OF A VAR IABLE DECLARAT ION

variable-declaration → variable-declaration-head pattern-initializer-listvariable-declaration → variable-declaration-head variable-name type-annotation code-

blockvariable-declaration → variable-declaration-head variable-name type-annotation getter-

setter-blockvariable-declaration → variable-declaration-head variable-name type-annotation getter-

setter-keyword-blockvariable-declaration → variable-declaration-head variable-name initializer willSet-didSet-

blockvariable-declaration → variable-declaration-head variable-name type-annotation

initializer opt willSet-didSet-blockvariable-declaration-head → attributes opt declaration-modifiers opt varvariable-name → identifiergetter-setter-block → code-blockgetter-setter-block → { getter-clause setter-clause opt }getter-setter-block → { setter-clause getter-clause }getter-clause → attributes opt mutation-modifier opt get code-blocksetter-clause → attributes opt mutation-modifier opt set setter-name opt code-blocksetter-name → ( identifier )getter-setter-keyword-block → { getter-keyword-clause setter-keyword-clause opt }getter-setter-keyword-block → { setter-keyword-clause getter-keyword-clause }getter-keyword-clause → attributes opt mutation-modifier opt getsetter-keyword-clause → attributes opt mutation-modifier opt setwillSet-didSet-block → { willSet-clause didSet-clause opt }willSet-didSet-block → { didSet-clause willSet-clause opt }willSet-clause → attributes opt willSet setter-name opt code-blockdidSet-clause → attributes opt didSet setter-name opt code-block

GRAMMAR OF ATYPE AL I AS DECLARAT ION

typealias-declaration → attributes opt access-level-modifier opt typealias typealias-name generic-parameter-clause opt typealias-assignment

typealias-name → identifiertypealias-assignment → = type

GRAMMAR OF A FUNCT ION DECLARAT ION

function-declaration → function-head function-name generic-parameter-clause optfunction-signature generic-where-clause opt function-body opt

function-head → attributes opt declaration-modifiers opt func

function-name → identifier | operatorfunction-signature → parameter-clause throwsopt function-result optfunction-signature → parameter-clause rethrows function-result optfunction-result → -> attributes opt typefunction-body → code-blockparameter-clause → ( ) | ( parameter-list )parameter-list → parameter | parameter , parameter-listparameter → external-parameter-name opt local-parameter-name type-annotation

default-argument-clause optparameter → external-parameter-name opt local-parameter-name type-annotationparameter → external-parameter-name opt local-parameter-name type-annotation ...external-parameter-name → identifierlocal-parameter-name → identifierdefault-argument-clause → = expression

GRAMMAR OF AN ENUMERAT ION DECLARAT ION

enum-declaration → attributes opt access-level-modifier opt union-style-enumenum-declaration → attributes opt access-level-modifier opt raw-value-style-enumunion-style-enum → indirectopt enum enum-name generic-parameter-clause opt

type-inheritance-clause opt generic-where-clause opt { union-style-enum-members

opt }union-style-enum-members → union-style-enum-member union-style-enum-members optunion-style-enum-member → declaration | union-style-enum-case-clause | compiler-

control-statementunion-style-enum-case-clause → attributes opt indirectopt case union-style-enum-

case-listunion-style-enum-case-list → union-style-enum-case | union-style-enum-case , union-

style-enum-case-listunion-style-enum-case → enum-case-name tuple-type optenum-name → identifierenum-case-name → identifierraw-value-style-enum → enum enum-name generic-parameter-clause opt type-

inheritance-clause generic-where-clause opt { raw-value-style-enum-members }raw-value-style-enum-members → raw-value-style-enum-member raw-value-style-enum-

members optraw-value-style-enum-member → declaration | raw-value-style-enum-case-clause |

compiler-control-statementraw-value-style-enum-case-clause → attributes opt case raw-value-style-enum-case-

listraw-value-style-enum-case-list → raw-value-style-enum-case | raw-value-style-enum-

case , raw-value-style-enum-case-listraw-value-style-enum-case → enum-case-name raw-value-assignment optraw-value-assignment → = raw-value-literalraw-value-literal → numeric-literal | static-string-literal | boolean-literal

GRAMMAR OF A STRUCTURE DECLARAT ION

struct-declaration → attributes opt access-level-modifier opt struct struct-namegeneric-parameter-clause opt type-inheritance-clause opt generic-where-clause optstruct-body

struct-name → identifierstruct-body → { struct-members opt }struct-members → struct-member struct-members optstruct-member → declaration | compiler-control-statement

GRAMMAR OF ACLASS DECLARAT ION

class-declaration → attributes opt access-level-modifier opt finalopt class class-name generic-parameter-clause opt type-inheritance-clause opt generic-where-clause

opt class-bodyclass-declaration → attributes opt final access-level-modifier opt class class-

name generic-parameter-clause opt type-inheritance-clause opt generic-where-clause

opt class-bodyclass-name → identifierclass-body → { class-members opt }class-members → class-member class-members optclass-member → declaration | compiler-control-statement

GRAMMAR OF A PROTOCOLDECLARAT ION

protocol-declaration → attributes opt access-level-modifier opt protocol protocol-name type-inheritance-clause opt generic-where-clause opt protocol-body

protocol-name → identifierprotocol-body → { protocol-members opt }protocol-members → protocol-member protocol-members optprotocol-member → protocol-member-declaration | compiler-control-statementprotocol-member-declaration → protocol-property-declarationprotocol-member-declaration → protocol-method-declarationprotocol-member-declaration → protocol-initializer-declarationprotocol-member-declaration → protocol-subscript-declarationprotocol-member-declaration → protocol-associated-type-declarationprotocol-member-declaration → typealias-declaration

GRAMMAR OF A PROTOCOLPROPERTY DECLARAT ION

protocol-property-declaration → variable-declaration-head variable-name type-annotationgetter-setter-keyword-block

GRAMMAR OF A PROTOCOLMETHOD DECLARAT ION

protocol-method-declaration → function-head function-name generic-parameter-clauseopt function-signature generic-where-clause opt

GRAMMAR OF A PROTOCOL I N I T I A L I ZER DECLARAT ION

protocol-initializer-declaration → initializer-head generic-parameter-clause optparameter-clause throwsopt generic-where-clause opt

protocol-initializer-declaration → initializer-head generic-parameter-clause optparameter-clause rethrows generic-where-clause opt

GRAMMAR OF A PROTOCOLSUBSCR IPT DECLARAT ION

protocol-subscript-declaration → subscript-head subscript-result generic-where-clauseopt getter-setter-keyword-block

GRAMMAR OF A PROTOCOLASSOC IATED TYPE DECLARAT ION

protocol-associated-type-declaration → attributes opt access-level-modifier optassociatedtype typealias-name type-inheritance-clause opt typealias-assignment

opt generic-where-clause opt

GRAMMAR OF AN I N I T I A L I ZER DECLARAT ION

initializer-declaration → initializer-head generic-parameter-clause opt parameter-clausethrowsopt generic-where-clause opt initializer-body

initializer-declaration → initializer-head generic-parameter-clause opt parameter-clauserethrows generic-where-clause opt initializer-body

initializer-head → attributes opt declaration-modifiers opt initinitializer-head → attributes opt declaration-modifiers opt init ?initializer-head → attributes opt declaration-modifiers opt init !initializer-body → code-block

GRAMMAR OF ADE IN I T I A L I ZER DECLARAT ION

deinitializer-declaration → attributes opt deinit code-block

GRAMMAR OF AN EXTENS ION DECLARAT ION

extension-declaration → attributes opt access-level-modifier opt extension type-identifier type-inheritance-clause opt generic-where-clause opt extension-body

extension-body → { extension-members opt }extension-members → extension-member extension-members optextension-member → declaration | compiler-control-statement

GRAMMAR OF A SUBSCR IPT DECLARAT ION

subscript-declaration → subscript-head subscript-result generic-where-clause opt code-block

subscript-declaration → subscript-head subscript-result generic-where-clause opt getter-setter-block

subscript-declaration → subscript-head subscript-result generic-where-clause opt getter-setter-keyword-block

subscript-head → attributes opt declaration-modifiers opt subscript generic-parameter-clause opt parameter-clause

subscript-result → -> attributes opt type

GRAMMAR OF AN OPERATOR DECLARAT ION

operator-declaration → prefix-operator-declaration | postfix-operator-declaration | infix-operator-declaration

prefix-operator-declaration → prefix operator operatorpostfix-operator-declaration → postfix operator operatorinfix-operator-declaration → infix operator operator infix-operator-group optinfix-operator-group → : precedence-group-name

GRAMMAR OF A PRECEDENCE GROUP DECLARAT ION

precedence-group-declaration → precedencegroup precedence-group-name {precedence-group-attributes opt }

precedence-group-attributes → precedence-group-attribute precedence-group-attributesopt

precedence-group-attribute → precedence-group-relationprecedence-group-attribute → precedence-group-assignmentprecedence-group-attribute → precedence-group-associativityprecedence-group-relation → higherThan : precedence-group-namesprecedence-group-relation → lowerThan : precedence-group-namesprecedence-group-assignment → assignment : boolean-literalprecedence-group-associativity → associativity : leftprecedence-group-associativity → associativity : rightprecedence-group-associativity → associativity : noneprecedence-group-names → precedence-group-name | precedence-group-name ,

precedence-group-namesprecedence-group-name → identifier

GRAMMAR OF ADECLARAT ION MOD IF I ER

declaration-modifier → class | convenience | dynamic | final | infix | lazy |optional | override | postfix | prefix | required | static | unowned |unowned ( safe ) | unowned ( unsafe ) | weak

declaration-modifier → access-level-modifierdeclaration-modifier → mutation-modifier

declaration-modifiers → declaration-modifier declaration-modifiers optaccess-level-modifier → private | private ( set )access-level-modifier → fileprivate | fileprivate ( set )access-level-modifier → internal | internal ( set )access-level-modifier → public | public ( set )access-level-modifier → open | open ( set )mutation-modifier → mutating | nonmutating

Attributes

GRAMMAR OF AN ATTR IBUTE

attribute → @ attribute-name attribute-argument-clause optattribute-name → identifierattribute-argument-clause → ( balanced-tokens opt )attributes → attribute attributes optbalanced-tokens → balanced-token balanced-tokens optbalanced-token → ( balanced-tokens opt )balanced-token → [ balanced-tokens opt ]balanced-token → { balanced-tokens opt }balanced-token → Anyidentifier,keyword,literal,oroperatorbalanced-token → Anypunctuationexcept ( , ) , [ , ] , { ,or }

Patterns

GRAMMAR OF A PATTERN

pattern → wildcard-pattern type-annotation optpattern → identifier-pattern type-annotation optpattern → value-binding-patternpattern → tuple-pattern type-annotation optpattern → enum-case-patternpattern → optional-patternpattern → type-casting-patternpattern → expression-pattern

GRAMMAR OF AW I LDCARD PATTERN

wildcard-pattern → _

GRAMMAR OF AN I DENT I F I ER PATTERN

identifier-pattern → identifier

GRAMMAR OF A VALUE -B IND ING PATTERN

value-binding-pattern → var pattern | let pattern

GRAMMAR OF ATUPLE PATTERN

tuple-pattern → ( tuple-pattern-element-list opt )tuple-pattern-element-list → tuple-pattern-element | tuple-pattern-element , tuple-

pattern-element-listtuple-pattern-element → pattern | identifier : pattern

GRAMMAR OF AN ENUMERAT ION CASE PATTERN

enum-case-pattern → type-identifier opt . enum-case-name tuple-pattern opt

GRAMMAR OF AN OPT IONALPATTERN

optional-pattern → identifier-pattern ?

GRAMMAR OF ATYPE CAST ING PATTERN

type-casting-pattern → is-pattern | as-patternis-pattern → is typeas-pattern → pattern as type

GRAMMAR OF AN EXPRESS ION PATTERN

expression-pattern → expression

GenericParametersandArguments

GRAMMAR OF AGENER IC PARAMETER CLAUSE

generic-parameter-clause → < generic-parameter-list >generic-parameter-list → generic-parameter | generic-parameter , generic-parameter-

listgeneric-parameter → type-namegeneric-parameter → type-name : type-identifiergeneric-parameter → type-name : protocol-composition-type

generic-where-clause → where requirement-listrequirement-list → requirement | requirement , requirement-listrequirement → conformance-requirement | same-type-requirementconformance-requirement → type-identifier : type-identifierconformance-requirement → type-identifier : protocol-composition-typesame-type-requirement → type-identifier == type

GRAMMAR OF AGENER IC ARGUMENT CLAUSE

generic-argument-clause → < generic-argument-list >generic-argument-list → generic-argument | generic-argument , generic-argument-listgeneric-argument → type

RevisionHistory

DocumentRevisionHistory

2019-09-10

UpdatedforSwift5.1.

Addedinformationaboutfunctionsthatspecifyaprotocolthattheirreturnvalueconformsto,insteadofprovidingaspecificnamedreturntype,totheOpaqueTypeschapter.

AddedinformationaboutpropertywrapperstothePropertyWrapperssection.

Addedinformationenumerationsandstructuresthatarefrozenforlibraryevolutiontothefrozensection.

AddedtheFunctionsWithanImplicitReturnandShorthandGetterDeclarationsectionswithinformationaboutfunctionsthatomitreturn.

AddedinformationaboutusingsubscriptsontypestotheTypeSubscriptssection.

UpdatedtheEnumerationCasePatternsection,nowthatanenumerationcasepatterncanmatchanoptionalvalue.

UpdatedtheMemberwiseInitializersforStructureTypessection,nowthatmemberwiseinitializerssupportomittingparametersforpropertiesthathaveadefaultvalue.

AddedinformationaboutdynamicmembersthatarelookedupbykeypathatruntimetothedynamicMemberLookupsection.

AddedmacCatalysttothelistoftargetenvironmentsinConditionalCompilationBlock.

UpdatedtheSelfTypesection,nowthatSelfcanbeusedtorefertothetypeintroducedbythecurrentclass,structure,orenumerationdeclaration.

2019-03-25

UpdatedforSwift5.0.

AddedtheExtendedStringDelimiterssectionandupdatedtheStringLiteralssectionwithinformationaboutextendedstringdelimiters.

AddedthedynamicCallablesectionwithinformationaboutdynamicallycallinginstancesasfunctionsusingthedynamicCallableattribute.

AddedtheunknownandSwitchingOverFutureEnumerationCasessectionswithinformationabouthandlingfutureenumerationcasesinswitchstatementsusingtheunknownswitchcaseattribute.

Addedinformationabouttheidentitykeypath(\.self)totheKey-PathExpressionsection.

Addedinformationaboutusingthelessthan(<)operatorinplatformconditionstotheConditionalCompilationBlocksection.

2018-09-17

UpdatedforSwift4.2.

Addedinformationaboutaccessingallofanenumeration’scasestotheIteratingoverEnumerationCasessection.

Addedinformationabout#errorand#warningtotheCompile-TimeDiagnosticStatementsection.

AddedinformationaboutinliningtotheDeclarationAttributessectionundertheinlinableandusableFromInlineattributes.

AddedinformationaboutmembersthatarelookedupbynameatruntimetotheDeclarationAttributessectionunderthedynamicMemberLookupattribute.

Addedinformationabouttherequires_stored_property_initsandwarn_unqualified_accessattributestotheDeclarationAttributessection.

AddedinformationabouthowtoconditionallycompilecodedependingontheSwiftcompilerversionbeingusedtotheConditionalCompilationBlocksection.

Addedinformationabout#dsohandletotheLiteralExpressionsection.

2018-03-29

UpdatedforSwift4.1.

AddedinformationaboutsynthesizedimplementationsofequivalenceoperatorstotheEquivalenceOperatorssection.

AddedinformationaboutconditionalprotocolconformancetotheExtensionDeclarationsectionoftheDeclarationschapter,andtotheConditionallyConformingtoaProtocolsectionoftheProtocolschapter.

AddedinformationaboutrecursiveprotocolconstraintstotheUsingaProtocolinItsAssociatedType’sConstraintssection.

AddedinformationaboutthecanImport()andtargetEnvironment()platformconditionstoConditionalCompilationBlock.

2017-12-04

UpdatedforSwift4.0.3.

UpdatedtheKey-PathExpressionsection,nowthatkeypathssupportsubscriptcomponents.

2017-09-19

UpdatedforSwift4.0.

AddedinformationaboutexclusiveaccesstomemorytotheMemorySafetychapter.

AddedtheAssociatedTypeswithaGenericWhereClausesection,nowthatyoucanusegenericwhereclausestoconstrainassociatedtypes.

AddedinformationaboutmultilinestringliteralstotheStringLiteralssectionoftheStringsandCharacterschapter,andtotheStringLiteralssectionoftheLexicalStructurechapter.

UpdatedthediscussionoftheobjcattributeinDeclarationAttributes,nowthatthisattributeisinferredinfewerplaces.

AddedtheGenericSubscriptssection,nowthatsubscriptscanbegeneric.

UpdatedthediscussionintheProtocolCompositionsectionoftheProtocolschapter,andintheProtocolCompositionTypesectionoftheTypeschapter,nowthatprotocolcompositiontypescancontainasuperclassrequirement.

UpdatedthediscussionofprotocolextensionsinExtensionDeclarationnowthatfinalisn’tallowedinthem.

AddedinformationaboutpreconditionsandfatalerrorstotheAssertionsandPreconditionssection.

2017-03-27

UpdatedforSwift3.1.

AddedtheExtensionswithaGenericWhereClausesectionwithinformationaboutextensionsthatincluderequirements.

AddedexamplesofiteratingoverarangetotheFor-InLoopssection.

AddedanexampleoffailablenumericconversionstotheFailableInitializerssection.

AddedinformationtotheDeclarationAttributessectionaboutusingtheavailableattributewithaSwiftlanguageversion.

UpdatedthediscussionintheFunctionTypesectiontonotethatargumentlabelsarenotallowedwhenwritingafunctiontype.

UpdatedthediscussionofSwiftlanguageversionnumbersintheConditionalCompilationBlocksection,nowthatanoptionalpatchnumber

isallowed.

UpdatedthediscussionintheFunctionTypesection,nowthatSwiftdistinguishesbetweenfunctionsthattakemultipleparametersandfunctionsthattakeasingleparameterofatupletype.

RemovedtheDynamicTypeExpressionsectionfromtheExpressionschapter,nowthattype(of:)isaSwiftstandardlibraryfunction.

2016-10-27

UpdatedforSwift3.0.1.

UpdatedthediscussionofweakandunownedreferencesintheAutomaticReferenceCountingchapter.

Addedinformationabouttheunowned,unowned(safe),andunowned(unsafe)declarationmodifiersintheDeclarationModifierssection.

AddedanotetotheTypeCastingforAnyandAnyObjectsectionaboutusinganoptionalvaluewhenavalueoftypeAnyisexpected.

UpdatedtheExpressionschaptertoseparatethediscussionofparenthesizedexpressionsandtupleexpressions.

2016-09-13

UpdatedforSwift3.0.

UpdatedthediscussionoffunctionsintheFunctionschapterandtheFunctionDeclarationsectiontonotethatallparametersgetanargumentlabelbydefault.

UpdatedthediscussionofoperatorsintheAdvancedOperatorschapter,nowthatyouimplementthemastypemethodsinsteadofasglobalfunctions.

Addedinformationabouttheopenandfileprivateaccess-levelmodifierstotheAccessControlchapter.

UpdatedthediscussionofinoutintheFunctionDeclarationsectiontonotethatitappearsinfrontofaparameter’stypeinsteadofinfrontofaparameter’sname.

Updatedthediscussionofthe@noescapeand@autoclosureattributesintheEscapingClosuresandAutoclosuressectionsandtheAttributeschapternowthattheyaretypeattributes,ratherthandeclarationattributes.

AddedinformationaboutoperatorprecedencegroupstothePrecedenceforCustomInfixOperatorssectionoftheAdvancedOperatorschapter,andtothePrecedenceGroupDeclarationsectionoftheDeclarationschapter.

UpdateddiscussionthroughouttousemacOSinsteadofOSX,ErrorinsteadofErrorProtocol,andprotocolnamessuchasExpressibleByStringLiteralinsteadofStringLiteralConvertible.

UpdatedthediscussionintheGenericWhereClausessectionoftheGenericschapterandintheGenericParametersandArgumentschapter,nowthatgenericwhereclausesarewrittenattheendofadeclaration.

UpdatedthediscussionintheEscapingClosuressection,nowthatclosuresarenonescapingbydefault.

UpdatedthediscussionintheOptionalBindingsectionoftheTheBasicschapterandtheWhileStatementsectionoftheStatementschapter,nowthatif,while,andguardstatementsuseacomma-separatedlistofconditionswithoutwhereclauses.

AddedinformationaboutswitchcasesthathavemultiplepatternstotheSwitchsectionoftheControlFlowchapterandtheSwitchStatementsectionoftheStatementschapter.

UpdatedthediscussionoffunctiontypesintheFunctionTypesectionnowthatfunctionargumentlabelsarenolongerpartofafunction’stype.

UpdatedthediscussionofprotocolcompositiontypesintheProtocolCompositionsectionoftheProtocolschapterandintheProtocolCompositionTypesectionoftheTypeschaptertousethenewProtocol1&Protocol2syntax.

UpdatedthediscussionintheDynamicTypeExpressionsectiontousethenewtype(of:)syntaxfordynamictypeexpressions.

Updatedthediscussionoflinecontrolstatementstousethe#sourceLocation(file:line:)syntaxintheLineControlStatementsection.

UpdatedthediscussioninFunctionsthatNeverReturntousethenewNevertype.

AddedinformationaboutplaygroundliteralstotheLiteralExpressionsection.

UpdatedthediscussionintheIn-OutParameterssectiontonotethatonlynonescapingclosurescancapturein-outparameters.

UpdatedthediscussionaboutdefaultparametersintheDefaultParameterValuessection,nowthattheycan’tbereorderedinfunctioncalls.

UpdatedattributeargumentstouseacolonintheAttributeschapter.

AddedinformationaboutthrowinganerrorinsidethecatchblockofarethrowingfunctiontotheRethrowingFunctionsandMethodssection.

AddedinformationaboutaccessingtheselectorofanObjective-Cproperty’sgetterorsettertotheSelectorExpressionsection.

AddedinformationtotheTypeAliasDeclarationsectionaboutgenerictypealiasesandusingtypealiasesinsideofprotocols.

UpdatedthediscussionoffunctiontypesintheFunctionTypesectiontonotethatparenthesesaroundtheparametertypesarerequired.

UpdatedtheAttributeschaptertonotethatthe@IBAction,@IBOutlet,and@NSManagedattributesimplythe@objcattribute.

Addedinformationaboutthe@GKInspectableattributetotheDeclarationAttributessection.

UpdatedthediscussionofoptionalprotocolrequirementsintheOptionalProtocolRequirementssectiontoclarifythattheyareusedonlyincodethatinteroperateswithObjective-C.

RemovedthediscussionofexplicitlyusingletinfunctionparametersfromtheFunctionDeclarationsection.

RemovedthediscussionoftheBooleanprotocolfromtheStatementschapter,nowthattheprotocolhasbeenremovedfromtheSwiftstandardlibrary.

Correctedthediscussionofthe@NSApplicationMainattributeintheDeclarationAttributessection.

2016-03-21

UpdatedforSwift2.2.

AddedinformationabouthowtoconditionallycompilecodedependingontheversionofSwiftbeingusedtotheConditionalCompilationBlocksection.

AddedinformationabouthowtodistinguishbetweenmethodsorinitializerswhosenamesdifferonlybythenamesoftheirargumentstotheExplicitMemberExpressionsection.

Addedinformationaboutthe#selectorsyntaxforObjective-CselectorstotheSelectorExpressionsection.

UpdatedthediscussionofassociatedtypestousetheassociatedtypekeywordintheAssociatedTypesandProtocolAssociatedTypeDeclarationsections.

UpdatedinformationaboutinitializersthatreturnnilbeforetheinstanceisfullyinitializedintheFailableInitializerssection.

AddedinformationaboutcomparingtuplestotheComparisonOperatorssection.

AddedinformationaboutusingkeywordsasexternalparameternamestotheKeywordsandPunctuationsection.

Updatedthediscussionofthe@objcattributeintheDeclarationAttributessectiontonotethatenumerationsandenumerationcasescanusethisattribute.

UpdatedtheOperatorssectionwithdiscussionofcustomoperatorsthatcontainadot.

AddedanotetotheRethrowingFunctionsandMethodssectionthatrethrowingfunctionscan’tdirectlythrowerrors.

AddedanotetothePropertyObserverssectionaboutpropertyobserversbeingcalledwhenyoupassapropertyasanin-outparameter.

AddedasectionabouterrorhandlingtotheASwiftTourchapter.

UpdatedfiguresintheWeakReferencessectiontoshowthedeallocationprocessmoreclearly.

RemoveddiscussionofC-styleforloops,the++prefixandpostfixoperators,andthe--prefixandpostfixoperators.

Removeddiscussionofvariablefunctionargumentsandthespecialsyntaxforcurriedfunctions.

2015-10-20

UpdatedforSwift2.1.

UpdatedtheStringInterpolationandStringLiteralssectionsnowthatstringinterpolationscancontainstringliterals.

AddedtheEscapingClosuressectionwithinformationaboutthe@noescapeattribute.

UpdatedtheDeclarationAttributesandConditionalCompilationBlocksectionswithinformationabouttvOS.

Addedinformationaboutthebehaviorofin-outparameterstotheIn-OutParameterssection.

AddedinformationtotheCaptureListssectionabouthowvaluesspecifiedinclosurecapturelistsarecaptured.

UpdatedtheAccessingPropertiesThroughOptionalChainingsectiontoclarifyhowassignmentthroughoptionalchainingbehaves.

ImprovedthediscussionofautoclosuresintheAutoclosuressection.

Addedanexamplethatusesthe??operatortotheASwiftTourchapter.

2015-09-16

UpdatedforSwift2.0.

AddedinformationabouterrorhandlingtotheErrorHandlingchapter,theDoStatementsection,theThrowStatementsection,theDeferStatementsection,andtheTryOperatorsection.

UpdatedtheRepresentingandThrowingErrorssection,nowthatalltypescanconformtotheErrorTypeprotocol.

Addedinformationaboutthenewtry?keywordtotheConvertingErrorstoOptionalValuessection.

AddedinformationaboutrecursiveenumerationstotheRecursiveEnumerationssectionoftheEnumerationschapterandtheEnumerationswithCasesofAnyTypesectionoftheDeclarationschapter.

AddedinformationaboutAPIavailabilitycheckingtotheCheckingAPIAvailabilitysectionoftheControlFlowchapterandtheAvailabilityConditionsectionoftheStatementschapter.

AddedinformationaboutthenewguardstatementtotheEarlyExitsectionoftheControlFlowchapterandtheGuardStatementsectionoftheStatementschapter.

AddedinformationaboutprotocolextensionstotheProtocolExtensionssectionoftheProtocolschapter.

AddedinformationaboutaccesscontrolforunittestingtotheAccessLevelsforUnitTestTargetssectionoftheAccessControlchapter.

AddedinformationaboutthenewoptionalpatterntotheOptionalPatternsectionofthePatternschapter.

UpdatedtheRepeat-Whilesectionwithinformationabouttherepeat-whileloop.

UpdatedtheStringsandCharacterschapter,nowthatStringnolongerconformstotheCollectionTypeprotocolfromtheSwiftstandardlibrary.

AddedinformationaboutthenewSwiftstandardlibraryprint(_:separator:terminator)functiontothePrintingConstantsandVariablessection.

AddedinformationaboutthebehaviorofenumerationcaseswithStringrawvaluestotheImplicitlyAssignedRawValuessectionoftheEnumerationschapterandtheEnumerationswithCasesofaRaw-ValueTypesectionoftheDeclarationschapter.

Addedinformationaboutthe@autoclosureattribute—includingits@autoclosure(escaping)form—totheAutoclosuressection.

UpdatedtheDeclarationAttributessectionwithinformationaboutthe@availableand@warn_unused_resultattributes.

UpdatedtheTypeAttributessectionwithinformationaboutthe@conventionattribute.

AddedanexampleofusingmultipleoptionalbindingswithawhereclausetotheOptionalBindingsection.

AddedinformationtotheStringLiteralssectionabouthowconcatenatingstringliteralsusingthe+operatorhappensatcompiletime.

AddedinformationtotheMetatypeTypesectionaboutcomparingmetatypevaluesandusingthemtoconstructinstanceswithinitializerexpressions.

AddedanotetotheDebuggingwithAssertionssectionaboutwhenuser-definedassertionsaredisabled.

Updatedthediscussionofthe@NSManagedattributeintheDeclarationAttributessection,nowthattheattributecanbeappliedtocertaininstancemethods.

UpdatedtheVariadicParameterssection,nowthatvariadicparameterscanbedeclaredinanypositioninafunction’sparameterlist.

AddedinformationtotheOverridingaFailableInitializersectionabouthowanonfailableinitializercandelegateuptoafailableinitializerbyforce-unwrappingtheresultofthesuperclass’sinitializer.

AddedinformationaboutusingenumerationcasesasfunctionstotheEnumerationswithCasesofAnyTypesection.

AddedinformationaboutexplicitlyreferencinganinitializertotheInitializerExpressionsection.

AddedinformationaboutbuildconfigurationandlinecontrolstatementstotheCompilerControlStatementssection.

AddedanotetotheMetatypeTypesectionaboutconstructingclassinstancesfrommetatypevalues.

AddedanotetotheWeakReferencessectionaboutweakreferencesbeingunsuitableforcaching.

UpdatedanoteintheTypePropertiessectiontomentionthatstoredtypepropertiesarelazilyinitialized.

UpdatedtheCapturingValuessectiontoclarifyhowvariablesandconstantsarecapturedinclosures.

UpdatedtheDeclarationAttributessectiontodescribewhenyoucanapply

the@objcattributetoclasses.

AddedanotetotheHandlingErrorssectionabouttheperformanceofexecutingathrowstatement.AddedsimilarinformationaboutthedostatementintheDoStatementsection.

UpdatedtheTypePropertiessectionwithinformationaboutstoredandcomputedtypepropertiesforclasses,structures,andenumerations.

UpdatedtheBreakStatementsectionwithinformationaboutlabeledbreakstatements.

UpdatedanoteinthePropertyObserverssectiontoclarifythebehaviorofwillSetanddidSetobservers.

AddedanotetotheAccessLevelssectionwithinformationaboutthescopeofprivateaccess.

AddedanotetotheWeakReferencessectionaboutthedifferencesinweakreferencesbetweengarbagecollectedsystemsandARC.

UpdatedtheSpecialCharactersinStringLiteralssectionwithamoreprecisedefinitionofUnicodescalars.

2015-04-08

UpdatedforSwift1.2.

SwiftnowhasanativeSetcollectiontype.Formoreinformation,seeSets.

@autoclosureisnowanattributeoftheparameterdeclaration,notitstype.Thereisalsoanew@noescapeparameterdeclarationattribute.Formoreinformation,seeDeclarationAttributes.

Typemethodsandpropertiesnowusethestatickeywordasadeclarationmodifier.FormoreinformationseeTypeVariableProperties.

Swiftnowincludestheas?andas!failabledowncastoperators.Formoreinformation,seeCheckingforProtocolConformance.

AddedanewguidesectionaboutStringIndices.

Removedtheoverflowdivision(&/)andoverflowremainder(&%)operatorsfromOverflowOperators.

Updatedtherulesforconstantandconstantpropertydeclarationandinitialization.Formoreinformation,seeConstantDeclaration.

UpdatedthedefinitionofUnicodescalarsinstringliterals.SeeSpecialCharactersinStringLiterals.

UpdatedRangeOperatorstonotethatahalf-openrangewiththesamestartandendindexwillbeempty.

UpdatedClosuresAreReferenceTypestoclarifythecapturingrulesforvariables.

UpdatedValueOverflowtoclarifytheoverflowbehaviorofsignedandunsignedintegers

UpdatedProtocolDeclarationtoclarifyprotocoldeclarationscopeandmembers.

UpdatedDefiningaCaptureListtoclarifythesyntaxforweakandunownedreferencesinclosurecapturelists.

UpdatedOperatorstoexplicitlymentionexamplesofsupportedcharactersforcustomoperators,suchasthoseintheMathematicalOperators,MiscellaneousSymbols,andDingbatsUnicodeblocks.

Constantscannowbedeclaredwithoutbeinginitializedinlocalfunctionscope.Theymusthaveasetvaluebeforefirstuse.Formoreinformation,seeConstantDeclaration.

Inaninitializer,constantpropertiescannowonlyassignavalueonce.Formoreinformation,seeAssigningConstantPropertiesDuringInitialization.

Multipleoptionalbindingscannowappearinasingleifstatementasacomma-separatedlistofassignmentexpressions.Formoreinformation,see

OptionalBinding.

AnOptional-ChainingExpressionmustappearwithinapostfixexpression.

Protocolcastsarenolongerlimitedto@objcprotocols.

Typecaststhatcanfailatruntimenowusetheas?oras!operator,andtypecaststhatareguaranteednottofailusetheasoperator.Formoreinformation,seeType-CastingOperators.

2014-10-16

UpdatedforSwift1.1.

AddedafullguidetoFailableInitializers.

AddedadescriptionofFailableInitializerRequirementsforprotocols.

ConstantsandvariablesoftypeAnycannowcontainfunctioninstances.UpdatedtheexampleinTypeCastingforAnyandAnyObjecttoshowhowtocheckforandcasttoafunctiontypewithinaswitchstatement.

EnumerationswithrawvaluesnowhavearawValuepropertyratherthanatoRaw()methodandafailableinitializerwitharawValueparameterratherthanafromRaw()method.Formoreinformation,seeRawValuesandEnumerationswithCasesofaRaw-ValueType.

AddedanewreferencesectionaboutFailableInitializers,whichcantriggerinitializationfailure.

Customoperatorscannowcontainthe?character.UpdatedtheOperatorsreferencetodescribetherevisedrules.RemovedaduplicatedescriptionofthevalidsetofoperatorcharactersfromCustomOperators.

2014-08-18

NewdocumentthatdescribesSwift1.0,Apple’snewprogramminglanguageforbuildingiOSandOSXapps.

AddedanewsectionaboutInitializerRequirementsinprotocols.

AddedanewsectionaboutClass-OnlyProtocols.

AssertionsandPreconditionscannowusestringinterpolation.Removedanotetothecontrary.

UpdatedtheConcatenatingStringsandCharacterssectiontoreflectthefactthatStringandCharactervaluescannolongerbecombinedwiththeadditionoperator(+)oradditionassignmentoperator(+=).TheseoperatorsarenowusedonlywithStringvalues.UsetheStringtype’sappend(_:)methodtoappendasingleCharactervalueontotheendofastring.

AddedinformationabouttheavailabilityattributetotheDeclarationAttributessection.

Optionalsnolongerimplicitlyevaluatetotruewhentheyhaveavalueandfalsewhentheydonot,toavoidconfusionwhenworkingwithoptionalBoolvalues.Instead,makeanexplicitcheckagainstnilwiththe==or!=operatorstofindoutifanoptionalcontainsavalue.

SwiftnowhasaNil-CoalescingOperator(a??b),whichunwrapsanoptional’svalueifitexists,orreturnsadefaultvalueiftheoptionalisnil.

UpdatedandexpandedtheComparingStringssectiontoreflectanddemonstratethatstringandcharactercomparisonandprefix/suffixcomparisonarenowbasedonUnicodecanonicalequivalenceofextendedgraphemeclusters.

Youcannowtrytosetaproperty’svalue,assigntoasubscript,orcallamutatingmethodoroperatorthroughOptionalChaining.TheinformationaboutAccessingPropertiesThroughOptionalChaininghasbeenupdatedaccordingly,andtheexamplesofcheckingformethodcallsuccessinCallingMethodsThroughOptionalChaininghavebeenexpandedtoshowhowtocheckforpropertysettingsuccess.

AddedanewsectionaboutAccessingSubscriptsofOptionalTypethroughoptionalchaining.

UpdatedtheAccessingandModifyinganArraysectiontonotethatyoucannolongerappendasingleitemtoanarraywiththe+=operator.Instead,usetheappend(_:)method,orappendasingle-itemarraywiththe+=operator.

AddedanotethatthestartvalueafortheRangeOperatorsa...banda..<bmustnotbegreaterthantheendvalueb.

RewrotetheInheritancechaptertoremoveitsintroductorycoverageofinitializeroverrides.Thischapternowfocusesmoreontheadditionofnewfunctionalityinasubclass,andthemodificationofexistingfunctionalitywithoverrides.Thechapter’sexampleofOverridingPropertyGettersandSettershasbeenrewrittentoshowhowtooverrideadescriptionproperty.(Theexamplesofmodifyinganinheritedproperty’sdefaultvalueinasubclassinitializerhavebeenmovedtotheInitializationchapter.)

UpdatedtheInitializerInheritanceandOverridingsectiontonotethatoverridesofadesignatedinitializermustnowbemarkedwiththeoverridemodifier.

UpdatedtheRequiredInitializerssectiontonotethattherequiredmodifierisnowwrittenbeforeeverysubclassimplementationofarequiredinitializer,andthattherequirementsforrequiredinitializerscannowbesatisfiedbyautomaticallyinheritedinitializers.

InfixOperatorMethodsnolongerrequirethe@infixattribute.

The@prefixand@postfixattributesforPrefixandPostfixOperatorshavebeenreplacedbyprefixandpostfixdeclarationmodifiers.

AddedanoteabouttheorderinwhichPrefixandPostfixOperatorsareappliedwhenbothaprefixandapostfixoperatorareappliedtothesameoperand.

OperatorfunctionsforCompoundAssignmentOperatorsnolongerusethe@assignmentattributewhendefiningthefunction.

TheorderinwhichmodifiersarespecifiedwhendefiningCustomOperatorshaschanged.Younowwriteprefixoperatorratherthanoperatorprefix,forexample.

AddedinformationaboutthedynamicdeclarationmodifierinDeclarationModifiers.

AddedinformationabouthowtypeinferenceworkswithLiterals.

Addedmoreinformationaboutcurriedfunctions.

AddedanewchapteraboutAccessControl.

UpdatedtheStringsandCharacterschaptertoreflectthefactthatSwift’sCharactertypenowrepresentsasingleUnicodeextendedgraphemecluster.IncludesanewsectiononExtendedGraphemeClustersandmoreinformationaboutUnicodeScalarValuesandComparingStrings.

UpdatedtheStringLiteralssectiontonotethatUnicodescalarsinsidestringliteralsarenowwrittenas\u{n},wherenisahexadecimalnumberbetween0and10FFFF,therangeofUnicode’scodespace.

TheNSStringlengthpropertyisnowmappedontoSwift’snativeStringtypeasutf16Count,notutf16count.

Swift’snativeStringtypenolongerhasanuppercaseStringorlowercaseStringproperty.ThecorrespondingsectioninStringsandCharactershasbeenremoved,andvariouscodeexampleshavebeenupdated.

AddedanewsectionaboutInitializerParametersWithoutArgumentLabels.

AddedanewsectionaboutRequiredInitializers.

AddedanewsectionaboutOptionalTupleReturnTypes.

UpdatedtheTypeAnnotationssectiontonotethatmultiplerelatedvariablescanbedefinedonasinglelinewithonetypeannotation.

The@optional,@lazy,@final,and@requiredattributesarenowtheoptional,lazy,final,andrequiredDeclarationModifiers.

Updatedtheentirebooktoreferto..<astheHalf-OpenRangeOperator(ratherthanthe“half-closedrangeoperator”).

UpdatedtheAccessingandModifyingaDictionarysectiontonotethatDictionarynowhasaBooleanisEmptyproperty.

ClarifiedthefulllistofcharactersthatcanbeusedwhendefiningCustomOperators.

nilandtheBooleanstrueandfalsearenowLiterals.

Swift’sArraytypenowhasfullvaluesemantics.UpdatedtheinformationaboutMutabilityofCollectionsandArraystoreflectthenewapproach.Alsoclarifiedtheassignmentandcopybehaviorforstringsarraysanddictionaries.

ArrayTypeShorthandSyntaxisnowwrittenas[SomeType]ratherthanSomeType[].

AddedanewsectionaboutDictionaryTypeShorthandSyntax,whichiswrittenas[KeyType:ValueType].

AddedanewsectionaboutHashValuesforSetTypes.

ExamplesofClosureExpressionsnowusetheglobalsorted(_:_:)functionratherthantheglobalsort(_:_:)function,toreflectthenewarrayvaluesemantics.

UpdatedtheinformationaboutMemberwiseInitializersforStructureTypestoclarifythatthememberwisestructureinitializerismadeavailableevenifastructure’sstoredpropertiesdonothavedefaultvalues.

Updatedto..<ratherthan..fortheHalf-OpenRangeOperator.

AddedanexampleofExtendingaGenericType.

CopyrightandNotices

AppleInc.Copyright©2019AppleInc.

ThisdocumentismadeavailableunderaCreativeCommonsAttribution4.0International(CCBY4.0)License:https://creativecommons.org/licenses/by/4.0/

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SwiftandtvOSaretrademarksofAppleInc.

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