Computer Notes - Constructor

8/3/2019 Computer Notes - Constructor

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ConstructorConstructor

http://ecomputernotes.com


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ReviewReview

Member functions implementationMember functions implementation

ConstructorsConstructors

Constructors overloadingConstructors overloadingCopy constructorsCopy constructors



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Copy ConstructorCopy Constructor

Copy constructor are used when:Copy constructor are used when:

Initializing an object at the time ofInitializing an object at the time of

creationcreation

When an object is passed by value to aWhen an object is passed by value to a

functionfunction



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ExampleExample

void func1(Student student){void func1(Student student){

}}

intintmain(){main(){StudentStudent studentA(studentA(AhmadAhmad););

StudentStudent studentBstudentB == studentAstudentA;;func1(studentA);func1(studentA);

}}http://ecomputernotes.com


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Copy Constructor (contd.)Copy Constructor (contd.)

Student::Student(Student::Student(const Student &const Student &objobj){){

rollNorollNo == obj.rollNoobj.rollNo;;name =name = obj.nameobj.name;;

GPA =GPA = obj.GPAobj.GPA;;}}



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Shallow CopyShallow Copy

When we initialize one object withWhen we initialize one object with

another then the compiler copies stateanother then the compiler copies state

of one object to the otherof one object to the other

This kind of copying is called shallowThis kind of copying is called shallow

copyingcopying



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ExampleExampleStudent studentA(Ahmad);

GPAGPA

NameName

RollNnRollNn

studentBstudentB

Heap

A

H

M

A

D

GPAGPA

NameName

RollNnRollNn

studentAstudentA

Student studentB = studentA;



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Exampleint main(){

Student studentA(Ahmad,1);

{


GPAGPA

NameName

RollNnRollNn

studentBstudentB

Heap

A

H

M

A

D

GPAGPA

NameName

RollNnRollNn

studentAstudentA



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ExampleExampleint main(){


{

Student studentB = studentA;}

}

HeapGPAGPA

NameName

RollNnRollNn

studentAstudentA



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Student::Student(Student::Student(const Student &const Student & objobj){){

intint lenlen == strlen(obj.namestrlen(obj.name););

name = new char[len+1]name = new char[len+1]

strcpy(namestrcpy(name,, obj.nameobj.name););

/*copy rest of the data members*//*copy rest of the data members*/

}}



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Exampleint main(){Student studentA(Ahmad,1);

{


A

H

MA

D

GPAGPANameName

RollNnRollNn

studentAstudentA

GPAGPA

NameName

RollNnRollNnstudentBstudentB

A

HM

A

D


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ExampleExampleint main(){


{Student studentB = studentA;}

}

HeapGPAGPANameName

RollNnRollNn

studentAstudentA

A

HM

A

D



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Copy constructor is normally used toCopy constructor is normally used toperform deep copyperform deep copy

If we do not make a copyIf we do not make a copyconstructor then the compilerconstructor then the compiler

performs shallow copyperforms shallow copy



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DestructorDestructor

Destructor is used to free memoryDestructor is used to free memory

that is allocated through dynamicthat is allocated through dynamic

allocationallocation

Destructor is used to perform houseDestructor is used to perform house

keeping operationskeeping operations



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Destructor (contd.)Destructor (contd.)

Destructor is a function withDestructor is a function withthe same name as that of class,the same name as that of class,

but preceded with a tildebut preceded with a tilde~~



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ExampleExampleclass Studentclass Student

{{

public:public:

~Student(){~Student(){if(name){if(name){

delete []name;delete []name;

}}

}}

}}



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OverloadingOverloading

Destructors cannot beDestructors cannot beoverloadedoverloaded



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Sequence of CallsSequence of Calls

Constructors and destructors areConstructors and destructors arecalled automaticallycalled automatically

Constructors are called in theConstructors are called in the

sequence in which object is declaredsequence in which object is declaredDestructors are called in reverse orderDestructors are called in reverse order



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ExampleExampleStudent::Student(char *Student::Student(char * aNameaName){){

coutcout


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ExampleExampleintintmain()main()

{{

StudentStudent studentB(studentB(AliAli););

StudentStudent studentA(studentA(AhmadAhmad););

return 0;return 0;

}}



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ExampleExample

Output:Output:

Ali ConsAli Cons

Ahmad ConsAhmad ConsAhmadAhmad DestDest

AliAli DestDest



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Accessor FunctionsAccessor Functions

Usually the data member are defined inUsually the data member are defined in

private part of a classprivate part of a class information hidinginformation hiding

Accessor functions are functions that areAccessor functions are functions that are

used to access these private dataused to access these private datamembersmembers

Accessor functions also useful in reducingAccessor functions also useful in reducingerrorerror


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ExampleExampleAccessing Data MemberAccessing Data Member

class Student{class Student{

intint rollNorollNo;;

public:public:

voidvoidsetRollNo(intsetRollNo(int aRollNoaRollNo){){

rollNorollNo == aRollNoaRollNo;;}}

};}; http://ecomputernotes.com


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ExampleExampleAvoiding ErrorAvoiding ErrorvoidvoidStudent::setRollNo(intStudent::setRollNo(int aRollNoaRollNo){){

if(aRollNoif(aRollNo< 0){< 0){rollNorollNo = 0;= 0;

}}

elseelse

{{

rollNorollNo == aRollNoaRollNo;;}}

}}



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ExampleExample -- GetterGetterclass Student{class Student{

intint rollNorollNo;;

public:public:

intint getRollNogetRollNo(){(){

returnreturn rollNorollNo;;

}}

};};http://ecomputernotes.com


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thisthis PointerPointerclass Student{class Student{

intint rollNorollNo;;char *name;char *name;

float GPA;float GPA;

public:public:

intint getRollNogetRollNo();();

voidvoidsetRollNo(intsetRollNo(int aRollNoaRollNo););

};};http://ecomputernotes.com


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thisthis PointerPointerThe compiler reserves space for theThe compiler reserves space for the

functions defined in the classfunctions defined in the class

Space for data is not allocated (Space for data is not allocated (since nosince no

object is yet createdobject is yet created))

Function SpacegetRollNo(),



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thisthis PointerPointerStudent sStudent s11,, s2s2,, s3s3;;

Function SpacegetRollNo(),

s1(rollNo,)

s2(rollNo,)

s3(rollNo,)


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thisthis PointerPointerFunction space is common for everyFunction space is common for every

variablevariable

Whenever a new object is created:Whenever a new object is created:

Memory is reserved for variables onlyMemory is reserved for variables only Previously defined functions are usedPreviously defined functions are used

over and over againover and over again



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thisthis PointerPointerMemory layout for objects created:Memory layout for objects created:

s1

rollNo,

Function Space

getRollNo(),

s2

rollNo,

s3

rollNo,

s4

rollNo,

How does the functions know on which

object to act?


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thisthis PointerPointerAddress of each object is passed to the callingAddress of each object is passed to the calling

functionfunctionThis address isThis address is deferenceddeferenced by the functions andby the functions and

hence they act on correct objectshence they act on correct objects

address

s1

rollNo,

s2

rollNo,

s3

rollNo,

s4

rollNo,

address address address

The variable containing the self-address is

called this pointer


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PassingPassing thisthisPointerPointerWhenever a function is called theWhenever a function is called the thisthispointer ispointer is

passed as a parameter to that functionpassed as a parameter to that functionFunction withFunction with nnparameters is actually calledparameters is actually called

withwith n+1n+1parametersparameters



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ExampleExamplevoidvoidStudent::setName(charStudent::setName(char *)*)

is internally represented asis internally represented as

voidvoidStudent::setName(charStudent::setName(char *,*,

const Student *)const Student *)



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Declaration of thisDeclaration of thisDataTypeDataType * const this;* const this;



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Compiler Generated CodeCompiler Generated CodeStudent::Student(){Student::Student(){

rollNorollNo = 0;= 0;

}}

Student::Student(){Student::Student(){

thisthis-->>rollNorollNo = 0;= 0;}}


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