Classes: Part 2

Static vs. Dynamic Variables

Global and local variables Locals are automatic variables from the ‘stack’ of the memory,

therefore called ‘static’ Dynamic variables are from the ‘heap’ of the memory.

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‘static’ variables are ‘global’ with the file scope

In C and C++, ‘static’ is well defined:

Generally speaking: ‘static’ is meant to be ‘non-dynamic’

Static Variables in C and C++ Static variables are put somewhere ‘permanently’ or ‘globally’ in memory The static variable ‘s’ can only be accessed within the function but it is not

deleted with the function A ‘local’ static variable is a ‘global’ variable for the function, not the others.

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int f(){ static int s = 0; return s++;}

int main() { cout << f() << endl; cout << f() << endl;}

Don’t do it unless you have a good reason!

Static Class Members

A variable that is part of a class, yet not part of an object of that class, is called a static member. There is exactly one copy of a static member per class, instead of one copy per object, as for non-

static members. Only one copy of a variable shared by all objects of a class

“Class-wide” information A property of the class shared by all instances, not a property of a specific object of the class

A function that needs to access to members of a class, yet does not need to be invoked for a particular object, is called a static member function.

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like a kind of global variables but they have class scope (outside the class, they cannot be accessed)

Static Class Members

Can be declared public, private or protected Primitive (Fundamental-type) static data members

Initialized by default to 0 If you want a different initial value, a static data member can be initialized

once (and only once) A const static data member of int or enum type can be initialized in

its declaration in the class definition Alternatively, you can also initialize it in file scope

All other static data members must be defined at file scope (i.e., outside the body of the class definition)

static data members of class types (i.e., static member objects) that have default constructors need not be initialized because their default constructors will be called

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Static Class Members To access a public static class member when no objects of the

class exist: Prefix the class name and the binary scope resolution operator (::) to the

name of the data member Employee::count

Also accessible through any object of that class Use the object’s name, the dot operator and the name of the member Employee_object.count

static member function Is a service of the class, not of the object of the class

Example: SEmployee.h, SEmployee.cpp, static.cpp

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Private ‘static’ members still can only be accessed by member functions!

Static Member functions

Declare a member function static It cannot access non-static data members or non-static member

functions of the class (because the object may not exist when the function is called)

A static member function does not have a this pointer static data members and static member functions exist

independently of any objects of a class, i.e., when a static member function is called, there might not be any objects of its class in memory

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SEmployee.h Employee has a static function and a static data member

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class Employee {public: Employee(const char *const, const char *const); ~Employee(); const char* getFirstName() const; const char* getLastName() const; static int getCount(); private: char* firstName; char* lastName; static int count; // number of objects instantiated};

Static data member keeps track of number of Employee objects that currently exist;

Static member function may be called even the object does not exist.

SEmployee.cpp (1/3)

static data member is defined and initialized at file scope in the .cpp file

static member function can access only static data, because the function might be called when no objects exists

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// define and initialize static data member at file scopeint Employee::count = 0; // cannot include keyword static

int Employee::getCount() { return count; }

Even static count is private!

SEmployee.cpp (2/3)

Non-static member function (e.g., constructor) can modify the class’s static data members

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Employee::Employee(const char *const first, const char *const last){ firstName = new char[ strlen( first ) + 1 ]; strcpy(firstName, first);

lastName = new char[ strlen( last ) + 1 ]; strcpy( lastName, last );

count++;

cout << "Employee constructor for " << firstName << ' ' << lastName << " called." << endl;}

SEmployee.cpp (3/3)

Remember to deallocate memory reserved for arrays

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Employee::~Employee(){ cout << "~Employee() called for " << firstName << ' ' << lastName << endl;

delete[] firstName; delete[] lastName;

count--;}

static.cpp (1/2)

Calling static member function using class name and binary scope resolution operator

Calling a static member function through a pointer to an object returns the value of the static variable Same as getting the value of Employee::count or calling Employee::getCount()

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cout << "Number of employees before instantiation of any objects is " << Employee::getCount() << endl;

Employee* e1Ptr = new Employee( "Susan", "Baker" );Employee* e2Ptr = new Employee( "Robert", "Jones" );

cout << "Number of employees after objects are instantiated is “ << e1Ptr->getCount();

static.cpp (2/2)

Even when no object exists, we can still call static member function getCount()

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cout << "\n\nEmployee 1: " << e1Ptr->getFirstName() << " " << e1Ptr->getLastName() << "\nEmployee 2: " << e2Ptr->getFirstName() << " " << e2Ptr->getLastName() << "\n\n";

delete e1Ptr; e1Ptr = 0; // e1Ptr = NULL;delete e2Ptr; e2Ptr = 0;

cout << "Number of employees after objects are deleted is " << Employee::getCount() << endl;

static.cpp Sample Output

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Number of employees before instantiation of any objects is 0Employee constructor for Susan Baker called.Employee constructor for Robert Jones called.Number of employees after objects are instantiated is 2 (same as calling Employee::getCount() = 2) Employee 1: Susan BakerEmployee 2: Robert Jones ~Employee() called for Susan Baker~Employee() called for Robert JonesNumber of employees after objects are deleted is 0

Constant Static Variable

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#include <iostream>

using namespace std;

class F {public: static int getcount(); // static member function cannot have `const' method qualifierprivate: const static int count;};

// initialization of constant static variable: must be here; not in main()const int F::count = 2;

int F::getcount() { cout << count;}

int main() { F::getcount(); // print out 2 F::getcount(); // print out 2 cout << F::count; // wrong as 'const int F::count' is private return 0;}

Constructors with Member Initializers

How to initialize a ‘const’ private member?

const data member increment must be initialized using a member initializer

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class Increment {public: Increment(int c=0, int i=1); void addIncrement(){ count += increment; } void print() const; // prints count and incrementprivate: int count; const int increment; // const data member};

Colon (:) marks the start of a member initializer list c is the initial count, increment is the increment step Member initializer for non-const member count Required member initializer for const member increment Not providing a member initializer for a const data member is

a compilation error See Increment.h, Increment.cpp and const2.cpp

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Increment::Increment(int c, int i) : count(c), // initializer for non-const member increment(i) // required initializer for const member { // empty body}

Constructors with Member Initializer

Required for initializing ‘reference’ data members ‘const’ data members

Member initializer list Appears between a constructor’s parameter list and the left brace that begins the

constructor’s body Separated from the parameter list with a colon (:) Each member initializer consists of the data member name followed by

parentheses containing the member’s construction and its initial value Multiple member initializers are separated by commas Executes before the body of the constructor executes

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X::X(parameter_list) : member_initializer_list{

// body of constructor definition}

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class F {public: F() : i(j), m(3), k(m), j(4) { cout << i << j << k << m << endl; }private: const int& i; const int j; // ANSI C++ cannot have const int j = 4; int& k; int m; // ANSI C++ cannot have int m = 3;};

class F {public: F() : i(j), k(m), j(4) { m=3; cout << i << j << k << m << endl; }private: const int& i; const int j; int& k; int m;};

class F {public: F() : i(j), k(m) { m=3; j = 4; // compiler complains: assignment of read-only member `F::j' cout << i << j << k << m << endl; }private: const int& i; const int j; int& k; int m;};

OK OK

NOT OK

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Examples of member initialization

Example: Time Class

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Information hiding (Time.h and Time.cpp) Two types of constructors

class Time { public: Time(); Time(unsigned initHours, unsigned initMinutes, char initAMPM);

void set(unsigned hours, unsigned minutes, char am_pm);

void display(ostream& out) const; ... private: unsigned myHours, myMinutes; char myAMorPM; // 'A' or 'P' unsigned myMilTime; // military time equivalent};

Default Constructor

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Time::Time() : myHours(12), myMinutes(0), myAMorPM('A'), myMilTime(0){

// void}

mealTime

myHours 12

myMinutes 0

myAMorPM A

myMilTime 0

Time mealTime = Time();

Explicit-Value Constructor

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Time::Time(unsigned initHours, unsigned initMinutes, char initAMPM){ set(initHours, initMinutes, initAMPM); //a member function}

bedTime

myHours 11

myMinutes 30

myAMorPM P

myMilTime 2330

Time bedTime = Time(11,30,’P’);

(a random value if myMilTime is not set in set())

Constructors with Default Arguments

Constructors can specify default arguments Can initialize data members to a consistent state

Even if no values are provided in a constructor call

Constructor that defaults all its arguments is also a default constructor Can be invoked with no arguments Maximum of one default constructor per class

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Possible to specify default values for constructor arguments

t1

myHours 12

myMinutes 0

myAMorPM A

myMilTime 0

t2

myHours 5

myMinutes 0

myAMorPM A

myMilTime 500

t3

myHours 6

myMinutes 30

myAMorPM A

myMilTime 630

t4

myHours 8

myMinutes 15

myAMorPM P

myMilTime 2015

Time(unsigned initHours = 12, unsigned initMinutes = 0, char initAMPM = 'A');

Time t1, t2(5), t3(6,30), t4(8,15,'P');

Copy Operations

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During initialization

Time t = bedTime;

During assignment

t = midnight;

bedTime

myHours 11

myMinutes 30

myAMorPM P

myMilTime 2330

t

myHours 11

myMinutes 30

myAMorPM P

myMilTime 2330

midnight

myHours 12

myMinutes 0

myAMorPM A

myMilTime 0

t

myHours 12

myMinutes 0

myAMorPM A

myMilTime 0

Same as: Time t(bedTime); and calls ‘copy constructor’.

‘assignment’, by default, memberwise copy of the left into the right object.

Other Class Operations

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Accessors: "get" functions

unsigned Time::getMinutes() const {return myMinutes;

}

unsigned Time::getHours() const { return myHours;}

unsigned Time::getAMPM() const { return myAMorPM;}

unsigned Time::getMilTime() const { return myMilTime;}

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Mutators: "set" functions

void Time::set(unsigned hours, unsigned minutes, char am_pm){ // Check class invariant if (hours >= 1 && hours <= 12 && minutes >= 0 && minutes <= 59 && (am_pm == 'A' || am_pm == 'P')) { myHours = hours; myMinutes = minutes; myAMorPM = am_pm; myMilTime = toMilitary(hours, minutes, am_pm); } else cerr << "*** Can't set time with these values ***\n"; }

Display and Operator Overloading: do it later …

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Explicit constructors (not explicit-value constructor

Initialization T t = v; // assignment syntax

Where v is of type V, the t is initilized by calling the constructor T::T(V v).

T t(v); // explicit syntax If v is not of type T, convert v to a temporary T object temp_t; Initialize t using the copy constructor T::T(const T&) with temp_t

as argument.

In most cases, the compiler may optimize such that the effect of the two is the same. This is the case, e.g. if there exisit a (non-explicity) constructor T::T(V), where v has type V.

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class IntCell{public:

explicit IntCell(int initialValue = 0) : storedValue(initialValue) {}

int read( ) const {return storedValue;}void write(int x) {storedValue = x;}

private:int storedValue;

}

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Avoid implicit type conversion

An explicit constructor will be invoked only explicitly. Where a copy constructor is needed, an explicit constructor will not be implicitly invoked.

main(){ int x = 4; // same as int x(4); IntCell z(5); // now 5 (call explicit-value constructor) IntCell t; // now 0 (call default constructor) IntCell u = IntCell(x); // now 4 (call explicit-value constructor, // then copy constructor) IntCell y = x; // invalid implicit conversion: y = IntCell(x)…}

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Try to implicitly call IntCell (x), which is ‘explicit’!

If no ‘explicit’ keywork, IntCell y = x; is OK.

Objects as class members

Objects as Members of Classes A class can have objects of other classes as members

Sometimes referred to as a ‘has-a’ relationship Example: Date.h, Date.cpp, Employee.h, Employee.cpp and

composition.cpp

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class Employee{public: Employee(const char *const, const char *const, const Date&, const Date&); ~Employee();

void print() const;private: char firstName[25]; char lastName[25]; const Date birthDate; const Date hireDate; };

const char *const: see ‘pointer’ slides.

Employee’s constructor

Member initializers pass arguments to Date’s implicit copy constructor (equivalent to const Date birthDate = dateOfBirth;)

A compilation error occurs if a const member object is not initialized with a member initializer in the constructor

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Employee::Employee(const char *const first, const char *const last, const Date& dateOfBirth, const Date& dateOfHire) : birthDate(dateOfBirth), hireDate(dateOfHire){ ...}

Initializing member objects Member initializers pass arguments from the object’s constructor to member-object constructors Before the enclosing class object (host object) is constructed If a member initializer is not provided, the member object’s default constructor will be called

implicitly

Object reference and self-reference: the this pointer

Pointers to Class Objects

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Possible to declare pointers to class objects

Access with

or

Call delete to free the memory

t

myHours 12

myMinutes

0

myAMorPM

A

myMilTime

0

timePtr

Time* timePtr = &t;

Time* timePtr = new Time(12, 0, ‘A’, 0);

timePtr->getMilTime()

(*timePtr).getMilTime()

delete timePtr; // call destructor

A Member Function Returning a Reference

We can have a member function which returns a reference. For example, if a member function returns an integer reference, there are 4

possibilities. int& f();

This is for non-constant objects. It returns an integer reference and hence can be subsequently changed.

E.g., for a non-constant object ncfoo, we can call ncfoo.f() = 10; or i = ncfoo.f(); const int& f();

This is for non-constant objects. It has to be a rvalue. i = ncfoo.f(); // good ncfoo.f() = 10; // wrong: compilation error

const int& f() const; This is for both constant and non-constant objects (constant object can call it only). It returns a constant

reference and hence can only be rvalue. i = cfoo.f(); // good; or i= ncfoo.f(); cfoo.f() = 10; //wrong; and nor ncfoo.f() = 10;

int& f() const; This returns a reference which can be a lvalue. However, because it can be called by a constant object

(which should never be a lvalue), this should not be used. Therefore, you can have

Either first or second for non-constant objects depending on what you want on the return value; and The third one for constant objects The compiler will make the call depending on whether the object is constant or not.

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The this Pointer

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Every class has a keyword, this a pointer whose value is the address of the object Value of *this would be the object itself

Function members

Data members

Class Object

this

*this

Using the this Pointer when you want to return the modified object:

Every object has access to its own address through a pointer called this (a C++ keyword)

Objects use the this pointer implicitly or explicitly Implicitly when accessing members directly Explicitly when using keyword this Type of the this pointer (i.e., whether it can be modified or not) depends on the

type of the object and whether the executing member function is declared const

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F& F::f(){ // … return *this;}

Members of the class, not that of the objects member data, non-static, so far belong to ‘objects’ or ‘instances’ of the class member data, static, are ‘variables’ of the class, not the objects.

Summary: static and non-static members

class A {…int l;static int g;

}

A::g

A a,b,c,d, … ;

a.lb.lc.ld.l (multiple copies)

a.g = b.g = c.g = d.g = … = A::g (one copy)

Have to use ‘member initializers’ for ‘reference’ data members ‘const’ data members

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X::X(…) : …{ …}

General form of ‘constructors’

class A {…int& r;const int c;

}

Separate compilation

Interface and Implementation

In C++ it is more common to separate the class interface from its implementation. Abstract data type

The interface lists the class and its members (data and functions).

The implementation provides implementations of the functions.

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What to do?

How to do?

Interface Describes what services a class’s clients can use and how to

request those services But does not reveal how the class carries out the services A class definition that lists only member function names, return types

and parameter types Function prototypes

A class’s interface consists of the class’s public member functions (services)

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Separate File for Reusability

.cpp source-code files .h header files

Separate files in which class definitions are placed Allow compiler to recognize the classes when used elsewhere Generally have .h filename extensions

Driver files Program used to test software (such as classes) Contains a main function so it can be executed

See GradeBook4.h and Gradebook4.cpp

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#include preprocessor directive

#include "GradeBook.h" include header files

Instructs C++ preprocessor to replace directive with a copy of the contents of the specified file

Quotes for user-defined header files Preprocessor first looks in current directory If the file is not found, looks in C++ Standard Library directory

Angle brackets for C++ Standard Library Preprocessor looks only in C++ Standard Library directory #include <iostream>

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class IntCell {

public:

explicit IntCell(int initialValue = 0 );

int read( ) const;

void write( int x );

private:

int storedValue;

}

IntCell::IntCell(int initialValue)

: storedValue(initialValue)

{ }

int IntCell::read( ) const

{return storedValue;}

void IntCell::write(int x)

{storedValue = x;}

The interface is typically placed in a file that ends with .h. The implementation file typically ends with .cpp, .cc, or .C.

IntCell.h IntCell.cpp

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Separate Class Interface from Implementation

Client code should not break if the implementation changes, as long as the interface stays the same

Define member functions outside the class definition, in a separate source-code file

In source-code file for a class Use binary scope resolution operator (::) to “tie” each member function

to the class definition

Implementation details are hidden Client code does not need to know the implementation

In the header file for a class Function prototypes describe the class’s public interface

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Class Libraries

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Class declarations placed in header file Given .h extension Contains data items and prototypes

Implementation file Same prefix name as header file Given .cpp extension

Programs which use this class library called client programs

Reality: the separation is not perfect

Header files do contain some portions of the implementation and hint about others

private members are listed in the class definition in the header file

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Typical C++ Development Environment

Edit Programmer writes program (and

stores source code on disk) Preprocess

Perform certain manipulations and file I/O before compilation

Compile Compiler translates C++ programs into

machine languages in object codes Link

Link object codes with missing functions and data

Load Transfer executable image to memory

Execute Execute the program one instruction at

a time

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The Compilation and Linking Process Source-code file is compiled to

create the class’s object code (source-code file must #include header file) Class implementation

programmer only needs to provide header file and object code to client

Client must #include header file in their own code So compiler can ensure that the

main function creates and manipulates objects of the class correctly

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Translating a Library

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Program Source File

Library Header File

Library Implementation File

C++ Compiler

C++ Compiler

Linker

Program Object File

Library Object File

Program Executable File

g++ -c

g++ -c

.o

.o e.g., g++ foo.cpp bar.o fb.o

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#include <iostream>#include "library.h"using namespace std;int TestInt=99;

int main(){ cout << "Hello"<<endl; cout << functionA(100) << endl; return 0;}

#include "library.h"

int functionA( int i ){ return TestInt* i;}

#ifndef ABC#define ABC

int TestInt = 99;

int functionA( int ); #endif

> g++ main.cpp source.cppld: fatal: symbol `TestInt' is multiply-defined: (file /var/tmp/ccvkmxE2.o type=OBJT; file /var/tmp/ccgj1SDu.o type=OBJT);ld: fatal: File processing errors. No output written to a.outcollect2: ld returned 1 exit status

main.cppsource.cpp

--------------- extern int TestInt;

Output:Hello9900

library.h

Why #ifndefine #define #endif Statement?

It is ok to have multiple declarations of a function prototype, but not for its definition In the .h file, put the prototypes there .h files are likely to be multiply-included

In creating the .o file, there may be nested #include statement The nested #include statement may be recursive

In main.cpp, #include “foo.h” In foo.h, #include “bar.h” In bar.h, #include “foo.h”

To break the infinite “recursive” inclusion, use #ifndefine #define to define a “variable” in the compilation process of .o file

If a variable has been defined, the compiler will skip the code segment between #ifndefine and #endif.

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Data Integrity Data integrity are not automatic by putting data members

as private The programmer must provide appropriate validity checking and

report the errors Member functions that set the values of private data

should verify that the intended new values are proper They should place the private data members into an appropriate

state set functions can be used to validate data besides simply

setting the value Known as validity checking Keeps object in a consistent state

The data member contains a valid value Can return message indicating that attempts were made to assign

invalid data

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Gradebook6.h (with Validity Checking)

Same as Gradebook4.h, but with the interface, implementation and driver separated into three files

Interface contains data members and member function prototypes only Note the #ifndef statements to prevent multiple inclusions 59

#ifndef GRADEBOOK_H #define GRADEBOOK_H

class GradeBook{public: GradeBook( string ); void setCourseName( string ); string getCourseName(); void displayMessage();

private: string courseName;};

#endif

Gradebook6.cpp

set functions perform validity checking to keep courseName in a consistent state

GradeBook implementation is placed in a separate file Include the header file to access the class name GradeBook Binary scope resolution operator :: “ties” a function to its class 60

#include "GradeBook6.h" // include definition of class GradeBook

void GradeBook::setCourseName( string name ){ // if name has 25 or fewer characters if ( name.length() <= 25 ) courseName = name;

// if name has more than 25 characters if ( name.length() > 25 ) { // set courseName to first 25 characters of parameter name courseName = name.substr( 0, 25 );

cout << "Name \"" << name << "\" exceeds maximum length (25).\n" << "Limiting courseName to first 25 characters."; }}

driver6.cpp (1/2)

Include the header file to use the class GradeBook

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#include "GradeBook6.h" // include definition of class GradeBook

int main(){ // initial course name of gradeBook1 is too long GradeBook gradeBook1( "COMP104 Introduction to Programming in C++" ); GradeBook gradeBook2( "COMP152 OOP and Data Structures" ); cout << "gradeBook1's initial course name is: " << gradeBook1.getCourseName() << "\ngradeBook2's initial course name is: " << gradeBook2.getCourseName() << endl;

driver6.cpp (2/2)

Call set function to perform validity checking directly In Linux, compile them all together using g++ Gradebook6.cpp driver6.cpp Or using object files: g++ -c Gradebook6.cpp; g++ -c driver6.cpp; g++ driver6.o driver6.o

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// modify myGradeBook's courseName (with a valid-length string) gradeBook1.setCourseName( "COMP104 C++ Programming" );

// display each GradeBook's courseName cout << "\ngradeBook1's course name is: " << gradeBook1.getCourseName() << "\ngradeBook2's course name is: " << gradeBook2.getCourseName() << endl; return 0;}

driver6.cpp Sample Output

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Name "COMP104 Introduction to Programming in C++" exceeds maximum length (25).Limiting courseName to first 25 characters.gradeBook1's initial course name is: COMP104 Introduction to PgradeBook2's initial course name is: COMP152 OOP and Data StrugradeBook1's course name is: COMP104 C++ ProgramminggradeBook2's course name is: COMP152 OOP and Data Stru