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By ILTAF MEHDI, IT LECTURER, MIHE, KATR-E-PARWAN BRANCH, KABUL

The word polymorphism is a combination of two words poly and Morphism.

Poly means many and Morphism means form. In OOP polymorphism is the ability of objects of different types of respond to functions of the same name. the user does not have know the exact type of the object in advance.

The behavior of the object can be implemented at the run time. It is called late binding or dynamic binding. Polymorphism is implemented by using virtual function.

It is one of the outstanding feature of OOP.

In it objects of different classes related by inheritance receive a same message (function call) and respond differently.

One interface & multiple methods. One thing having different shapes

A pointer can also refers to an object of a class. The member of an object can be accessed through pointers by the symbol ->

The saymbol is known as member access operator.

Syntax ; ptr -> member

#include <iostream.h> #include <conio.h> class Test { private: int *n; public: void in(void); { cout <<“enter number”;

cin>>n; }

void out(void);

{

cout <<“the value of n: “<<n<<endl;

cout <<“address”<<&n<<endl;

} };

Void main(void) { clrscr(); Test *ptr; ptr = new Test; ptr -> in(); ptr->out(); getch(); }

POLYMORPHISM

class drawing{public:void draw() { ----------}};

Class ball:public drawing{ public: ---------- void draw() { --------} };

Class ball:public drawing{ public: ---------- void draw() { --------} };

void main(){ball objball;rect objrect;P=&objball; pdraw(); pdraw();P=&objrect; pdraw(); pdraw();}

Class rect:public drawing{ public: void draw() {--------} };

Class rect:public drawing{ public: void draw() {--------} };

EARLY BINDING

class drawing{public:virtual void draw()=0 { ----------}};

class drawing{public:virtual void draw()=0 { ----------}};

Pure Virtual Function

LATE BINDING

ABSTRACT CLASS A class having one or more pure virtual function is called

Abstract class. Object of the abstract class can not be created .

CONCRETE CLASS

A class having no pure virtual function is called Concrete class.

Object of the concrete class can be created .

In principle, private and protected members of a class cannot be accessed from outside the same class in which they are declared. However, this rule does not affect friends.

Friends are functions or classes declared as such.

If we want to declare an external function as friend of a class, thus allowing this function to have access to the private and protected members of this class, we do it by declaring a prototype of this external function within the class, and preceding it with the keyword friend.

#include <iostream>

class CRectangle {

int width, height;

public:

void set_values (int, int);

int area () {return (width * height);}

friend CRectangle duplicate (CRectangle);

};

void CRectangle::set_values (int a, int b)

{

width = a;

height = b;

}

CRectangle duplicate (CRectangle rectparam){CRectangle rectres;rectres.width = rectparam.width*2;rectres.height = rectparam.height*2;return (rectres);}int main () {CRectangle rect, rectb;rect.set_values (2,3);rectb = duplicate (rect);cout << rectb.area();return 0;}

#include <iostream.h> class CSquare; class Crectangle { int width, height; public: int area () { return (width * height);} void convert (CSquare a); }; class CSquare { private: int side; public: void set_side (int a) {side=a;} friend class CRectangle; };

void CRectangle::convert (CSquare a) { width = a.side; height = a.side;}int main () { CSquare sqr; Crectangle rect; sqr.set_side(4); rect.convert(sqr); cout << rect.area();return 0;}

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Class Interface Diagram

Protected data:

hrs

mins

secs

ExtTime class

Set

Increment

Write

Time

Time

Set

Increment

Write

ExtTime

ExtTime

Private data:zone

Time class

When the type of a formal parameter is a parent class, the argument used can be:

the same type as the formal parameter,

or,

any derived class type.

Static binding is the compile-time determination of which function to call for a particular object based on the type of the formal parameter

When pass-by-value is used, static binding occurs

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Is the run-time determination of which function to call for a particular object of a derived class based on the type of the argument

Declaring a member function to be virtual instructs the compiler to generate code that guarantees dynamic binding

Dynamic binding requires pass-by-reference

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// SPECIFICATION FILE ( time.h )

class Time{

public :

. . .

virtual void Write ( ) ; // for dynamic bindingvirtual ~Time(); // destructor

private :

int hrs ; int mins ; int secs ;

} ;

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void Print (Time * someTime ) {

cout << “Time is “ ;someTime->Write ( ) ;cout << endl ;

}

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CLIENT CODE

Time startTime( 8, 30, 0 ) ; ExtTime endTime(10, 45, 0, CST) ;

Time *timeptr;timeptr = &startTime;Print ( timeptr ) ;

timeptr = &endTime;Print ( timeptr ) ;

OUTPUT

Time is 08:30:00 Time is 10:45:00 CST

Time::write()

ExtTime::write()

Virtual Functions overcome the problem of run time object determination

Keyword virtual instructs the compiler to use late binding and delay the object interpretation

How ? Define a virtual function in the base class. The word virtual appears

only in the base class If a base class declares a virtual function, it must implement that

function, even if the body is empty Virtual function in base class stays virtual in all the derived classes It can be overridden in the derived classes But, a derived class is not required to re-implement a virtual

function. If it does not, the base class version is used

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Some classes exist logically but not physically. Example : Shape

Shape s; // Legal but silly..!! : “Shapeless shape” Shape makes sense only as a base of some classes derived from it.

Serves as a “category” Hence instantiation of such a class must be prevented

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class Shape //Abstract { public : //Pure virtual Function virtual void draw() = 0;}

A class with one or more pure virtual functions is an Abstract Class

Objects of abstract class can’t be

createdShape s; // error : variable of an abstract class

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Shape

virtual void draw()

Circle

public void draw()

Triangle

public void draw()

A pure virtual function not defined in the derived class remains a pure virtual function.

Hence derived class also becomes abstract

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class Circle : public Shape { //No draw() - Abstractpublic :void print(){ cout << “I am a circle” << endl;}

class Rectangle : public Shape {public :void draw(){ // Override Shape::draw() cout << “Drawing Rectangle” << endl;}Rectangle r; // Valid

Circle c; // error : variable of an abstract class

When you use virtual functions, compiler store additional information about the types of object available and created

Polymorphism is supported at this additional overhead Important :

virtual functions work only with pointers/referenceswith objects even if the function is virtual If a class declares any virtual methods, the destructor of the

class should be declared as virtual as well.

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Pure virtual functions are useful because they make explicit the abstractness of a class

Tell both the user and the compiler how it was intended to be used

Note : It is a good idea to keep the common code as close as possible to the root of you hierarchy

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It is still possible to provide definition of a pure virtual function in the base class

The class still remains abstract and functions must be redefined in the derived classes, but a common piece of code can be kept there to facilitate reuse

In this case, they can not be declared inline

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class Shape { //Abstract public : virtual void draw() = 0;};

// OK, not defined inline void Shape::draw(){ cout << “Shape" << endl;}

class Rectangle : public Shape

{ public : void draw(){ Shape::draw(); //Reuse cout <<“Rectangle”<< endl;}