Virtual Function and Polymorphism in C++.

Virtual Function and Polymorphism

• Virtual functions are used in C++ to support polymorphic behavior. 
• We are modifying the above program and will introduce you the concept of virtual function by following example:

#include <iostream.h>

#include <conio.h>

class Shape

{

  protected:

    double width, height;

  public:

    void set_data (double a, double b)

    {

        width = a;

        height = b;

    }

    virtual double area()

    {return 0;}

}; 

class Rectangle: public Shape

{

  public:

    double area ()

    {

        return (width * height);

    }

}; 

int main ()

{

    Shape *sPtr;

    Rectangle Rect;

    sPtr = &Rect; 

    sPtr -> set_data (5,3);

    cout << sPtr -> area() << endl;

    return 0;

}

Expected Output :-

15

• A member of a class that can be redefined in its derived classes is known as a virtual member. 
• In order to declare a member of a class as virtual, we must precede its declaration with the keyword virtual. 
• The member function area() has been declared as virtual in the base class because it is later redefined in each derived class. 
• The advantage of having virtual function is that we are able to access area function of derived class by pointer variable of base class.

Pure Virtual Function and Abstract Class

• In above example, base class Shape member function area do not need any implementation because it is overriding in derived class. 
• If this is the case, the C++ language permits the programmer to declare the function a pure virtual function. 
• The C++ way of declaring a pure virtual function is to put the expression = 0 in the class declaration. 
• For example, if a member function double area() is being declared pure virtual, then its declaration in its class looks like

virtual double area() = 0;

• A pure virtual function is sometimes called an abstract function, and a class with at least one pure virtual function is called an abstract class. 
• The C++ compiler will not allow you to instantiate an abstract class. 
• Abstract classes can only be sub-classed: that is, you can only use them as base classes from which to derive other classes.
• A class derived from an abstract class inherits all functions in the base class, and will itself be an abstract class unless it overrides all the abstract functions it inherits. 
• The usefulness of abstract classes lies in the fact that they define an interface that will then have to be supported by objects of all classes derived from it.

#include <iostream.h>

#include <conio.h>

class Shape

{

  protected:

    double width, height;

  public:

    void set_data (double a, double b)

    {

        width = a;

        height = b;

    }

    virtual double area() = 0;

}; 

class Rectangle: public Shape

{

public:

    double area ()

    {

        return (width * height);

    }

}; 

class Triangle: public Shape

{

public:

    double area ()

    {

        return (width * height)/2;

    }

}; 

int main ()

{

    Shape *sPtr;

    Rectangle Rect;

    sPtr = &Rect;

    sPtr -> set_data (5,3);

    cout << "Area of Rectangle is " << sPtr -> area() << endl; 

    Triangle Tri;

    sPtr = &Tri;

    sPtr -> set_data (4,6);

    cout << "Area of Triangle is " << sPtr -> area() << endl;

    return 0;

}

Expected Output :-

Area of Rectangle is 15

Area of Triangle is 12