Pure Virtual Function

Pure Virtual Function

• Definition : A pure virtual function is a virtual function which is to be implemented by derived class. The class that contains the pure virtual function is called the abstract class.

• The pure virtual functions are declared using pure specifier i.e. = 0

• Following program demonstrate the use of pure virtual function.

• The class A is an abstract class in which the pure virtual function Display() is declared. This function is overridden by the two derived classes ‒ class B and class C.

/**************************************************************************

Program for demonstrating the pure virtual function

**************************************************************************/

#include<iostream>

using namespace std;

class A

{

public:

   virtual void Display()=0;          // Pure Virtual Function

};

class B: public A

{

public:

void Display()

{

   cout<<"\n In Derived Class B"<<endl;

}

 };

class C: public A

{

public:

void Display()

{

   cout<<"\n In Derived Class C"<<endl;

}

};

int main()

{

   A* p

   B b;

   C c;

    p = &b;

   p‒>Display(); //using pointer the method of derived class is accessed

   p = &c;

   p‒>Display();

   return 0;

}

Output

In Derived Class B

In Derived Class C

Program Explanation :

• The base class A derived two classes namely ‒ B and C. In class A, the pure virtual function Display is defined by assigning = 0.

• The Display function is overridden in class B and class C.

• In main function, the pointer object to the base class is declared. It is denoted by variable p. By assigning the address of class B to the pointer variable p, the function Display of class B is invoked. Similarly by assigning the address of class C the Display function of class C is invoked.

Review Question

1. What is called pure virtual function ?