Virtual Function is a function that is declared within a base class and redefined in the derived class. Virtual functions are declared by preceding the class declaration with a keyword "virtual". When a virtual function is declared C++ decides to execute a function based on the type of object pointed by the base pointer and not on the type of pointer.

#include <iostream.h>
class Bclass
  {
    public:
       void  disp() { cout  << " BASE BASE\n" ; }
       virtual void sh() { cout << "base base\n"; }
  };
class Dclass : public Bclass
  {
    public:
      void disp() { cout << "DERIVED DERIVED\n"; }
      void sh() { cout << "derived derived\n"; }
  };
int main()
  {
     
     Bclass B;
     Dclass D;
     Bclass *ptr;
     
     cout << "ptr points to base class\n" ;
     ptr =  &B;
     ptr->disp();
     ptr->sh();
   
     cout << "ptr points derived class\n";
     ptr = &D;
     ptr->disp();
     ptr->sh();
     return 0;
  }

   ptr points to base class
   BASE BASE
   base base
   ptr points derived class
    BASE BASE
   derived derived

In the above example, the base ptr is used point the object D. Since the "sh()" function has a derived version in the form of a virtual function, it displays the base class version of "sh()".