Expert: Prince M. Premnath - 10/24/2006

Question
sir i wanna know about the concept of virtual pointer..?
It has been asked in interviews as well as a new concept for me...! i m working in C/C++ environment since last 6 years.so i have a good relation with C/C++.Please rectify my doubts...?

Answer
 
Dear Neeraj!

 I'm not sure to what you are referring.( may be pointer to virtual functions) if so follow the answer !

In order to achieve run time polymorphism its necessary to use a single pointer that can refer object of different classes  , Here we use the pointer to BASE Class to refer to all the derived objects !

How to do that ?

Use same function name in both base and derived classes
The function in the base class is declared as virtual using "virtual" key word
Upon doing this The C   Compiler determine which function to be executed during runtime based on the object pointed by the base pointer rather than the type of pointer ,  by  making pointing to different objects we can invoke different version of virtual functions.

Eg:
 class B // Class Base
 {
   void d(){ cout<<"Display base D"; }
   virtual void c(){ cout<<"Display  base C"; }
 };

 class D : public B // Derived class
 {
   void d() { cout<<" Display derived D" ; }
   void c() { cout<<" Display derived C" ; }
 };

 void main()
{

   B obj1;
   D obj2;
   B* bptr ;
   cout<<" Base pointer pointing to base :";
   bptr = &B; // ASSIGN BASE ADDRESS
   bptr-> d();// INVOKES BASE
   bptr -> c(); // INVOKES BASE

   bptr = &D;// ASSIGN DERIVED CLASS ADDRESS
   bptr-> d();// INVOKE BASE
   bptr -> c();// INVOKE DERIVED
}



Regards!
Prince M. Premnath