Virtual function

#include<iostream>
using namespace std;
class P1{
public:
    virtual void display(){
    cout<<"Count the valu"<<endl;
    }
    };
    class P2: public P1{
        void display(){
        cout<<"Count the second value"<<endl;
        }
   };
   class P3:public P1{
   void display(){
   cout<<"Count value"<<endl;
   }
   };
   int main(){
   P1 *p;
   P2 q;
   P3 r;
   p=&q;
   p->display();
   p=&r;
   p->display();
   }