/* here ,a base class reference is used 
/* to access a vitual  function.

#include  < iostream >
class base {
public: 
       virtual void vfunc ( ) { 
           cout <<  "this is base's vfunc ( ). \n";
       }
};
class derived1 : public base { 
public:
       void vfunc ( ) { 
           cout << " this is derived1's vfunc ( ) .\n " ;
       }
};
class derived2 :public base {
public:
       void vfunc ( ) {
           cout << "this is derived2's vfunc ( ) .\n ";       
       }
};
//use a base class reference parameter .
void  f (base &r) {
       r.vfunc ( );
}
int main ( ) 
{       
       base b;
       derived1 d1;
       derived2 d2;
       f(b); //pass a base object to f ( ) .
       f(d1); //pass a derived1 object to f ( ).
       f(d2); //pass a derived2 object to f ( ).
       return  0;
}

 
#include  < iostream >
class base {
public: 
       virtual void vfunc ( ) { 
           cout <<  "this is base's vfunc ( ). \n";
       }
};
class derived1 : public base { 
public:
       void vfunc ( ) { 
           cout << " this is derived1's vfunc ( ) .\n " ;
       }
};

//derived2 inherits  virtual function vfunc ( )
// from derived1.
class derived2 :public derived1 {
public:
//vfunc ( ) is still virtual 
       void vfunc ( ) {
           cout << "this is derived2's vfunc ( ) .\n ";
       }
};

//use a base class reference parameter .
void  f (base &r) {
       r.vfunc ( );
}
int main ( ) 
{
       base  *p , b;
       derived1 d1;
       derived2 d2;
// point to base 
       p =&b;
       p->vfunc ( );//access base's vfunc ( )
//point to derived1
       p =&d1;
       p->vfunc ( ); //access derived1's vfunc ( )
//point to derived2
       p =&d2;
       p->vfunc ( ) ;//access derived2's vfunc ( )
       return 0 ;

This is base's vfunc ( ).
This is derived1's vfunc ( ).
This is derived2's vfunc ( ).