#include <iostream>
using namespace std;

class A{
public:
	A() { cout<<"A ctor"<<endl; }
	virtual ~A()=0 { cout<<"A dtor"<<endl; }
	virtual void f(A*) { cout<<"A's f"<<endl; }
};

class B: public A{
public:
	B():A() { cout<<"B ctor"<<endl; }
	virtual ~B() { cout<<"B dtor"<<endl; }

	virtual void f(A*) {cout<<"B's f"<<endl;}
};

class C: public A{
public:
	C():A() { cout<<"C ctor"<<endl; }
	virtual void f(A*) {cout<<"C's f"<<endl;}
	virtual ~C() { cout<<"C dtor"<<endl; }
};

int main()
{
	//A *ap= new A(); - compilation error
	A *bp= new B();
	A *cp= new C();

	bp->f(cp);

	delete bp;
	delete cp;

	return 0;
}

OUTPUT:

A ctor
B ctor
A ctor
C ctor
B's f
B dtor
A dtor
C dtor
A dtor