#include <iostream>

using std::cout;
using std::endl;

struct X     { virtual void f() { cout << "X::f()" << endl; } };
struct Y : X { virtual void f() { cout << "Y::f()" << endl; } };

int main()
{
	Y* y = new Y();
	X* x = y;

	x->f(); // ohne virtual in X: X::f()
	y->f();
}