#include <iostream>

class A {
public:
  virtual void f();
};

class B : public A {
public:
  void f();
};

void A::f() {
  std::cout << "this is A::f" << "\n";
}

void B::f() {
  std::cout << "this is B::f" << "\n";
}

int main() {
  A *a = new A();
  a->f();
  a = new B();
  a->f();
}