class P;
  class C;
  
  void f(P* p) { cout << "f(P*)"; } 
 (1)
  void f(C* c) { cout << "f(C*)"; } 
 (2)
  
  class P {
  public:
  virtual void f() { cout << "P::f"; }
 (3)
  };
  
  class C : public P {
  public:
  virtual void f() { cout << "C::f"; } 
 (4)
  };