class A
{
public:
  virtual void a() { cout << "A" << endl; }
  virtual void a2();
};
 
class B : public A
{
public:
  void a() { cout << "B" << endl; }
  virtual void a2();
};
 
A  a, * pA;  B  b, * pB;
pA = &a;
pB = &b;
 
pA->a();
pB->a();
pA = & b;
pA->a();