#include <iostream>

class A
{
 protected:
  virtual void do_thing_impl() = 0;
 public:
  virtual ~A(){}
  static void do_thing(A * _ptr){ _ptr->do_thing_impl(); }
};

class B : public A
{
 protected:
  void do_thing_impl(){ std::cout << "B impl" << std::endl; }
};

class C : public A
{
 protected:
  void do_thing_impl(){ std::cout << "C impl" << std::endl; }
};

int main() 
{
 B b_;
 C c_;

 A::do_thing(&b_);
 A::do_thing(&c_);	
 
 return (0);
}