#include <iostream> 

using namespace std;

struct A {
  virtual void invoke() { std::cout << "Goodbye from A" << endl;}
};

struct Invoker {
  Invoker (A *a) { a->invoke();};
};

class B: A, Invoker {
public:
  B() : A(), Invoker(this) {}
  void invoke() { cout << "Hello World from B" << endl;}
};

class C: A, Invoker {
public:
  C() : Invoker(this), A() {}  
  void invoke() { cout << "Hello World from C" << endl;}
};

// this undefined becuase A::invoke is called, before A is built
class D: Invoker, A {
public:
  D() : Invoker(this), A() {}  
  void invoke() { cout << "Hello World from D" << endl;}
};



int main() {
  B b;

  C c;  

  b.invoke();
  c.invoke();

  cout << "creating D is undefined becuase A::invoke is called, before A is built" << endl;
  cout << "BOMBS AWAY!!!" << endl;
  D d;
  d.invoke();

  return 0;
}