TITLE: Callbacks and multiple inheritance, part IV

This example is based upon "Subobject Members", Stephen Dewhurst, 
C++ Report, V5 N3. This is part 4 of a multipart example.

The final part of this example is shown below:

.................................................................

class Engine {
    // ...
  public:
    void start ();
    void stop  ();
  private:
    
    class StartButton : public Button {
	void press ();
    } start_b;
    friend StartButton;

    class StopButton : public Button {
	void press ();
    } stop_b;
    friend StopButton;
};

void Engine :: StartButton :: press () 
{
    C_addr (Engine, start_b)->start ();
}

void Engine :: StopButton :: press () 
{
    C_addr (Engine, stop_b)->stop ();
}

.................................................................

This solution relies on the "offsetof" facility defined in stddef.h.
An appropriate define would be:

#define C_addr(C, M) ((C*)(((char*)this)-offsetof(C, M)))

"An important feature of this solution is that, unlike the previous
solution, the [...] members do not require explicit initialization
from the containing object".