Question

I have a constructor for a window in my project setup such that it looks like this. There are many windows in my project and I’m constantly adding or removing things.

LRESULT CPicture::Msg(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch(uMsg)
    { 
        HANDLE_MSG(hWnd, WM_CREATE, OnCreate);
        HANDLE_MSG(hWnd, WM_PAINT, OnPaint);
        HANDLE_MSG(hWnd, WM_SIZE, OnSize);
        HANDLE_MSG(hWnd, WM_CLOSE, OnDestroy);

        default:
            return (DefWindowProc(hWnd, uMsg, wParam, lParam));
    }
}

Here is an example of all the valid events;

// Csx / Dsx
virtual BOOL OnCreate(HWND hWnd, LPCREATESTRUCT lpCreateStruct) {return FALSE;};
virtual void OnDestroy(HWND hWnd){};

// Painting
virtual void OnPaint(HWND hWnd){};
virtual BOOL OnEraseBkgnd(HWND hWnd, HDC hDC) {return FALSE;};

// Movement / Sizing
virtual void OnMove(HWND hWnd, int x, int y) {};
virtual void OnSize(HWND hWnd, UINT state, int nWidth, int nHeight){};
virtual void OnGetMinMaxInfo(HWND hWnd, LPMINMAXINFO lpMinMaxInfo){};

// Focus
virtual void OnSetFocus(HWND hWnd, HWND hWndOldFocus){};
virtual void OnKillFocus(HWND hWnd, HWND hWndNewFocus){};

… and lots more. Is it possible using preprocessor macros to detect when one of these functions is overridden in a child class and automatically add a handler for it to the message loop?

I was told by someone that It was a really bad idea for my Msg() to handle every message even if it wasn’t defined, so I’m looking for an easy alternative that isn’t user-heavy on constantly adding/removing event definitions.

Answer 1

It’s not possible to do what you want exactly using the standard preprocessor, But following Bo Persson’s comment, you could do something like this:

class MessageHandler
{
public:
  virtual void Handle () = 0;
  static void Dispatch (int i) { m_messages [i]->Handle (); }
protected:
  MessageHandler (int i) { m_messages [i] = this; }
private:
  static map <int, MessageHandler *> m_messages;
};

map <int, MessageHandler *> MessageHandler::m_messages;

template <int i>
class MessageHandlerInstance : MessageHandler
{
protected:
  MessageHandlerInstance () : MessageHandler (i) { }
};

#define Handler(name,i) class name : public MessageHandlerInstance <i> { void Handle (); } name##Instance;

// these are the actual handlers for the messages
Handler (Msg1, 1);
void Msg1::Handle () { cout << "Msg1" << endl; }

Handler (Msg2, 2);
void Msg2::Handle () { cout << "Msg2" << endl; }

Handler (Msg3, 3);
void Msg3::Handle () { cout << "Msg3" << endl; }


int main ()
{
  // and this is how they're called
  MessageHandler::Dispatch (1);
  MessageHandler::Dispatch (2);
  MessageHandler::Dispatch (3);
}

It needs a bit of error checking – the Dispatch call will fail if there’s no message handler defined. Also, you could probably move the template class into the #define, but that would make the #define bigger.