Question

I was just wondering what (if any) the difference was between the following two message traps in MFC for the function, OnSize(..).

1 – Via Message map:

BEGIN_MESSAGE_MAP(CClassWnd, CBaseClassWnd) ...     ON_WM_SIZE() .. END_MESSAGE_MAP() 

2 – Via afx_message:

afx_msg type OnSize(...); 

They seem to be used interchangeably, which one should be used or does it depend on other factors?

Answer 1

Both parts are necessary to add a message handler to a class. The message map should be declared inside your class, together with declarations for any message handler functions (e.g, OnSize).

class CClassWnd : public CBaseClassWnd {     ...     afx_msg void OnSize(UINT nType, int cx, int cy);     DECLARE_MESSAGE_MAP }; 

afx_msg is just an empty placeholder macro – it doesn’t actually do anything, but is always included by convention.

The message map is then defined in the class’s .cpp file:

BEGIN_MESSAGE_MAP(CClassWnd, CBaseClassWnd)     ON_WM_SIZE() END_MESSAGE_MAP() 

These macros generate a lookup table for the class which allows messages received by the window to be dispatched to the corresponding handler functions. The ON_WM_SIZE macro allows the wParam and lParam message parameters in the WM_SIZE message to be decoded into more meaningful values for the message handler function (nType, cx, and cy in this case). MFC provides macros for most window messages (WM_LBUTTONDOWN, WM_DESTROY, etc).

You can find more information on how message maps work in MFC here on MSDN.