The CallWindowProc function passes message information to the specified window procedure.
LRESULT CallWindowProc(
WNDPROC lpPrevWndFunc,
HWND hWnd,
UINT Msg,
WPARAM wParam,
LPARAM lParam
);
The return value specifies the result of the message processing and depends on the message sent.
Use the CallWindowProc function for window subclassing. Usually, all windows with the same class share one window procedure. A subclass is a window or set of windows with the same class whose messages are intercepted and processed by another window procedure (or procedures) before being passed to the window procedure of the class.
The SetWindowLong function creates the subclass by changing the window procedure associated with a particular window, causing the system to call the new window procedure instead of the previous one. An application must pass any messages not processed by the new window procedure to the previous window procedure by calling CallWindowProc. This allows the application to create a chain of window procedures.
If STRICT is defined, the lpPrevWndFunc parameter has the data type WNDPROC. The WNDPROC type is declared as follows:
LRESULT (CALLBACK* WNDPROC) (HWND, UINT, WPARAM, LPARAM);
If STRICT is not defined, the lpPrevWndFunc parameter has the data type FARPROC. The FARPROC type is declared as follows:
int (FAR WINAPI * FARPROC) ()
In C, the FARPROC declaration indicates a callback function that has an unspecified parameter list. In C++, however, the empty parameter list in the declaration indicates that a function has no parameters. This subtle distinction can break careless code. Following is one way to handle this situation:
#ifdef STRICT WNDPROC MyWindowProcedure #else FARPROC MyWindowProcedure #endif ... lResult = CallWindowProc(MyWindowProcedure, ...) ;
For further information about functions declared with empty argument lists, refer to The C++ Programming Language, Second Edition, by Bjarne Stroustrup.
Windows NT/2000/XP: The CallWindowProc function handles Unicode-to-ANSI conversion. You cannot take advantage of this conversion if you call the window procedure directly.
Windows 95/98/Me: CallWindowProcW is supported by the Microsoft Layer for Unicode (MSLU). Also, the ANSI version is supported, to provide more consistent behavior across all Microsoft Windows operating systems. To use this, you must add certain files to your application, as outlined in Microsoft Layer for Unicode on Windows 95/98/Me Systems .