CallWindowProc

The CallWindowProc function passes message information to the specified window procedure.

LRESULT CallWindowProc(

WNDPROC lpPrevWndFunc,	// pointer to previous procedure
HWND hWnd,	// handle to window
UINT Msg,	// message
WPARAM wParam,	// first message parameter
LPARAM lParam	// second message parameter
);

Parameters

lpPrevWndFunc

Pointer to the previous window procedure.

If this value is obtained by calling the GetWindowLong function with the nIndex parameter set to GWL_WNDPROC or DWL_DLGPROC, it is actually either the address of a window or dialog box procedure, or a handle representing that address.

hWnd

Identifies the window procedure to receive the message.

Msg

Specifies the message.

wParam

Specifies additional message-specific information. The contents of this parameter depend on the value of the Msg parameter.

lParam

Specifies additional message-specific information. The contents of this parameter depend on the value of the Msg parameter.

Return Values

The return value specifies the result of the message processing and depends on the message sent.

Remarks

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 Windows 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: The CallWindowProc function handles Unicode-to-ANSI conversion. You don't get the conversion if you call the window procedure directly.

See Also

FARPROC, GetWindowLong, SetClassLong, SetWindowLong, WNDPROC

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