Other Windows Messages

The MFC library directly supports hundreds of Windows message-handling functions. In addition, you can define your own messages. You will see plenty of message-handling examples in later chapters, including handlers for menu items, child window controls, and so forth. In the meantime, five special Windows messages deserve special attention: WM_CREATE, WM_CLOSE, WM_QUERYENDSESSION, WM_DESTROY, and WM_NCDESTROY.

The WM_CREATE Message

This is the first message that Windows sends to a view. It is sent when the window's Create function is called by the framework, so the window creation is not finished and the window is not visible. Therefore, your OnCreate handler cannot call Windows functions that depend on the window being completely alive. You can call such functions in an overridden OnInitialUpdate function, but you must be aware that in an SDI application OnInitialUpdate can be called more than once in a view's lifetime.

The WM_CLOSE Message

Windows sends the WM_CLOSE message when the user closes a window from the system menu and when a parent window is closed. If you implement the OnClose message map function in your derived view class, you can control the closing process. If, for example, you need to prompt the user to save changes to a file, you do it in OnClose. Only when you have determined that it is safe to close the window do you call the base class OnClose function, which continues the close process. The view object and the corresponding window are both still active.

When you're using the full application framework, you probably won't use the WM_CLOSE message handler. You can override the CDocument::SaveModified virtual function instead, as part of the application framework's highly structured program exit procedure.


Windows sends the WM_QUERYENDSESSION message to all running applications when the user exits Windows. The OnQueryEndSession message map function handles it. If you write a handler for WM_CLOSE, write one for WM_QUERYENDSESSION too.

The WM_DESTROY Message

Windows sends this message after the WM_CLOSE message, and the OnDestroy message map function handles it. When your program receives this message, it should assume that the view window is no longer visible on the screen but that it is still active and its child windows are still active. Use this message handler to do cleanup that depends on the existence of the underlying window. Be sure to call the base class OnDestroy function. You cannot "abort" the window destruction process in your view's OnDestroy function. OnClose is the place to do that.


This is the last message that Windows sends when the window is being destroyed. All child windows have already been destroyed. You can do final processing in OnNcDestroy that doesn't depend on a window being active. Be sure to call the base class OnNcDestroy function.

Do not try to destroy a dynamically allocated window object in OnNcDestroy. That job is reserved for a special CWnd virtual function, PostNcDestroy, that the base class OnNcDestroy calls. MFC Technical Note #17 in the online documentation gives hints on when it's appropriate to destroy a window object.