What is the difference between WM_QUIT, WM_CLOSE, and WM_DESTROY in a windows program?

Question

I was wondering what the difference between the WM_QUIT, WM_CLOSE, and WM_DESTROY messages in a windows program, essentially: when are they sent, and do they have any automatic effects besides what's defined by the program?

Answer 1

They are totally different.

WM_CLOSE is sent to the window when "X" is pressed or "Close" is chosen from window menu. If you catch this message this is your call how to treat it - ignore it or really close the window. By default, WM_CLOSE passed to DefWindowProc causes window to be destroyed. When the window is being destroyed WM_DESTROY message is sent. In this stage, in opposition toWM_CLOSE, you cannot stop the process, you can only make a necessary cleanup. But remember that when you catch WM_DESTROY just before all child windows are already destroyed. WM_NCDESTROY is send just after all child windows have been destroyed.

WM_QUIT message is not related to any window (the hwnd got from GetMessage is NULL and no window procedure is called). This message indicates that the message loop should be stopped and application should be closed. When GetMessage reads WM_QUIT it returns 0 to indicate that. Take a look at typical message loop snippet - the loop is continued while GetMessage returns non-zero. WM_QUIT can be sent by PostQuitMessage function. This function is usually called when main window receives WM_DESTROY (see typical window procedure snippet).

Answer 2

First of all, the WM_CLOSE and WM_DESTROY messages are associated with particular windows whereas the WM_QUIT message is applicable to the whole application (well thread) and the message is never received through a window procedure (WndProc routine), but only through the GetMessage or PeekMessage functions.

In your WndProc routine the DefWindowProc function takes care of the default behavoir of these messages. The WM_CLOSE messages requests that the application should close and the default behavoir for this is to call the DestroyWindow function. Its when this DestroyWindow function is called that the WM_DESTROY message is sent. Notice that the WM_CLOSE is only a message requesting that you close (like WM_QUIT) - you don't actually have to exit/quit. But the WM_DESTROY message tells you that your window IS being closed and destroyed so you must cleanup any resources, handles etc.

Answer 3

Just so it doesn't get lost in the comments... don't forget about WM_CANCEL. When you click the close (x) button on an MFC dialog, it will certainly send WM_CLOSE. The default OnClose() function will then call the default (base class) OnCancel() function.

However, if you simply type the ESC key, this will lead to the closure of the dialog, but (as far as I can tell) without generating the WM_CLOSE event - it goes directly to the WM_CANCEL/OnCancel() mechanism.

I hereby invite the community to elaborate on this... or edit that elaboration into the accepted answer.

Answer 4

At first let's discuss WM_QUIT - the difference from another messages that this is not associated with window. It is used by application. For example this can be handled by non-visible standalone OLE server (.exe, but not in-proc as .dll)

WM_CLOSE - per msdn: "An application can prompt the user for confirmation, prior to destroying a window" - it is used as notification about intention to close (you can reject this intention).

WM_DESTROY - is a fact that window is closing and all resources must(!) be deallocated.