Unparented Window cant have HMENU id set? - c

Question

I have a custom window class (that I created with RegisterClassEx()). If I create a window instance of this class and set its HMENU property the CreateWindowEx() function fails.
Why can I not set this kind of window's id/HMENU id?
// hwnd = NULL
hwnd = CreateWindowEx(0, WND_CLASS_NAME.c_str(), wndTitle.c_str(), wndFlags,
wndDimensions.left, wndDimensions.top, wndDimensions.right, wndDimensions.bottom,
NULL, (HMENU)50001, hinstance, NULL);
// hwnd is valid
hwnd = CreateWindowEx(0, WND_CLASS_NAME.c_str(), wndTitle.c_str(), wndFlags,
wndDimensions.left, wndDimensions.top, wndDimensions.right, wndDimensions.bottom,
NULL, 0, hinstance, NULL);
The whole purpose is to be able to call GetDlgCtrlId(hwnd);.

Answer 1

This cannot work. The CreateWindowEx function is a little bit confusing in that the interpretation of the parameters differs, depending on which type of window you are creating. You have to read the documentation carefully to avoid making erroneous assumptions.
There are two fundamental types of windows: overlapped/pop-up windows (I think there used to be a distinction back in 16-bit Windows, but that distinction is no longer relevant; these are practically identical) and child windows. The first type are the ones you intuitively think of, since they're the ones that look like windows. They're what an application uses for its main window, dialog boxes, floating tool windows, etc. The second type are a specific type of windows that can only be used as children of another window. Controls are child windows—things like buttons, static controls, listviews, etc. Child windows are hosted by a parent window (they always have a parent), which can either be another child window or an overlapped/pop-up window.
Only child windows have application-defined IDs. You specify this ID when calling the CreateWindowEx function with the WS_CHILD style flag (which requests the creation of a child window). When that flag is present, the hMenu parameter is not interpreted as a handle to a menu. Rather, it is interpreted as the child window's ID.†
When calling the CreateWindowEx function without the WS_CHILD style flag (which means you are passing either WS_OVERLAPPED or WS_POPUP, which we have already seen are essentially interchangeable), the hMenu parameter is interpreted as a handle to a menu. If it is a valid handle to a menu, then this menu is associated with the window. If it is NULL, then the window uses the class menu (the one specified when RegisterClassEx was called during creation of the window class).‡
The function's documentation tries to make this clear in the description of the hMenu parameter.
It says:
hMenu [in, optional]
Type: HMENU
A handle to a menu, or specifies a child-window identifier, depending on the window style. For an overlapped or pop-up window, hMenu identifies the menu to be used with the window; it can be NULL if the class menu is to be used. For a child window, hMenu specifies the child-window identifier, an integer value used by a dialog box control to notify its parent about events. The application determines the child-window identifier; it must be unique for all child windows with the same parent window.
The implication should be obvious. A window cannot have both a menu and an ID. It has either a menu (if it is an overlapped/pop-up window) or a child-window ID (if it is a child window). Child windows never have menus—this is technically impossible because they have IDs. So what you are trying to do is impossible: overlapped/pop-up windows cannot have an ID set.
You state in the question that your intent is to be able to call the GetDlgCtrlID function, but that function's documentation is pretty clear about the fact that it works only for child windows. For starters, the very name of the function implies that it works for controls (abbreviated to "ctrl"), which by definition must be child windows. Reading further (italicized annotations are mine):
GetDlgCtrlID accepts child window handles as well as handles of controls in dialog boxes. [Technically, this is an unnecessary distinction. As we have seen, controls in dialog boxes are child windows. But, presumably, the author was taking special care to be as clear as possible.] An application sets the identifier for a child window when it creates the window by assigning the identifier value to the hmenu parameter when calling the CreateWindow or CreateWindowEx function.
Although GetDlgCtrlID may return a value if hwndCtl is a handle to a top-level window, top-level windows [by this is meant either overlapped or pop-up windows] cannot have identifiers and such a return value is never valid.
Note in particular that final sentence.
Of course, there is some way to identify overlapped and pop-up windows: their handle. This is the value returned to you by the CreateWindowEx function (assuming it is successful), and you can save this handle as the window's ID. It is guaranteed to be unique system-wide (although it may be reused for another window after your window has been destroyed), and is the most reliable way of identifying a window.
If, for some reason, you are unable to save the window handle but still need to find a top-level window later, you can call the FindWindow function. This uses the window class name and the window caption to find a matching window. If it finds a match, it returns the window handle. (Note again the distinction between top-level and child windows. FindWindow does not work on child windows. If you want to search child windows, you must call FindWindowEx instead.)
† Note that, once a child window has been created, its ID can be set or retrieved by passing the GWL_ID index to the GetWindowLongPtr or SetWindowLongPtr functions, respectively. This index only has meaning for child windows, because no other type of window has an ID.
‡ Note that, once an overlapped/pop-up window has been created, its menu handle can be retrieved with GetMenu or set with SetMenu. Again, the documentation for those functions is forced to emphasize the fact that child windows cannot have menus. GetMenu fails when called on a child window, returning an "undefined" result; SetMenu similarly fails, setting an error code.