CODEWITHSUNDEEP
c++winapi
devjeetroy
devjeetroy

Reputation: 1945

The difference between the handle received in the WNDPROC window callback procedure vs the one returned by CreateWindow?

So i was wondering if there is any difference between the handle returned when creating the window using CreateWindow/CreateWindowEx, and the one that is passed as a parameter to the WNDPROC window callback procedure for the concerned window. The context for this question is that I need to use certain API functions that require a handle to the window, so I thought that instead of passing the handle received by the window callback procedures, I could just save a copy of the handle returned by CreateWindow once, and then use it for successive operations, in case both handles are the same.

Regards,

Devjeet

Upvotes: 0

Views: 534

Answers (2)

Hans Passant
Hans Passant

Reputation: 941485

Be careful here, the window procedure is used by multiple instances of a window. The callback is determined by the value of WNDCLASSEX.lpfnWndProc so any CreateWindow/Ex() call that uses the same window class name uses the same callback. If you are doing this to map the callback to a class instance method then you first have to go through a thunk that maps the callback's hwnd argument to a class instance. Making a special case for WM_CREATE of course.

Upvotes: 2

rodrigo
rodrigo

Reputation: 98368

Yes they are the same, that's the whole point of having an HWND. Every window has only one HWND that identifies it within the system and remains valid until DestroyWindow is called with it. It is returned by CreateWindow(EX) and sent in with every message so you can use that as a unique identifier for your window, even if you create several windows of the same class.

But note that some messages are sent to the window procedure even before the CreateFunction returns, so if you use a global variable to hold your HWND:

HWND globalHwnd = NULL;

int main()
{
    //...
    globalHwnd = CreateWindow(...);
}

LRESULT WndProc(HWND hWnd, ...)
{
    assert(hWnd == globalHwnd); //fail!
}

The assertion will fail because several messages such as WM_CREATE are sent before CreateWindow returns, so the global variable is not yet assigned.

Upvotes: 2

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