Strange access violation

This is a discussion on Strange access violation within the Windows Programming forums, part of the Platform Specific Boards category; Hi, I have put the windows procedure inside a class as a static member function and I get a pointer ...

  1. #1
    Registered User
    Join Date
    Apr 2004
    Posts
    7

    Strange access violation

    Hi,

    I have put the windows procedure inside a class as a static member function and I get a pointer to the class containing the function using GetWindowLong(), and everything works fine, like calling member functions trough the pointer and reading variables through the pointer, but when I try to write through the pointer, I get an Access violation writing location... Does anyone know why?

    the code:
    LRESULT CALLBACK Win32Window::WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
    {
    // Get a pointer to the class which this window belongs to
    Win32Window *w32Window;
    if (msg != WM_CREATE)
    w32Window = (Win32Window*)GetWindowLongPtr( hWnd, 0 );

    switch (msg)
    {
    case WM_CREATE:
    {
    CREATESTRUCT *cs = (CREATESTRUCT*)lParam;
    w32Window = (Win32Window*)cs->lpCreateParams;
    SetWindowLongPtr(hWnd, GWL_USERDATA, (LONG_PTR) w32Window);
    return 0;
    break;
    }

    case WM_KEYDOWN:
    {
    w32Window->aMemberVariable = 0; // this causes a Access violation writing...
    break;
    }


    }

    return DefWindowProc(hWnd, msg, wParam, lParam);
    }

  2. #2
    erstwhile
    Join Date
    Jan 2002
    Posts
    2,227
    Please use code tags

    If you use 'GWL_USERDATA' to store your c++ class pointer with SetWindowLongPtr then use the same flag to retrieve it with GetWindowLongPtr.

    Also note that WM_CREATE is not the first message sent to your window procedure but WM_CREATE is where you first store your c++ class pointer in your window (most use WM_NCCREATE in this respect because it's sent to your window procedure before WM_CREATE). Checking the return value of GetWindowLongPtr is probably a good idea, too, to ensure that the GWL_USERDATA returned is valid.

    This is the most recent of a long line of threads on c++ class message handling which may be of some interest to you.
    CProgramming FAQ
    Caution: this person may be a carrier of the misinformation virus.

  3. #3
    Registered User
    Join Date
    Apr 2004
    Posts
    7
    Thanks, using GWL_USERDATA when retrieving the pointer did the trick, I should have thought about that. Code tags will be used in my next post .

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