Re: Why does this cause "data abort" ?

"Lisa Pearlson" <no@spam.plz> ha scritto nel messaggio
news:OrgRWwmFIHA.4196@TK2MSFTNGP04.phx.gbl...


It does not make sense to store the hWnd as a parameter for GWL_USERDATA,
you should store the "this" instance pointer into GWL_USERDATA slot, so you
can retrieve "this" instance in later calls.

You may define a pair of static member functions like these
(Get/SetInstancePtr):

<code>

---- [header] ----

  class MyWnd
  {
    ...
    // Retrieve class instance
    static MyWnd * GetInstancePtr( HWND hwnd );

    // Set class instance
    static void SetInstancePtr( HWND hwnd, MyWnd * pThisWnd );
  };

---- [implementation] ----

  /* static */ MyWnd * MyWnd::GetInstancePtr( HWND hwnd )
  {
    _ASSERTE( hwnd != NULL );

    return (MyWnd *) ::GetWindowLong( hwnd, GWL_USERDATA );
  }

  /* static */ void MyWnd::SetInstancePtr( HWND hwnd, MyWnd * pThisWnd )
  {
    _ASSERTE( hwnd != NULL );
    _ASSERTE( pThisWnd != NULL );

    ::SetWindowLong( hwnd, GWL_USERDATA, (LONG)pThisWnd );
  }

</code>

Note that, as I wrote in my previous post, there are other options different
from GWL_USERDATA. For example, you can use window extra bytes (in
WNDCLASSEX.cbWndExtra) to store the "this" pointer.
e.g.

<code>

  // When you register window class
  // WNDCLASSEX wndClass;
  ...
  wndClass.cbWndExtra = sizeof( MyWnd * );

</code>

And you should accordingly change the implementations of Get/SetInstancePtr
like so (note that the 0-offset accesses the extra window bytes with
::Get/SetWindowLong):

<code>

  /* static */ MyWnd * MyWnd::GetInstancePtr( HWND hwnd )
  {
    _ASSERTE( hwnd != NULL );

    return (MyWnd *) ::GetWindowLong( hwnd, 0 );
  }

  /* static */ void MyWnd::SetInstancePtr( HWND hwnd, MyWnd * pThisWnd )
  {
    _ASSERTE( hwnd != NULL );
    _ASSERTE( pThisWnd != NULL );

    ::SetWindowLong( hwnd, 0, (LONG)pThisWnd );
  }

</code>

Then you can have the following static window proc, which routes the
messages to proper class methods. The "this" pointer is extracted from the
HWND (using GWL_USERDATA or extra bytes):

<code>

/* static */ LRESULT CALLBACK MyWnd::Proc(
        HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam )
{
    // Retrieve the C++ this pointer for this window
    MyWnd * pWnd = GetInstancePtr( hwnd );

    switch(msg)
    {
    // Setup
    case WM_NCCREATE:

        // Create a new window object onto the heap
        pWnd = new MyWnd();

        // Set the "this" pointer, so we can retrieve it back from HWND
        SetInstancePtr( hwnd, pWnd );

        // Continue with the window creation process
        return TRUE;

    // Release resources when the window is destroyed
    case WM_NCDESTROY:
        delete pWnd;
        break;

    ...
    ...
    // Can handle other messages here, routing to class specific methods,
    // e.g.:
    case WM_SOME_MESSAGE:
        pWnd->OnSomeMessage(...);
        break;
        // or:
        // return pWnd->OnSomeMessage(...);
    ...
    ...
    // or you can implement a "local" *non-static* window proc, and just
call it:
    // return pWnd->WndProc( hwnd, msg, wParam, lParam );

    default:
        break;
    }

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

</code>

Note that I used WM_NCCREATE and WM_NCDESTROY to get kind of the "very
first" and the "very last" window messages.

Giovanni