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Access violation when deleting CFrameWnd

Posted on 2006-11-02
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Last Modified: 2013-11-20
Ah hello.

I have the following code in a handler for a button on a dialog:

void CDialogAppDlg::OnBnClickedButton1()
{
      if ( m_pFrameWnd )
      {
            m_pFrameWnd->DestroyWindow();
            delete m_pFrameWnd;
      }

      m_pFrameWnd = new CFrameWnd();
      m_pFrameWnd->Create ( NULL, _T("Frame Window") );
      m_pFrameWnd->UpdateWindow();
      m_pFrameWnd->ShowWindow ( SW_SHOW );

}

Assume

CFrameWnd* m_pFrameWnd;

Now, I can click the button once and create the window, but if I click it again I get access violation when calling

m_pFrameWnd->DestroyWindow()

Strangely enough, if I just create the window once, then close the app, I get no memory leaks, which I would expect from

      m_pFrameWnd = new CFrameWnd();

What is going on ?

TIA
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Question by:mrwad99
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Accepted Solution

by:
AlexFM earned 125 total points
ID: 17857559
CFrameWnd detetes itself in PostNCDestroy, it is enough to call DestroyWindow.
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Author Comment

by:mrwad99
ID: 17857571
>> CFrameWnd detetes itself in PostNCDestroy

OK so that explains why I get no memory leaks

>> it is enough to call DestroyWindow.

I cannot call DestroyWindow, that is what crashes.  Or are you saying I don't need to ?

Thanks.
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LVL 48

Expert Comment

by:AlexFM
ID: 17857611
How exactly does it crash?
Try this:

m_pFrameWnd->SendMessage(WM_CLOSE, 0, 0);
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LVL 19

Author Comment

by:mrwad99
ID: 17857635
Thanks.
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LVL 19

Author Comment

by:mrwad99
ID: 17857638
>> How exactly does it crash?

Not sure what you were getting at there.  Can you elaborate ?

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Expert Comment

by:AlexFM
ID: 17857724
Code line, message etc. - as usual. But I think it is enough to remove delete m_pFrameWnd line.
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Author Comment

by:mrwad99
ID: 17857825
>> But I think it is enough to remove delete m_pFrameWnd line.

I have stepped though the code and found that I don't even need the

m_pFrameWnd->DestroyWindow(), as this gets called when I close the window via the 'X' button.  But I would need to call this if the user closes the dialog before the main window, otherwise I would get leaks.

Interestingly so, when I do close the dialog with the frame window still active, the frame window does go away, and I get the leak on

m_pFrameWnd = new CFrameWnd();

detected.

I assume the OS destroys the CFrameWnd, but still, I only get one call into

CWnd::DestroyWindow()

which is called for the modal dialog.  Not for the CFrameWnd...

Do you know why this is ?

(Sorry to go on after I have closed this, but I read your last comment a bit late)
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LVL 48

Expert Comment

by:AlexFM
ID: 17857959
When you close dialog, application exits immediately. In this case possibly CFrameWnd is not deleted properly, because OnNCDestroy is not called, and there is memory leak. It is possible to debug this or read MFC code, but closing frame when dialog is closed fixes this problem in any case.
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Author Comment

by:mrwad99
ID: 17858057
Thanks again :o)
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