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Will deleting a CWnd through it's own PostNcDestroy message cause a memory leak if it was allocated on the heap?

Posted on 2006-10-31
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Last Modified: 2013-11-20
I have a splash screen window that is allocated on the heap and deleted in it's PostNcDestroy message handler.  Will this cause a memory leak?

The reason I'm asking is because a memory leak detection program pointed to the spot where I create a new Window on the heap with out deleting it before the function ended.  It may be wrong in this case.

Details below, if needed:

It is created like this:

BOOL CMainFrame::ShowSplashScreen()
{            
      pSplash=new CSplashScreen();      

      pSplash->Create(this,_T("My Splash"),1700,CSS_FADE | CSS_CENTERSCREEN | CSS_SHADOW);

      pSplash->Show();
}

And deleted like this:

void CSplashScreen::PostNcDestroy()
{
      CWnd::PostNcDestroy();
      delete this;
}


This is preferable because the splash screen is killed as soon as a timer runs out:

void CSplashScreen::OnTimer(UINT nIDEvent)
{
      KillTimer(0);
      ShowWindow(SW_HIDE);
      DestroyWindow();
}
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Question by:bigsteve87
2 Comments
 
LVL 86

Accepted Solution

by:
jkr earned 2000 total points
ID: 17843587
This is absolutely OK, see http://msdn2.microsoft.com/zh-tw/library/5zba4hah.aspx resp. http://msdn.microsoft.com/library/en-us/vcmfc98/html/_mfcnotes_tn017.asp ("TN017: Destroying Window Objects"):

Auto Cleanup with CWnd::PostNcDestroy

When destroying a Windows window, the last Windows message sent to the window is WM_NCDESTROY. The default CWnd handler for that message (CWnd::OnNcDestroy) will detach the HWND from the C++ object and call the virtual function PostNcDestroy. Some classes override this function to delete the C++ object.

The default implementation of CWnd::PostNcDestroy does nothing which is appropriate for window objects allocated on the stack frame or embedded in other objects. This is not appropriate for window objects that are designed to be allocated by themselves on the heap (not embedded in other C++ object).

Those classes that are designed to be allocated by themselves on the heap override the PostNcDestroy member function to perform a "delete this". This statement will free any C++ memory associated with the C++ object. Even though the default CWnd destructor calls DestroyWindow if m_hWnd is non-NULL, this does not lead to infinite recursion since the handle will be detached and NULL during the cleanup phase
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Author Comment

by:bigsteve87
ID: 17843811
Great answer, exactly what I needed.  Thanks!
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