Deleting a C++ object that is associated with an ATL dialog box causes an assert in Atlwin.h, Line 2281 in Visual C++ 6.0 (202110)


The information in this article applies to:

  • The Microsoft Active Template Library (ATL) 3.0, when used with:
    • Microsoft Visual C++, 32-bit Enterprise Edition 6.0
    • Microsoft Visual C++, 32-bit Professional Edition 6.0
    • Microsoft Visual C++, 32-bit Learning Edition 6.0
This article was previously published under Q202110

SYMPTOMS

Deleting the C++ object associated with an ATL dialog box by calling "delete this" in the WM_NCDESTROY handler or OnFinalMessage() results in an assert in Atlwin.h, line 2281.

CAUSE

Line 2281 of Atlwin.h is: 
ATLASSERT(pThis->m_pCurrentMsg == &msg);
The object being referred to by "pThis" has already been deleted. Consider ATL's default dialog box procedure: 
template <class TBase>
LRESULT CALLBACK CDialogImplBaseT< TBase >::DialogProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
	CDialogImplBaseT< TBase >* pThis = (CDialogImplBaseT< TBase >*)hWnd;
	// set a ptr to this message and save the old value
	MSG msg = { pThis->m_hWnd, uMsg, wParam, lParam, 0, { 0, 0 } };
	const MSG* pOldMsg = pThis->m_pCurrentMsg;
	pThis->m_pCurrentMsg = &msg;
	// pass to the message map to process
	LRESULT lRes;
	BOOL bRet = pThis->ProcessWindowMessage(pThis->m_hWnd, uMsg, wParam, lParam, lRes, 0);
	// restore saved value for the current message
	ATLASSERT(pThis->m_pCurrentMsg == &msg);
	pThis->m_pCurrentMsg = pOldMsg;
	// set result if message was handled
	if(bRet)
	{
		switch (uMsg)
		{
		case WM_COMPAREITEM:
		case WM_VKEYTOITEM:
		case WM_CHARTOITEM:
		case WM_INITDIALOG:
		case WM_QUERYDRAGICON:
		case WM_CTLCOLORMSGBOX:
		case WM_CTLCOLOREDIT:
		case WM_CTLCOLORLISTBOX:
		case WM_CTLCOLORBTN:
		case WM_CTLCOLORDLG:
		case WM_CTLCOLORSCROLLBAR:
		case WM_CTLCOLORSTATIC:
			return lRes;
			break;
		}
		::SetWindowLong(pThis->m_hWnd, DWL_MSGRESULT, lRes);
		return TRUE;
	}
	if(uMsg == WM_NCDESTROY)
	{
		// clear out window handle
		HWND hWnd = pThis->m_hWnd;
		pThis->m_hWnd = NULL;
		// clean up after dialog is destroyed
		pThis->OnFinalMessage(hWnd);
	}
	return FALSE;
}
Typically, you will be calling DestroyWindow()/EndDialog() in a WM_CLOSE or a WM_COMMAND handler. Here's the sequence of events when you close a dialog box:

  1. DialogProc() is called with WM_CLOSE.
  2. ProcessWindowMessage() calls your WM_CLOSE handler.
  3. In your WM_CLOSE handler, you call DestroyWindow().
  4. This ends up calling DialogProc again with WM_NCDESTROY.
  5. ProcessWindowMessage() calls your WM_NCDESTROY handler.
  6. You call "delete this" in your WM_NCDESTROY handler.
RESULTS: When you come back from ProcessWindowMessage(), because the C++ class has been deleted, pThis no longer points to a valid object, and therefore the assert is returned.

The following is another possible scenario (note that the first four steps are the same as above):
  1. DialogProc has a case for WM_NCDESTROY and calls OnFinalMessage().
  2. You call "delete this" in OnFinalMessage().
  3. The stack unwinds to the original DialogProc call (with a WM_CLOSE).
RESULTS: Same problem. After the call to ProcessWindowMessage, try to use pThis, but it no longer points to valid memory.

RESOLUTION

Notify DialogProc when the dialog box class has been deleted; this can be accomplished by adding a member variable called m_bAutoDelete to the dialog box class. Setting this to TRUE causes the dialog box class to delete itself when the window is destroyed. Use the following code:
// Constant value used to determine if we should delete ourselves later.
#define DEFERDELETE 2
class CMyDlg : public CAxDialogImpl<CMyDlg>
{
public:
   // Variable that tells us if we want to auto-delete ourselves.
   BYTE m_bAutoDelete;
   // Set m_bAutoDelete to TRUE to automatically delete ourselves.
   CMyDlg() : m_bAutoDelete (TRUE)
   {
   }
   // Override GetDialogProc to provide our own DialogProc.
   WNDPROC GetDialogProc()
   {
      return MyDialogProc;
   }
   // Our own dialog procedure that is mostly copied from
   // CDialogImplBaseT<>::DialogProc() in Atlwin.h.
   static LRESULT CALLBACK MyDialogProc(HWND hWnd, UINT uMsg,
                                        WPARAM wParam, LPARAM lParam)
   {
      CMyDlg* pThis = (CMyDlg*)hWnd;
      // Set a ptr to this message and save the old value.
      MSG msg = { pThis->m_hWnd, uMsg, wParam, lParam, 0, { 0, 0 } };
      const MSG* pOldMsg = pThis->m_pCurrentMsg;
      pThis->m_pCurrentMsg = &msg;
      // Pass to the message map to process.
      LRESULT lRes;
      BOOL bRet = pThis->ProcessWindowMessage(pThis->m_hWnd, uMsg, wParam,
                                              lParam, lRes, 0);
      // If window has been destroyed and this is the last message,
      // then delete ourselves.
      if (DEFERDELETE == pThis->m_bAutoDelete && pOldMsg == NULL)
      {
         delete pThis;
         return FALSE;
      }
      // Restore saved value for the current message.
      ATLASSERT(pThis->m_pCurrentMsg == &msg);
      pThis->m_pCurrentMsg = pOldMsg;
      // Set result if message was handled.
      if(bRet)
      {
         switch (uMsg)
         {
            case WM_COMPAREITEM:
            case WM_VKEYTOITEM:
            case WM_CHARTOITEM:
            case WM_INITDIALOG:
            case WM_QUERYDRAGICON:
            case WM_CTLCOLORMSGBOX:
            case WM_CTLCOLOREDIT:
            case WM_CTLCOLORLISTBOX:
            case WM_CTLCOLORBTN:
            case WM_CTLCOLORDLG:
            case WM_CTLCOLORSCROLLBAR:
            case WM_CTLCOLORSTATIC:
               return lRes;
               break;
         }
         ::SetWindowLong(pThis->m_hWnd, DWL_MSGRESULT, lRes);
         return TRUE;
      }
      if(uMsg == WM_NCDESTROY)
      {
         // Clear out window handle.
         HWND hWnd = pThis->m_hWnd;
         pThis->m_hWnd = NULL;
         // Clean up after dialog box is destroyed.
         pThis->OnFinalMessage(hWnd);
         // If we want to automatically delete ourselves...
         if (pThis->m_bAutoDelete)
         {
            // If no outstanding messages to process in call stack,
            // m_pCurrentMsg will be NULL so we can delete ourselves.
            if (pThis->m_pCurrentMsg == NULL)
               delete pThis;
            // Else set a flag so we can delete ourselves later.
            else
               pThis->m_bAutoDelete = DEFERDELETE;
         }
      }
      return FALSE;
   }
   ...
};

STATUS

This behavior is by design.

MORE INFORMATION

In CDialogImplBaseT<>::DialogProc(), m_pCurrentMsg is set so you can call GetCurrentMessage() to retrieve the current message from any method in your dialog class. This problem and solution applies to any CWindowImplRoot-derived class.

REFERENCES

(c) Microsoft Corporation Samson Tanrena, All Rights Reserved. Contributions by 1999, Microsoft Corporation.

Modification Type:MajorLast Reviewed:4/28/2005
Keywords:kbtshoot kbATLWC kbDlg kbprb KB202110 kbAudDeveloper
©2004 Microsoft Corporation. All rights reserved.