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

View products that this article applies to.

This article was previously published under Q202110

Expand all | Collapse all

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.

Back to the top

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:

DialogProc() is called with WM_CLOSE.
ProcessWindowMessage() calls your WM_CLOSE handler.
In your WM_CLOSE handler, you call DestroyWindow().
This ends up calling DialogProc again with WM_NCDESTROY.
ProcessWindowMessage() calls your WM_NCDESTROY handler.
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):

DialogProc has a case for WM_NCDESTROY and calls OnFinalMessage().
You call "delete this" in OnFinalMessage().
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.

Back to the top

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;
   }
   ...
};

Back to the top