TN017: Destroying Window Objects
This note describes the use of the CWnd::PostNcDestroy method. Use this method if you want to do customized allocation of CWnd-derived objects. This note also explains why you should use CWnd::DestroyWindow to destroy a C++ Windows object instead of the delete operator.
If you follow the guidelines in this topic, you will have few cleanup problems. These problems can result from issues such as forgetting to delete/free C++ memory, forgetting to free system resources like HWNDs, or freeing objects too many times.
The Problem
Each windows object (object of a class derived from CWnd) represents both a C++ object and an HWND. C++ objects are allocated in the application's heap and HWNDs are allocated in system resources by the window manager. Because there are several ways to destroy a window object, we must provide a set of rules that prevent system resource or memory leaks. These rules must also prevent objects and Windows handles from being destroyed more than one time.
Destroying Windows
The following are the two permitted ways to destroy a Windows object:
Calling CWnd::DestroyWindow or the Windows API DestroyWindow.
Explicitly deleting with the delete operator.
The first case is by far the most common. This case applies even if your code does not call DestroyWindow directly. When the user directly closes a frame window, this action generates the WM_CLOSE message, and the default response to this message is to call DestroyWindow. When a parent window is destroyed, Windows calls DestroyWindow for all its children.
The second case, the use of the delete operator on Windows objects, should be rare. The following are some cases where using delete is the correct choice.
Auto Cleanup with CWnd::PostNcDestroy
When the system destroys a Windows window, the last Windows message sent to the window is WM_NCDESTROY. The default CWnd handler for that message is CWnd::OnNcDestroy. 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 that are allocated on the stack frame or embedded in other objects. This is not appropriate for window objects that are designed to be allocated on the heap without any other objects. In other words, it is not appropriate for window objects that are not embedded in other C++ objects.
Those classes that are designed to be allocated alone on the heap override the PostNcDestroy method to perform a delete this. This statement will free any 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 because the handle will be detached and NULL during the cleanup phase.
Notes
The system usually calls CWnd::PostNcDestroy after it processes the Windows WM_NCDESTROY message and the HWND and the C++ window object are no longer connected. The system will also call CWnd::PostNcDestroy in the implementation of most CWnd::Create calls if failure occurs. The auto cleanup rules are described later in this topic.
Auto Cleanup Classes
The following classes are not designed for auto-cleanup. They are typically embedded in other C++ objects or on the stack:
All standard Windows controls (CStatic, CEdit, CListBox, and so on).
Any child windows derived directly from CWnd (for example, custom controls).
Splitter windows (CSplitterWnd).
Default control bars (classes derived from CControlBar, see Technical Note 31 for enabling auto-delete for control bar objects).
Dialogs (CDialog) designed for modal dialogs on the stack frame.
All the standard dialogs except CFindReplaceDialog.
The default dialogs created by ClassWizard.
The following classes are designed for auto-cleanup. They are typically allocated by themselves on the heap:
Main frame windows (derived directly or indirectly from CFrameWnd).
View windows (derived directly or indirectly from CView).
If you want to break these rules, you must override the PostNcDestroy method in your derived class. To add auto-cleanup to your class, call your base class and then do a delete this. To remove auto-cleanup from your class, call CWnd::PostNcDestroy directly instead of the PostNcDestroy method of your direct base class.
The most common use of changing auto cleanup behavior is to create a modeless dialog that can be allocated on the heap.
When to Call delete
We recommend that you call DestroyWindow to destroy a Windows object, either the C++ method or the global DestroyWindow API.
Do not call the global DestroyWindow API to destroy a MDI Child window. You should use the virtual method CWnd::DestroyWindow instead.
For C++ Window objects that do not perform auto-cleanup, using the delete operator can cause a memory leak when you try to call DestroyWindow in the CWnd::~CWnd destructor if the VTBL does not point to the correctly derived class. This occurs because the system cannot find the appropriate destroy method to call. Using DestroyWindow instead of delete avoids these problems. Because this can be a subtle error, compiling in debug mode will generate the following warning if you are at risk.
Warning: calling DestroyWindow in CWnd::~CWnd
OnDestroy or PostNcDestroy in derived class will not be called
In the case of C++ Windows objects that do perform auto-cleanup, you must call DestroyWindow. If you use the delete operator directly, the MFC diagnostic memory allocator will notify you that you are freeing memory two times. The two occurrences are your first explicit call and the indirect call to delete this in the auto-cleanup implementation of PostNcDestroy.
After calling DestroyWindow on a non-auto-cleanup object, the C++ object will still be around, but m_hWnd will be NULL. After calling DestroyWindow on an auto-cleanup object, the C++ object will be gone, freed by the C++ delete operator in the auto-cleanup implementation of PostNcDestroy.