TN003: Mapping of Windows Handles to Objects
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This note describes the MFC routines that support mapping Windows object handles to C++ objects.
Windows objects are normally represented by HANDLEs. The MFC classes wrap Windows object handles with C++ objects. The handle wrapping functions of the MFC class library provide a way to find the C++ object that is wrapping the Windows object with a particular handle. There are times when a Windows object does not have a C++ wrapper object, however, and at these times a temporary object is created to act as the C++ wrapper.
The Windows objects that use handle maps are:
HWND (CWnd and CWnd-derived classes)
HDC (CDC and CDC-derived classes)
Given a handle to any of these objects, you can find the MFC object that wraps the handle by calling the static member function FromHandle. For example, given an HWND called hWnd:
will return a pointer to the CWnd that wraps the hWnd. If that hWnd does not have a specific wrapper object, then a temporary CWnd is created to wrap the hWnd. This makes it possible to get a valid C++ object from any handle.
Once you have a wrapper object, you can get to its handle through a public member variable. In the case of a CWnd, m_hWnd contains the HWND for that object.
Given a newly created handle-wrapper object and a handle to a Windows object, you can associate the two by calling Attach. For example:
CWnd myWnd; myWnd.Attach(hWnd);
This makes an entry in the permanent map associating myWnd and hWnd. Calling CWnd::FromHandle(hWnd) will now return a pointer to myWnd. When myWnd is deleted, the destructor will automatically destroy the hWnd by calling the Windows DestroyWindow function. If this is not desired, the hWnd must be detached from myWnd before the myWnd object is destroyed (normally when leaving the scope at which myWnd was defined). The Detach member function does this.
Temporary objects are created whenever FromHandle is given a handle that does not already have a wrapper object. These temporary objects are detached from their handle and deleted by the DeleteTempMap functions. The default OnIdle processing in CWinThread automatically calls DeleteTempMap for each class that supports temporary handle maps. This means that you cannot assume a pointer to a temporary object will be valid past the point of exit from the function where the pointer was obtained, as the temporary object will be deleted during the Windows message-loop idle time.
Both temporary and permanent objects are maintained on a per-thread basis. That is, one thread cannot access another threads C++ wrapper objects, regardless of whether it is temporary or permanent. As stated above, temporary objects are deleted when the thread which that temporary object belongs enters OnIdle.
To pass these objects from one thread to another, always send them as their native HANDLE type. Passing a C++ wrapper object from one thread to another will often result in unexpected results.