TN003: Mapping of Windows Handles to Objects
This note describes the MFC routines that support mapping Windows object handles to C++ objects.
The Problem
Windows objects are typically represented by various HANDLE objects The MFC classes wrap Windows object handles with C++ objects. The handle wrapping functions of the MFC class library let you find the C++ object that is wrapping the Windows object that has a particular handle. However, sometimes an object does not have a C++ wrapper object and at these times the system creates a temporary object to act as the C++ wrapper.
The Windows objects that use handle maps are as follows:
HWND (CWnd and CWnd-derived classes)
HDC (CDC and CDC-derived classes)
HMENU (CMenu)
HPEN (CGdiObject)
HBRUSH (CGdiObject)
HFONT (CGdiObject)
HBITMAP (CGdiObject)
HPALETTE (CGdiObject)
HRGN (CGdiObject)
HIMAGELIST (CImageList)
SOCKET (CSocket)
Given a handle to any one of these objects, you can find the MFC object that wraps the handle by calling the static method FromHandle. For example, given an HWND called hWnd, the following line will return a pointer to the CWnd that wraps hWnd:
CWnd::FromHandle(hWnd)
If hWnd does not have a specific wrapper object, a temporary CWnd is created to wrap hWnd. This makes it possible to obtain a valid C++ object from any handle.
After you have a wrapper object, you can retrieve its handle from a public member variable of the wrapper class. In the case of a CWnd, m_hWnd contains the HWND for that object.
Attaching Handles to MFC Objects
Given a newly created handle-wrapper object and a handle to a Windows object, you can associate the two by calling the Attach function as in this 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 hWnd by calling the Windows DestroyWindow function. If this is not desired, hWnd must be detached from myWnd before myWnd is destroyed (normally when leaving the scope at which myWnd was defined). The Detach method does this.
myWnd.Detach();
More About Temporary Objects
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. By default CWinThread::OnIdle 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.
Wrapper Objects and Multiple Threads
Both temporary and permanent objects are maintained on a per-thread basis. That is, one thread cannot access another thread's C++ wrapper objects, regardless of whether it is temporary or permanent.
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 cause unexpected results.