Using a Window
Class CWindow allows you to use a window. Once you attach a window to a CWindow object, you can then call CWindow methods to manipulate the window. CWindow also contains an HWND operator to convert a CWindow object to an HWND. Thus you can pass a CWindow object to any function that requires a handle to a window. You can easily mix CWindow method calls and Win32 function calls, without creating any temporary objects.
Because CWindow has only two data member (a window handle and the default dimensions), it does not impose an overhead on your code. In addition, many of the CWindow methods simply wrap corresponding Win32 API functions. By using CWindow, the HWND member is automatically passed to the Win32 function.
In addition to using CWindow directly, you can also derive from it to add data or code to your class. ATL itself derives three classes from CWindow: CWindowImpl, CDialogImpl, and CContainedWindowT.