MFC Windows 程序设计->最顶层窗口

The difference between a topmost window and a nontopmost window is an
extended window style bit. WS_EX_TOPMOST makes a window a topmost window. You
can create a topmost frame window by including a WS_EX_TOPMOST flag in the call
to Create, like this:

Create (NULL, _T ("MyWindow"), WS_OVERLAPPEDWINDOW, rectDefault, NULL, NULL, WS_EX_TOPMOST); [/code]

The alternative is to add the style bit after the window is created by
calling SetWindowPos with a &wndTopMost parameter, as shown
here:

SetWindowPos (&wndTopMost, 0, 0, 0, 0, SWP_NOMOVE ¦ SWP_NOSIZE); [/code]

You can convert a topmost window into a nontopmost window by calling
SetWindowPos with the first parameter equal to &wndNoTopMost
rather than &wndTopMost.

Clock uses SetWindowPos to make its window a topmost window when Stay
On Top is checked in the system menu and a nontopmost window when Stay On Top is
unchecked. The work is done by CMainWindow::SetTopMostState, which is
called by OnSysCommand. When Stay On Top is checked, Clock is visible on
the screen at all times, even if it's running in the background and it overlaps
the application running in the foreground.