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A window can be created as either a child or a popup. A child window can be moved only within its parent's rectangle area. Let's say one wants to do just the opposite, that is, to move a child window out of its parent's area, it is hard to switch its style without destroying and creating a new one. That is the original motivation for creating the CPopChildWnd.
CPopChildWnd is based on MFC's CWnd class, it actually uses two CWnd derivatives(one is created as child and the other as a popup) to give the appearence of its switchable style. To use it, user embeds a CPopChildWnd(or often time, its derivative) as a data member, and call its Create(...) function. When CPopChildWnd's create function is called, it creates two windows, as mentioned before, one is visible and the other is hidden. User then will be able to drag the window around by mouse, when it is in the parent's rectangle area, it is a child(actually the child window is shown), when it is outside of the parent's area, it is a popup window(the popup window shown). Because the drawing stuff is all handled by CPopChildWnd itself, therefore the two switchable windows appear to be the same.
I also used it as a banner where some text and graphs were shown, user can move that banner out of the way to have more screen real estate to work with still be able to see the banner.
I can also imagine it be used as some sort of short cut bar where user can conveniently place it anywhere on the desktop.
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