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Change:

// friend class WindowMgr;
friend void WindowMgr::clear(ScreenIndex);
To:

friend class WindowMgr;
// friend void WindowMgr::clear(ScreenIndex);

The class WindowMgr is not yet...

Incomplete types



And? Is there any problem in moving its initialisation to .cpp?

You must either move that initialisation to the .cpp file, or leave the definition of WindowMgr "below the...

You cannot put your WindowMgr "above that class", e.g., include its header file before including Screen's header file, because you attempt to use an incomplete type (Screen) when initialising your...