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c++mfcafx

windows.h and MFC

Why can't I include windows.h in afx(MFC) projects?

Solution

  • Typically, MFC application code includes afx.h or afxwin.h (the latter includes former). First two lines of windows.h are

    #ifndef _WINDOWS_
#define _WINDOWS_

    which means that _WINDOWS_ becomes defined if this header is included. Afx.h includes afxver_.h and this header includes afxv_w32.h which contains following code:

    #ifdef _WINDOWS_
    #error WINDOWS.H already included. MFC apps must not #include <windows.h>
#endif
...
#include <windows.h>

    So, if you include windows.h before MFC headers, you'll get this error generated in compile time and, as you can see, if you include afxwin.h you don't need to include windows.h yourself - it will already be included by afxv_w32.h.