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.
0
Péter Török
On
Because in MFC you are not supposed to use it directly. AFAIR you should include afx.h instead, which in turn indirectly includes windows.h the proper way.
0
A.J.
On
You can include windows.h; but you need to first include afx.h (or similar). If you got the error: "MFC apps must not #include <Windows.h>"; it is from including something like afx.h after including windows.h.
You might need to turn on 'show includes' if not sure how it got included.
Typically, MFC application code includes
afx.horafxwin.h(the latter includes former). First two lines ofwindows.harewhich means that
_WINDOWS_becomes defined if this header is included.Afx.hincludesafxver_.hand this header includesafxv_w32.hwhich contains following code:So, if you include
windows.hbefore MFC headers, you'll get this error generated in compile time and, as you can see, if you includeafxwin.hyou don't need to includewindows.hyourself - it will already be included byafxv_w32.h.