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[Reply #15 on:]

Hmm. I tried to boil it down to the following:

Code:

#include <bar.h>
#include <foo.h>

int main(){}

Code:

#ifndef bar
#define bar

#include <foo.h>

#endif

Code:

#ifndef foo
#define foo

void f();
int x = 0;

#endif

This does compile --> I thought I understood it. Everything works but I would expect a compiler error. --> I can not reproduce the issue I was asking about. But obviously I still do not understand it.

In my understanding both foo and bar should contain a definition. Hence they should create a compiler error when I include both of them. Obviously they do not. Any hints on this? Did I understand it wrong again or is this something else I do not know?

[Reply #16 on:]

What happens if you try to use x in you main() ?

BTW, foo will not be included twice, because of the inclusion guard (#ifndef/#define), this is acutally its purpose.