Is an #include before #ifdef/#define Include-Guard okay?

问题

I always placed my #include after the #ifdef/#define Include-Guard. Now the refactor mechanism of my IDE (Qt Creator) put it before the Include-Guard e.g.

#include "AnotherHeader.h"

#ifndef MYHEADER_H
#define MYHEADER_H

Can this cause any problems or can I leave it this way?

回答1:

If the header in question has include guards itself, you won't run into problems. Putting it inside the include guards may still speed up compilation. Something the compiler does not see takes less time to compile, even if it does not produce any errors.

回答2:

Although less common, I believe this is considered to be acceptable, however beware: if you have circular #include's, your includes will typically include each other forever (until the pre-parser complains about the maximum include depth being reached). This will not happen with the #include's after the include guards.

(Circular #include's is not considered good style in any case, but if it is used nevertheless, it might work with #include's after the include guards, but certainly won't with the #include's before them.)

回答3:

Simply check out, what will happen. Let's assume, that two different headers uses MyHeader.h.

1. AnotherHeader.h is included unconditionally
2. Include guard allows loading rest of your header file.

The next time:

1. AnotherHeader.h is included unconditionally again
2. Include guard prevents from loading rest of your header file.

If AnotherHeader.h is include-guarded, nothing bad should happen. But generally I'd put the include-guard at the top of your file - there's no point in loading AnotherHeader.h again, when it was already loaded once.

来源：https://stackoverflow.com/questions/20988705/is-an-include-before-ifdef-define-include-guard-okay