Does #pragma once add inclusion guards?

Question

#pragma once is not standard, but is supported by compilers like gcc and VC++. It helps to avoid inclusion guards.

But, internally, does the compiler add inclusion guards for #pragma once? If not, how does the compiler ensure that such a header is included only once?

Answer 1

I'm sure it works just like include_once in PHP - there is a table of "files that has been included". The compiler, in this case, looks in the list for the file it is about to include and if a file has already been included, don't include it again. If the compiler, while processign the file, sees a #pragma once, then add this file to "files that have already been included".

So it's not the same as inclusion guards on a detail level, but it has the same effect as inclusion guards. It also makes the code less portable, since there are plenty of compilers that doesn't support this.