CODEWITHSUNDEEP
c++c
Pranit Kothari
Pranit Kothari

Reputation: 9839

Does #pragma once add inclusion guards?

#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?

Upvotes: 6

Views: 521

Answers (3)

Lidong Guo
Lidong Guo

Reputation: 2857

When adding "#pragma once" to file "file.h",the compiler help us ensure the "file.h" will only be opened noce.

But if I have a duplication of "file.h" named "file_copy.h",if it is included, it will be opened.

Upvotes: -3

Mats Petersson
Mats Petersson

Reputation: 129454

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.

Upvotes: 10

David Rodríguez - dribeas
David Rodríguez - dribeas

Reputation: 208406

No, the compiler will not add include guards, but that should not matter, as it won't include the same file again, so it would never get a change to evaluate those guards in the first place.

Upvotes: 0

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