Is #pragma once a safe include guard?

Question

I\'ve read that there is some compiler optimization when using #pragma once which can result in faster compilation. I recognize that is non-standard, and thus

Answer 1

Using gcc 3.4 and 4.1 on very large trees (sometimes making use of distcc), I have yet to see any speed up when using #pragma once in lieu of, or in combination with standard include guards.

I really don't see how its worth potentially confusing older versions of gcc, or even other compilers since there's no real savings. I have not tried all of the various de-linters, but I'm willing to bet it will confuse many of them.

I too wish it had been adopted early on, but I can see the argument "Why do we need that when ifndef works perfectly fine?". Given C's many dark corners and complexities, include guards are one of the easiest, self explaining things. If you have even a small knowledge of how the preprocessor works, they should be self explanatory.

If you do observe a significant speed up, however, please update your question.