Why use pragma once instead of include guard?
In addition to reducing errors, #pragma once is typically optimized to reduce the use of the preprocessor and file openings, making it faster than include guards. Note that many compilers optimize for this already, so it is possible they are equally fast.
What is wrong with pragma once?
Additionally, #pragma once can do the wrong thing if the same file is intentionally copied into several parts of a project, e.g. when preparing the build. Whereas include guards would still protect from double definitions, #pragma once may or may not treat them as the same file in a compiler-dependent way.
Why do we need pragma once?
The use of #pragma once can reduce build times, as the compiler won’t open and read the file again after the first #include of the file in the translation unit. It’s called the multiple-include optimization.
Should you use pragma once or Ifndef?
The biggest con for #pragma once appears to be that it isn’t officially standard. However, I’ve read that it’s still widely supported in all modern IDEs. #ifndef, on the other hand, is officially supported but is a little more effort (granted, not much) and is not the default implementation in some IDEs.
Should I use pragma once or header guards?
It should never be used. If your #include search path is sufficiently complicated, the compiler may be unable to tell the difference between two headers with the same basename (e.g. a/foo. h and b/foo. h ), so a #pragma once in one of them will suppress both.
Is pragma once good practice?
Personally, I don’t bother with use of #pragma once . It is a solution to a non-existing problem: compilers can absolutely detect include guards to avoid opening files already included. #pragma once solves a real problem, and that real problem is keeping the symbols you define unique.
Is pragma once good?
Conclusion. I recommend using #pragma once instead of old macro-based header guards. It will reduce the number of preprocessor macros and prevent potential and hard to find compilation problems. You should also replace existing macro-based header guards with this statement if you do maintenance work on existing code.
Does G ++ support pragma once?
Clang, g++, Intel C++, and Visual C++ all support #pragma once .