In the C and C++ programming languages, #pragma once is a non-standard but widely supported preprocessor directive designed to cause the current header file to be included only once in a single compilation. Thus, #pragma once serves the same purpose as include guards, but with several advantages, including less code, avoidance of name clashes, and sometimes improvement in compilation speed.
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- en.wikipedia.org Pragma onceWhereas 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.
- azure.microsofts.workers.dev en-us/cpp/…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.
- 64.github.io Faqs What are 'Include Guards' and `#pragma once`?If you do measure a significant difference in compile times and want to use #pragma once anyways, you can use both include guards and #pragma once.
- stackoverflow.com questions/5776910/what-does-…This makes little to no sense in 2023 and beyond unless you are targeting some special compiler that you know does not support #pragma once.
- delftstack.com Cpp Cpp Pragma OnceThis article begins with an overview of pragma once as a preprocessor directive and a simple example of how it is used.
- hatchjs.com pragma-once-vs-ifndef/In this article, we will take a closer look at the `pragma once` and `ifndef` directives, and we will discuss the pros and cons of each directive.
- eksisozluk.com pragma-once--1018585microsoft compiler'ında gayet çalışırken, "#pragma once" içeren kodunuzu başka compiler ile derlemeye çalıştığınızda hata verebilir.
- peterbudai.medium.com using-pragma-once-together-…GCC and Clang claim that they are able to detect the header guard idiom and offer the same performance as #pragma once making it unnecessary.
- positioniseverything.net pragma-once/What Is “pragma once” and Why Is It Used in C++? – The Role of Header Files in C++ Programming. – Solution: Include Guards or “pragma once”.
- youtube.com watchPRAGMA ONCE EXPLAINED | how to use #pragma once | pragma c |pragma in c programming.19 bin görüntülemeYayınlandı8 Nis 2015
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It's used to replace the following preprocessor code:A good convention is adding both to support legacy compilers (which is rare though):So if fails the old method will still work.2023 updateI see some people in the comment section advocate for using guards instead of .
This makes little to no sense in 2023 and beyond unless you are targeting some special compiler that you know does not support .Today's best practice is to use only and don't bother with guards at all. Reasons being
#ifndef _MYHEADER_H_#define _MYHEADER_H_...#endif
#pragma once#ifndef _MYHEADER_H_#define _MYHEADER_H_...#endif
#pragma once
#pragma once
#pragma once
#pragma once
- All major compilers been supporting this forever and that is not going to change.
- Using allows the compiler to use its internal caches which is of course faster than using the pre-processor which will always include the contents of your file just to later stumble on your guards and dismiss the whole thing.
#pragma
- It's a lot shorter and easier to add/maintain