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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- 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.
- 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.
- 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.
- This article begins with an overview of pragma once as a preprocessor directive and a simple example of how it is used.
- 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.
- microsoft compiler'ında gayet çalışırken, "#pragma once" içeren kodunuzu başka compiler ile derlemeye çalıştığınızda hata verebilir.
- Using #pragma once will delegate the task, of detecting subsequent #include statements for the same file, to the compiler. It can do this efficiently and safely.
- To use #pragma once, you simply include it at the beginning of your header file, before any other code or declarations. Here’s an example
- 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.
- 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.
