C语言 #pragma once 在 C 中是什么意思？
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What does #pragma once mean in C?
提问by wireshark
Possible Duplicate:
#pragma - help understanding
可能的重复：
#pragma - 帮助理解
I saw the pragmamany times,but always confused, anyone knows what it does?Is it windows only?
看了pragma很多遍了，还是一头雾水，有谁知道是做什么用的吗？是不是只有windows？
回答by helpermethod
In the C and C++ programming languages, #pragma once is a non-standard but widely supported preprocessor directive designed to cause the current source 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, avoiding name clashes, and improved compile speed.
在 C 和 C++ 编程语言中，#pragma once 是一个非标准但受到广泛支持的预处理器指令，旨在使当前源文件在单个编译中只包含一次。因此，#pragma 曾经与#include 守卫服务相同的目的，但具有几个优点，包括：更少的代码、避免名称冲突和提高编译速度。
See the Wikipediaarticle for further details.
有关更多详细信息，请参阅维基百科文章。
回答by Alex Kremer
It's used to replace the following preprocessor code:
它用于替换以下预处理器代码：
#ifndef _MYHEADER_H_
#define _MYHEADER_H_
...
#endif
A good convention is adding both to support legacy compilers (which is rare tho):
一个好的约定是添加两者以支持遗留编译器（这种情况很少见）：
#pragma once
#ifndef _MYHEADER_H_
#define _MYHEADER_H_
...
#endif
So if #pragma once fails the old method will still work.
因此，如果#pragma once 失败，旧方法仍然有效。
回答by Alex Kremer
Generally, the #pragmadirectives are intended for implementing compiler-specific preprocessor instructions. They are not standardized, so you shouldn't rely on them too heavily.
通常，这些#pragma指令旨在实现特定于编译器的预处理器指令。它们没有标准化，因此您不应过分依赖它们。
In this case, #pragma once's purpose is to replace the include guards that you use in header files to avoid multiple inclusion. It works a little faster on the compilers that support it, so it may reduce the compilation time on large projects with a lot of header files that are #include'ed frequently.
在这种情况下，#pragma once的目的是替换您在头文件中使用的包含保护以避免多次包含。它在支持它的编译器上运行得更快一些，因此它可以减少大型项目的编译时间，这些项目有很多#include经常被'ed的头文件。
回答by danny_23
pragma is a directive to the preprocessor. It is usually used to provide some additional control during the compilation. For example do not include the same header file code. There is a lot of different directives. The answer depends on what follows the pragma word.
pragma 是预处理器的指令。它通常用于在编译期间提供一些额外的控制。例如不要包含相同的头文件代码。有很多不同的指令。答案取决于 pragma 词后面的内容。
