在同一个头文件中包含一次防护和#pragma [英] Include guard and #pragma once in the same header file

问题描述

引用 Microsoft文档，There is no advantage to use of both the #include guard idiom and #pragma once in the same file.

先前有关stackoverflow的相关问题的答案也证实，同时拥有这两者毫无意义.例如，请参见下文:

Answers to previous related questions on stackoverflow also confirm that it is pointless to have both. See below, for instance:

头防卫和杂物一次

boost库的vector.hpp文件由此开始:

#ifndef BOOST_ASSIGN_STD_VECTOR_HPP
#define BOOST_ASSIGN_STD_VECTOR_HPP

#if defined(_MSC_VER)
# pragma once
#endif
...
#endif

也就是说，它既包括后卫习语，也包括一次实用语. boost头文件同时具有这两个原因吗?

That is, it includes both the guard idiom as well as the pragma once. Is there any reason why boost header files have both?

推荐答案

技术上#pragma once不是标准的C ++，而标头保护符则是.如果两者都有，它们不会互相冲突.

Technically #pragma once is not standard C++, whereas header guards are. They will not conflict with each other if you have both.

#if defined(_MSC_VER)暗示，boost可能兼有的原因是，如果您不使用MSVC，则需要某物作为标头后卫，因此它们会退回到另一种方法.

The reason boost likely has both, as alluded to by the #if defined(_MSC_VER) is that if you're not using MSVC then you need something to act as your header guard, so they fall back to the other method.

由于boost致力于跨平台，因此他们试图确保其代码在不支持#pragma once的编译器上运行，尽管我能想到的所有大型现代编译器都支持它，例如在维基百科上进行枚举.

Since boost strives to be cross-platform they are trying to ensure their code works on compilers that don't support #pragma once, though all of the big modern compilers I can think of do support it, as enumerated on wikipedia.

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