#pragma 一旦进入头护卫的原因是什么？
刚刚在
#ifndef BOOST_ASIO_HPP
#define BOOST_ASIO_HPP
#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
/// ....
#endif // BOOST_ASIO_HPP
忽略 _MSC_VER 预处理器检查，在中使用 #pragma Once 有什么好处这个案子？预处理器标头保护是否不能确保在所有情况下和所有平台上，标头内容仅包含一次？
Just seen this inside <boost/asio.hpp>
#ifndef BOOST_ASIO_HPP
#define BOOST_ASIO_HPP
#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
/// ....
#endif // BOOST_ASIO_HPP
Disregarding the _MSC_VER preprocessor checks, what is the benefit of having the #pragma once in this case? Doesn't the preprocessor header guard ensure in all cases and on all platforms, the header contents are only ever included once?
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#pragma Once指定编译器在编译源代码文件时将仅包含（打开）该文件一次。这可以减少构建时间，因为编译器不会在模块的第一个#include之后打开和读取文件。如果您不
#pragma Once，该文件将每次需要时打开，并且编译器将在#ifndef BOOST_ASIO_HPP上停止解析它，如果已定义。#pragma oncespecifies that the file will be included (opened) only once by the compiler when compiling a source code file. This can reduce build times as the compiler will not open and read the file after the first#includeof the module.If you don't
#pragma once, the file will be opened each time it is needed and compiler will stop parsing it on#ifndef BOOST_ASIO_HPP, if it has been defined.指定编译器在构建中仅包含（打开）一次文件。这可以减少构建时间，因为编译器在模块的第一个 #include 之后不会打开和读取文件
还有一个来自 所以
Specifies that the file will be included (opened) only once by the compiler in a build. This can reduce build times as the compiler will not open and read the file after the first #include of the module
And one more related question from SO
是的标头防护确保标头内容仅包含一次。但在这里您使用 #pragma 检查另一个定义而不是包含文件。
以下链接是 SO 中关于标头保护的现有问题。
标头防护的目的
Yes header guards ensures that the header contents are included only once. but here you are using #pragma for checking another definition and not include file.
The below link is existing question on header guards in SO.
Purpose of Header guards
#pragma Once具有相同的目的，但包含防护旨在需要更深入的分析以确保文件仅包含一次 - 例如，除非编译器正确处理此类情况，否则它仍然需要打开文件并将其传递给预处理器，即使它之前已经包含过。
#pragma oncehas the same purpose, but include guards are intended to require a deeper analysis to ensure a file is included exactly once - e.g.Unless the compiler does handle such cases correctly, it still needs to open the file and pass it through the preprocessor even though it has been included before.
您可以使用以下命令以标准方式重现
#pragma Once的效果：尽管它更加冗长。正如其他人所说，它有助于缩短编译时间，因为不会搜索/打开文件，而不是打开文件并忽略其中的所有内容 - 文件仍然需要由预处理器解析。
You can reproduce the effect of the
#pragma oncein a standard way using the following:although it is much more verbose. As others have said, it helps with compilation times since the file is not searched for / opened instead of opening the file and ignoring everything inside it - the file still has to be parsed by the preprocessor.