在C/C ++预处理器中，在其自己的行上加一个井号/井号(#)的目的是什么? [英] What is the purpose of a single pound/hash sign (#) on its own line in the C/C++ preprocessor?

问题描述

我一直在查看 Boost 库的源代码，并且我已经注意到通常是单磅符号，没有附加任何预处理程序指令.我通读了GCC预处理程序手册和规格指南，但找不到任何东西.

I have been looking at the Boost libraries source code, and I have noticed that often there are single pound signs without any preprocessor directives attached to them. I read through the GCC preprocessor manual and specification guide and can't find anything about it.

(1) #ifndef BOOST_CONFIG_HPP
(2) #  include <boost/config.hpp>
(3) #endif
(4) #
(5) #if defined(BOOST_HAS_PRAGMA_ONCE)
(6) #  pragma once
(7) #endif

在第4行上，井号后面没有任何内容.这有什么作用?它是在C预处理程序(CPP)规范中定义的吗?

On line 4, there is nothing after the pound sign. What effect does this have? Is it defined in the C preprocessor (CPP) specification?

由于Boost是跨平台的库，因此我认为任何CPP都应正确解析它.在整个代码中具有随机的磅/哈希符号会产生什么影响/副作用?

As Boost is a cross-platform library, I would assume that any CPP should parse it correctly. What would the effect/side-effects be of having random pound/hash signs throughout the code?

推荐答案

#单独在一行上根本没有任何作用.我认为它是用于美学价值的.

A # on its own on a line has no effect at all. I assume it's being used for aesthetic value.

C标准说:

6.10.7空指令

语义

形式的预处理指令

# new-line

没有效果.

C ++标准说了同样的话:

The C++ standard says the same thing:

16.7空指令[cpp.null]

形式的预处理指令

# new-line

没有效果.

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