[英]What is the purpose of a single pound/hash sign (#) on its own line in the C/C++ preprocessor?
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. 我一直在查看Boost库的源代码,我注意到通常只有单个符号没有附加任何预处理器指令。 I read through the GCC preprocessor manual and specification guide and can't find anything about it.
我阅读了GCC预处理器手册和规范指南,但没有找到任何相关信息。
(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
On line 4, there is nothing after the pound sign. 在第4行,英镑符号后没有任何内容。 What effect does this have?
这有什么影响? Is it defined in the C preprocessor (CPP) specification?
它是在C预处理器(CPP)规范中定义的吗?
As Boost is a cross-platform library, I would assume that any CPP should parse it correctly. 由于Boost是一个跨平台的库,我认为任何CPP都应该正确解析它。 What would the effect/side-effects be of having random pound/hash signs throughout the code?
在整个代码中使用随机磅/哈希符号会产生什么影响/副作用?
A #
on its own on a line has no effect at all. 一条线路上的
#
根本没有任何影响。 I assume it's being used for aesthetic value. 我认为它被用于审美价值。
The C standard says: C标准说:
6.10.7 Null directive
6.10.7空指令
Semantics
语义
A preprocessing directive of the form
表单的预处理指令
# new-line
has no effect.
没有效果。
The C++ standard says the same thing: C ++标准说同样的事情:
16.7 Null directive [cpp.null]
16.7空指令[cpp.null]
A preprocessing directive of the form
表单的预处理指令
# new-line
has no effect.
没有效果。
It makes the source code look pretty, that's all. 它使源代码看起来很漂亮,就是这样。
Highlights the fact that the whole block is a preprocessor section. 突出显示整个块是预处理器部分的事实。
And indeed, both the C and C++ preprocessors must ignore #
on a line. 事实上,C和C ++预处理器都必须忽略一行中的
#
。
Always check an authoritative source instead of relying on other resources. 始终检查权威来源,而不是依赖其他资源。 C is standardised as ISO 9899::2011, C++ also has an ISO standard.
C标准化为ISO 9899 :: 2011,C ++也有ISO标准。 Both are well accepted and the final drafts available by a short search.
两者都被广泛接受,并通过简短搜索获得最终草稿。 The C standard states in 6.10.7 (C++ has much the same text):
C标准在6.10.7中声明 (C ++有相同的文本):
A preprocessing directive of the form
表单的预处理指令
# new-line
has no effect.
没有效果。
This is a null directive , as much as an ;
这是一个空指令 ,就像一个
;
without a preceeding expression in the core-language is a null statement . 在核心语言中没有前面的表达式是一个空语句 。
For the preprocessor it is just for formatting/readability to highlight that the lines belong semantically together. 对于预处理器,它只是为了格式化/可读性突出显示这些行在语义上属于一起。 (the semicolon OTOH is semantically relevant).
(分号OTOH在语义上相关)。
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