C++中__cplusplus宏介绍

爱被打了一巴掌 2022-09-04 03:48 266阅读 0赞

### 文章目录 ###

*   *  背景
     *  C++标准和编译器中的预定义宏
     *  查看\_\_cplusplus宏定义的值
     *  \_\_cplusplus宏定义的位置
     *  参考

## 背景 ##

[GCC中实现的C++标准库][GCC_C]中[\_\_cplusplus][cplusplus]是一个很常见的宏。

弄清这个宏之前，我们需要先明白**编程语言和编译器之间的关系**。C++是一门编程语言，它有其[标准][Link 1]。而[编译C++代码的编译器][C]有多种，如常见的[Gnu Compiler Collection][]、[Clang][]、[Visual C++ 2017 Community][Visual C_ 2017 Community]。不同的C++编译器需要遵守/支持C++的标准。

了解上述背景之后，我们来查看`__cplusplus`宏在C++标准的要求和编译器中的定义。

## C++标准和编译器中的预定义宏 ##

鉴于C++的标准文档[需要收费][Link 2]，我们在[cppreference][]中查看关于`__cplusplus`的标准信息。

# 标准规定支持该宏
    __cplusplus
    denotes the version of C++ standard that is being used, expands to value 199711L(until C++11), 201103L(C++11), 201402L(C++14), 201703L(C++17), or 202002L(C++20)
    (macro constant)

我使用的是Ubuntu系统，所以我们查看下[GCC标准预定义宏][GCC]。关于windows系统中`__cplusplus`宏定义的查看，可参考[/Zc:\_\_cplusplus（启用更新的 \_\_cplusplus 宏][Zc_cplusplus_ _cplusplus]

# GCC遵守C++标准支持该宏
    __cplusplus
    This macro is defined when the C++ compiler is in use. You can use __cplusplus to test whether a header is compiled by a C compiler or a C++ compiler. This macro is similar to __STDC_VERSION__, in that it expands to a version number. Depending on the language standard selected, the value of the macro is 199711L for the 1998 C++ standard, 201103L for the 2011 C++ standard, 201402L for the 2014 C++ standard, 201703L for the 2017 C++ standard, 202002L for the 2020 C++ standard, or an unspecified value strictly larger than 202002L for the experimental languages enabled by -std=c++23 and -std=gnu++23.

这里，我们知道`__cplusplus`宏的含义是“C++的版本“。接下来我们查看当前系统该宏的值是多少。

## 查看\_\_cplusplus宏定义的值 ##

参考[Which C++ standard is the default when compiling with g++?][Which C_ standard is the default when compiling with g]，我们使用下面命令查看宏的值。

# 输出编译器预定义的宏，grep进行过滤
    ➜  g++ -dM -E -x c++  /dev/null | grep -F __cplusplus
    #define __cplusplus 201402L

关于GCC的命令行参数，可通过[GCC手册][GCC 1]查看。上面的参数含义如下：

-dM -E # 输出预定义的宏
    Instead of the normal output, generate a list of ‘#define’ directives
    for all the macros defined during the execution of the preprocessor,
    including predefined macros. This gives you a way of finding out
    what is predefined in your version of the preprocessor. 
    If you use ‘-dM’ without the ‘-E’ option, ‘-dM’ is interpreted as a
    synonym for ‘-fdump-rtl-mach’
    
    -E # 预处理之后停止，预处理的结果输出到标准输出
    Stop after the preprocessing stage; do not run the compiler proper. The output
    is in the form of preprocessed source code, which is sent to the standard output.
    Input files that don’t require preprocessing are ignored
    
    -x language # 指定语言
    Specify explicitly the language

我们已经知道该宏的值，那这个宏在哪里/何时定义的呢？

## \_\_cplusplus宏定义的位置 ##

结论：**我暂时不知道，如果你知道的话，可以评论区留言**。

我知道[c++config][c_config]用于预定义C++库的宏。但是编译器预定义的宏的位置在哪呢？(或者说，这些宏本身的创建过程是什么样子的？)

我下载了[GCC源码][GCC 2]，它使用[autotools][]进行构建。我企图从这里找到答案。我使用关键字在源代码中进行搜素，但是并没有找到答案。

## 参考 ##

[Where do I find the current C or C++ standard documents?][Link 2]

[What does the “\_\_cplusplus” macro expand to?][What does the _cplusplus_ macro expand to]

[Which C++ standard is the default when compiling with g++?][Which C_ standard is the default when compiling with g]

[GCC_C]: https://blog.csdn.net/sinat_38816924/article/details/119207217#t3
[cplusplus]: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/allocator.h#L48
[Link 1]: https://isocpp.org/std
[C]: https://isocpp.org/get-started
[Gnu Compiler Collection]: http://gcc.gnu.org/
[Clang]: http://clang.llvm.org/get_started.html
[Visual C_ 2017 Community]: https://www.visualstudio.com/vs/cplusplus/
[Link 2]: https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-standard-documents
[cppreference]: https://en.cppreference.com/w/cpp/preprocessor/replace
[GCC]: https://gcc.gnu.org/onlinedocs/cpp/Standard-Predefined-Macros.html
[Zc_cplusplus_ _cplusplus]: https://docs.microsoft.com/zh-cn/cpp/build/reference/zc-cplusplus?view=msvc-160
[Which C_ standard is the default when compiling with g]: https://stackoverflow.com/questions/44734397/which-c-standard-is-the-default-when-compiling-with-g
[GCC 1]: https://gcc.gnu.org/onlinedocs/gcc-9.3.0/gcc/
[c_config]: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/c%2B%2Bconfig
[GCC 2]: https://github.com/gcc-mirror/gcc
[autotools]: https://blog.csdn.net/sinat_38816924/category_10907321.html
[What does the _cplusplus_ macro expand to]: https://stackoverflow.com/questions/49915424/what-does-the-cplusplus-macro-expand-to