[英]What is the value of __cplusplus for C++17?
We are trying to test some code under C++17 and its change to std::uncaught_exception
.我们正在尝试在 C++17 下测试一些代码及其对
std::uncaught_exception
更改。 I can't seem to get GCC to provide the value of __cplusplus
:我似乎无法让 GCC 提供
__cplusplus
的值:
$ /opt/local/bin/g++ -std=c++17 -dM -E - </dev/null | grep __cplusplus
cc1: warning: command line option '-std=c++1z' is valid for C++/ObjC++ but not for C
$
And:并且:
$ /opt/local/bin/g++ --version
g++-mp-6 (MacPorts gcc6 6.1.0_0) 6.1.0
Copyright (C) 2016 Free Software Foundation, Inc.
What is the value of __cplusplus
when using C++17?使用 C++17 时
__cplusplus
的值是什么?
__cplusplus
is 201703L
. __cplusplus
是201703L
。What is the value of
__cplusplus
when using C++17?使用 C++17 时
__cplusplus
的值是什么?
According to the draft standard N4594 §16.8/p1 Predefined macro names [cpp.predefined] ( Emphasis Mine ):根据草案标准N4594 §16.8/p1 预定义宏名称 [cpp.predefined] (强调我的):
The following macro names shall be defined by the implementation:
__cplusplus
The name__cplusplus
is defined to the value 201402L when compiling a C++ translation unit.以下宏名称应由实现定义:
__cplusplus
编译 C++ 翻译单元时,名称__cplusplus
定义为值201402L 。 156156
156) It is intended that future versions of this standard will replace the value of this macro with a greater value.
156) 本标准的未来版本打算用更大的值替换此宏的值。 Non-conforming compilers should use a value with at most five decimal digits.
不符合标准的编译器应使用最多五位十进制数字的值。
However the same value is appointed for the C++14 standard.但是,为 C++14 标准指定了相同的值。 Apparently it seems so, that there's no official/standard
__cplusplus
value set yet for the C++17 standard.显然,C++17 标准还没有官方/标准的
__cplusplus
值集。
In GCC versions 6.1 and 7.0 the value is changed to 201500在 GCC 版本 6.1 和 7.0 中,该值更改为201500
In Clang version 3.8 and 3.9 the value is unchanged 201406 .在 Clang 版本 3.8 和 3.9 中,该值保持不变201406 。
Consequently, you'll have to wait a little bit for the standard value to come out.因此,您必须稍等片刻才能得出标准值。
According to the C++ standard §19.8/p1 Predefined macro names [cpp.predefined] ( Emphasis Mine ):根据 C++ 标准§19.8/p1 预定义宏名称 [cpp.predefined] ( Emphasis Mine ):
1 The following macro names shall be defined by the implementation:
1以下宏名称应由实现定义:
__cplusplus
The integer literal 201703L .__cplusplus
整数文字201703L 。
Thus, the value of __cplusplus
when using C++17 shall be 201703L .因此,使用 C++17 时
__cplusplus
的值应为201703L 。
I would try:我会尝试:
#if __cplusplus > 201402L
// C++14 code here
...
#endif
In other words, testing for greater than C++14 should work as compilers add more features.换句话说,当编译器添加更多功能时,测试大于 C++14 应该可以工作。 As someone mentioned above, GCC uses
201500L
.正如上面提到的,GCC 使用
201500L
。 It looks like clang uses 201406L
(four months after C++14 I guess).看起来 clang 使用
201406L
(我猜是在 C++14 之后四个月)。
Using the above snippet should be cross-platform and will work even when C++17 comes out with real value for __cplusplus
.使用上面的代码片段应该是跨平台的,即使 C++17 为
__cplusplus
提供真正的价值也能工作。 For more details about evolving features try the feature test macros .有关不断发展的功能的更多详细信息,请尝试功能测试宏。
Normally you should use __cplusplus
define to detect c++17, but by default microsoft compiler does not define that macro properly, see https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/ - you need to either modify project settings to include /Zc:__cplusplus
switch, or you could use syntax like this:通常你应该使用
__cplusplus
定义来检测 c++17,但默认情况下,微软编译器没有正确定义该宏,请参阅https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/ - 你需要修改项目设置以包含/Zc:__cplusplus
开关,或者您可以使用如下语法:
#if ((defined(_MSVC_LANG) && _MSVC_LANG >= 201703L) || __cplusplus >= 201703L)
//C++17 specific stuff here
#endif
I realize you asked this question citing the Gnu C++ compiler as the one you're using, but you may want to have some awareness of what happens on the Visual C++ compiler, and strictly speaking, your question didn't ask about a specific compiler.我意识到你问这个问题时引用了 Gnu C++ 编译器作为你正在使用的编译器,但你可能想了解 Visual C++ 编译器上发生的事情,严格来说,你的问题没有询问特定的编译器.
Currently, as of the date of this posting, the VC++ 2017 compiler sets __cplusplus
to 199711L
rather than what you might expect if you set the compiler to use c++17.目前,截至本文发布之日,VC++ 2017 编译器将
__cplusplus
设置为199711L
而不是将编译器设置为使用 c++17 时所期望的值。
To get it to report correctly, you have to also set /Zc:__cplusplus
.要使其正确报告,您还必须设置
/Zc:__cplusplus
。
(source: https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=vs-2017 ) (来源: https : //docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=vs-2017 )
As to why?至于为什么? Well... in their words:
嗯……用他们的话来说:
We tried updating the macro by default and discovered that a lot of code doesn't compile correctly when we change the value of __cplusplus.
我们尝试默认更新宏,发现当我们更改 __cplusplus 的值时,很多代码无法正确编译。
(source: https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/ ) (来源: https : //devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/ )
I don't really know why __cplusplus
doesn't show up as a regular macro, but I'm guessing it's so you can't redefine it.我真的不知道为什么
__cplusplus
没有显示为常规宏,但我猜是这样你不能重新定义它。 This is how I determine the its value.这就是我确定它的价值的方式。
#include <iostream>
int main( int argc, char** argv )
{
std::cout << __cplusplus << std::endl;
return 0;
}
Then compilation shows the value.然后编译显示值。
$ g++-6 test.cpp && ./a.out
201402
$ g++-6 -std=c++17 test.cpp && ./a.out
201500
I would check whether it's >= 201500
as opposed to checking for any specific value.我会检查它是否
>= 201500
,而不是检查任何特定值。
现代编译时检查,您可以将其放入任何需要它的文件中:
static_assert(__cplusplus >= 201703L, "This file expects a C++17 compatible compiler.");
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