What is the value of __cplusplus for C++17?

Question

We are trying to test some code under C++17 and its change to std::uncaught_exception. I can't seem to get GCC to provide the value of __cplusplus:

$ /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:

$ /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?

Answer 1

tl;dr: For C++17, __cplusplus is 201703L.

What is the value of __cplusplus when using C++17?

According to the draft standard N4594 §16.8/p1 Predefined macro names [cpp.predefined] (Emphasis Mine):

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.¹⁵⁶

_{156) It is intended that future versions of this standard will replace the value of this macro with a greater value. Non-conforming compilers should use a value with at most five decimal digits.}

However the same value is appointed for the C++14 standard. Apparently it seems so, that there's no official/standard __cplusplus value set yet for the C++17 standard.

In GCC versions 6.1 and 7.0 the value is changed to 201500

Live Demo

In Clang version 3.8 and 3.9 the value is unchanged 201406.

Consequently, you'll have to wait a little bit for the standard value to come out.

--- Update ---

According to the C++ standard §19.8/p1 Predefined macro names [cpp.predefined] (Emphasis Mine):

₁ The following macro names shall be defined by the implementation:

__cplusplus The integer literal 201703L.

Thus, the value of __cplusplus when using C++17 shall be 201703L.

Answer 2

I would try:

#if __cplusplus > 201402L
  // C++14 code here
  ...
#endif

In other words, testing for greater than C++14 should work as compilers add more features. As someone mentioned above, GCC uses 201500L. It looks like clang uses 201406L (four months after C++14 I guess).

Using the above snippet should be cross-platform and will work even when C++17 comes out with real value for __cplusplus. For more details about evolving features try the feature test macros.

Answer 3

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:

#if ((defined(_MSVC_LANG) && _MSVC_LANG >= 201703L) || __cplusplus >= 201703L)
     //C++17 specific stuff here
#endif

Answer 4

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.

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.

To get it to report correctly, you have to also set /Zc:__cplusplus.

(source: https://learn.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=vs-2017 )

As to why? Well... in their words:

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.

(source: https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/)

Answer 5

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. This is how I determine the its value.

#include <iostream>
int main( int argc, char** argv )
{
  std::cout << __cplusplus << std::endl;
  return 0;
}

Then compilation shows the value.

$ 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.

Answer 6

A modern compile time check that you can put in any file that needs it:

static_assert(__cplusplus >= 201703L, "This file expects a C++17 compatible compiler.");