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

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.

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.

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://docs.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/)