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.