__cplusplus values: which number means which C++ standard (199711L to 202302L)

__cplusplus is a predefined macro that expands to a number of the form yyyymmL, and the number tells you which C++ standard the compiler is using for this build: 199711L for C++98, 201103L for C++11, 201402L for C++14, 201703L for C++17, 202002L for C++20 and 202302L for C++23 (some older compilers print 202100L for -std=c++23). The GCC manual lists these values under standard predefined macros. In a C compiler the macro is not defined, which is how headers tell the two languages apart.

Values

The GNU dialects (-std=gnu++17 and so on) keep the same __cplusplus value and add GNU extensions. For experimental modes the macro is a value larger than the last published standard.

Check it with your compiler

for std in c++98 c++11 c++14 c++17 c++20 c++23; do
  printf '%s: ' "$std"
  echo '__cplusplus' | g++ -std=$std -x c++ -E -P - | tail -n 1
done

c++98: 199711L
c++11: 201103L
c++14: 201402L
c++17: 201703L
c++20: 202002L
c++23: 202100L

On the GCC 13.3 used for this check, -std=c++23 prints 202100L, not the 202302L that the manual lists for the 2023 standard. A compiler that implements C++23 only partly can report a different value, so test __cplusplus >= 202100L when you need to accept such compilers, or use feature-test macros.

Reading the number

The first four digits are the year and the last two the month of the standard's version, so 201703L is 2017-03. Use >= rather than == so that code for C++17 also builds in C++20:

#if __cplusplus >= 201703L
// C++17 or later
#endif

Pitfalls

• The value follows the flags, not the compiler. A new compiler with an old -std still reports the old number. Print the real compile command (make V=1, cmake --build . --verbose).
• MSVC differs. It leaves __cplusplus at 199711L unless /Zc:__cplusplus is set; use _MSVC_LANG there.
• Feature-test macros are more precise. __cpp_if_consteval, __cpp_lib_expected and similar macros state whether one feature exists, even in a partial implementation.

The flag itself is set when you compile, see compile a C++ program. For what each standard added, read C++20 to C++23. A feature that is missing under the selected standard often appears as was not declared in this scope.

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