My understanding was the best practice is:

#if __cplusplus >= 202002L
// code
#endif

However, it does not work, even though I do compile with -std=c++20.

Also, the output of g++ -x c++ -std=c++20 -dM -E - </dev/null | grep __cplusplus is:

#define __cplusplus 201709L

Which really confuses me, because I assumed that the correct define for C++17 is 201703L.

Am i missing something?

I have tried to check if <=> support exists. So i can also try to do:

#if __cplusplus >= __cpp_impl_three_way_comparison && __cplusplus >= __cpp_lib_three_way_comparison

However, that does not work, as both of those constants say 201907L.

P.S.

$ g++ --version
g++ (Debian 10.2.1-6) 10.2.1 20210110
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.