I’m tinkering with C++ again.  I’m trying to use as much standard C++17 as possible because it’s a good reason to learn the C++17 standards.

Boost has a filesystem library. The library evolved into a core C++ library standard. GCC 7 has the filesystem module in the std::experimental namespace. GCC 7 moved the filesystem into the top level std:: namespace.

I’m trying to write a program that works in both recent Fedora and Ubuntu. Fedora29 uses GCC8, Ubuntu18.04 uses GCC7. Dang it. The experimental filesystem module still exists in GCC8 (/usr/include/c++/8/experimental/filesystem) but I’d like to use the latest and greatest available, if possible (/usr/include/c++/8/filesystem).

I can rename the namespace via the “namespace fs = ” which is really useful. Is a bit like Pythons “import module as newname” but I need to know which header and which namespace to rename. My first thought was duplicating the __cplusplus macro used in the filesystem header in the two aforementioned paths in Fedora.

#if __cplusplus >= 201703L
#include <filesystem> // gcc8 (Fedora29+)
namespace fs = std::experimental::filesystem;
#elif __cplusplus >= 201103L
#include <experimental/filesystem> // gcc7 (Ubuntu 18.04)
namespace fs = std::experimental::filesystem;
#endif


No such luck. The __cplusplus macro is set based on the -stdc++NN command line arg. So my build under Ubuntu also uses -stdc++17 and would hit the __cplusplus >= 201703L.

I don’t want to specifically test for GCC version in an #ifdef. I’d also like to eventually build under OSX (Clang+XCode) and Windows (VC++).

I’m using CMake. I think I’m going to have to do some test compiles with CMake to determine which header to use.