GCC 7.5.0 with --std=c++17: filesystem: No such file or directory

[ijulles]

Compilation fails using GCC 7.5.0 (default GCC compiler version in ubuntu 18.04) for a target which has --std=c++17 compiler option.

Please see: nlohmann/json#3090

I'd like to add a filesystem header file in include/mdf , add code like below from https://stackoverflow.com/questions/53365538/how-to-determine-whether-to-use-filesystem-or-experimental-filesystem

// We haven't checked which filesystem to include yet
#ifndef INCLUDE_STD_FILESYSTEM_EXPERIMENTAL

// Check for feature test macro for <filesystem>
#   if defined(__cpp_lib_filesystem)
#       define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 0

// Check for feature test macro for <experimental/filesystem>
#   elif defined(__cpp_lib_experimental_filesystem)
#       define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1

// We can't check if headers exist...
// Let's assume experimental to be safe
#   elif !defined(__has_include)
#       define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1

// Check if the header "<filesystem>" exists
#   elif __has_include(<filesystem>)

// If we're compiling on Visual Studio and are not compiling with C++17, we need to use experimental
#       ifdef _MSC_VER

// Check and include header that defines "_HAS_CXX17"
#           if __has_include(<yvals_core.h>)
#               include <yvals_core.h>

// Check for enabled C++17 support
#               if defined(_HAS_CXX17) && _HAS_CXX17
// We're using C++17, so let's use the normal version
#                   define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 0
#               endif
#           endif

// If the marco isn't defined yet, that means any of the other VS specific checks failed, so we need to use experimental
#           ifndef INCLUDE_STD_FILESYSTEM_EXPERIMENTAL
#               define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1
#           endif

// Not on Visual Studio. Let's use the normal version
#       else // #ifdef _MSC_VER
#           define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 0
#       endif

// Check if the header "<filesystem>" exists
#   elif __has_include(<experimental/filesystem>)
#       define INCLUDE_STD_FILESYSTEM_EXPERIMENTAL 1

// Fail if neither header is available with a nice error message
#   else
#       error Could not find system header "<filesystem>" or "<experimental/filesystem>"
#   endif


// We priously determined that we need the exprimental version
#   if INCLUDE_STD_FILESYSTEM_EXPERIMENTAL
// Include it
#       include <experimental/filesystem>
        namespace fs = std::experimental::filesystem;
// We have a decent compiler and can use the normal version
#   else
// Include it
#       include <filesystem>
        namespace fs = std::filesystem;
#   endif

#endif // #ifndef INCLUDE_STD_FILESYSTEM_EXPERIMENTAL

Then replace #include<filesystem> with #include "mdf/filesystem.h", and use fs:: instead of std::filesystem::

What do you think about this?

If you think it's okay, I can create a PR to fix it.

[ihedvall]

This filesystem issue have been dealt with before. GCC implemented in GCC 8. The last time, it was solved by updating the GCC compiler but he was running in a Linux container. It's OK to do the change. There is already a src/platform.cpp and a src/platform.h file that deal with different compiler issues. It might be better to use that files. Just ensure that the include/.. files, not are affected i.e. the modified platform.h should only be in src/.. files.