Compiler error with filesystem library: clang and g++ - c++

Question

I am writing a personal project in c++ which needs to access to files in some directories, hence I decided to use the filesystem library. I encountered some problems when I try to compile my project on MacOS and on Linux.
The code snippet is the following
#include <iostream>
#include <fstream>
int main(){
std::string path = "Inner";
std::cout << "Files in " << path << " directory :" << std::endl;
for (const auto & entry : std::filesystem::directory_iterator(path))
std::cout << entry.path() << std::endl;
return 0;
}
When I compile it on my MacBook Pro (clang version 11.0.3 (clang-1103.0.32.62)) with
g++ -o test test.cpp -std=c++17 -Wall
everything works fine. But as soon as I move to Linux (Ubuntu 19.04, g++ 8.3.0) I get the following error:
test.cpp: In function ‘int main()’:
test.cpp:8:33: error: ‘std::filesystem’ has not been declared
for (const auto & entry : std::filesystem::directory_iterator(path)){
I include then the filesystem library with #include <filesystem>:
#include <iostream>
#include <fstream>
#include <filesystem>
int main(){
std::string path = "Inner";
std::cout << "Files in " << path << " directory :" << std::endl;
for (const auto & entry : std::filesystem::directory_iterator(path))
std::cout << entry.path() << std::endl;
return 0;
}
compile it via g++ -o test test.cpp -std=c++17 -Wall -lstdc++fs and everything works fine on Linux too (note that I had to add -lstdc++fs).
Why is there this different behaviour on MacOS and on Linux? Does it depends on the compiler? What happens with Windows OS (I do not have any Windows PC at home)?
I found a related question and its answer here, but it does not seem to explain why in the first case (with clang) everything works fine also without including filesystem library.

Answer 1

Using 'g++' is not using clang you should use 'clang++'
Gcc should not be platform dependent but it might be different version
At any case, you should explicitly include header files needed, and std::filesystem is defined in "<filesystem>"
regarding the need to add "lstdc++fs' - this is a hint that actually g++ version is different and uses different llvm versions. As described in https://en.cppreference.com/w/cpp/filesystem
Notes:
Using this library may require additional compiler/linker options. GNU implementation prior to 9.1 requires linking with -lstdc++fs and LLVM implementation prior to LLVM 9.0 requires linking with -lc++fs