Link errors using <filesystem> members in C++17

Question

I'm using gcc 7.2 on Ubuntu 16.04, and I need to use the new filesystem library from C++17. Even though there is indeed a library called experimental/filesystem, I can't use any of its members. For example, when I try to compile this file:

#include <iostream>
#include <string>
#include <experimental/filesystem>
using namespace std;
namespace fs = std::experimental::filesystem::v1;

int main(){
    fs::path p1 = "/usr/share/";
}

I get a compilation error which looks like this:

$ g++-7 test.cpp -std=c++17
/tmp/ccfsMnlG.o: In function `std::experimental::filesystem::v1::__cxx11::path::path<char [12], std::experimental::filesystem::v1::__cxx11::path>(char const (&) [12])':
test.cpp:(.text._ZNSt12experimental10filesystem2v17__cxx114pathC2IA12_cS3_EERKT_[_ZNSt12experimental10filesystem2v17__cxx114pathC5IA12_cS3_EERKT_]+0x73): undefined reference to `st
d::experimental::filesystem::v1::__cxx11::path::_M_split_cmpts()'
collect2: error: ld returned 1 exit status

What am I doing wrong? I don't think there's anything wrong with the code, since I just copy-pasted it from a website. Am I using the wrong version of gcc? Also, why do I need <experimental/filesystem> instead of just <filesystem> in C++17? Thanks in advance.

Answer 1

Add the flag -lstdc++fs:

$ g++-7 test.cpp -std=c++17 -lstdc++fs

gcc 7.2 supports C++17 experimental filesystem namespace only. I do not know, maybe gcc 7.3 supports std filesystem namespace already.

Answer 2

You can also sudo apt install g++-8 and use #include <filesystem> as cppreference described instead of #include <experimental/filesystem> in older g++ and libstdc++ version.

If I install gcc 8 in Ubuntu, will I have 2 different libstdc++ library or merely the original one get updated?

you'll probably have two even though the newer one should work as a drop-in replacement for the old one.

I notice that a libstdc++-8-dev is installed along with g++-8.

This works for me:

g++-8 -g -Wall -std=c++17 test.cpp -lstdc++fs

It seems that even with g++-8, the filesystem library is not automatically linked, you still need to provide -lstdc++fs, and -std=c++17 is also needed in language level.

Answer 3

Following worked for me:

In code:

#include <filesystem>
namespace filesystem = std::filesystem;

In CMakeLists:

set (CMAKE_CXX_FLAGS "-lstdc++fs -std=c++17")

On Ubuntu 18.04 with GCC 10.