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.

Batruk

See gcc.gnu.org/projects/cxx-status.html – Colima

I would recommend using some external library like Boost or Poco or Qt, or more simply use POSIX calls like syscalls(2), notably stat(2)... – Colima

@BasileStarynkevitch the first link you are pointing to doesn't mention anything about file system support in GCC. In fact, I get zero matches when I search for the string "file". – Kaisership

This is because you need to check the page about the libstdc++, not g++ itself. It details that support for <filesystem> is scheduled to land with g++ 8: gcc.gnu.org/onlinedocs/libstdc++/manual/… – Ushaushant

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.

Choosey

I have gcc 7.3 on Arch Linux and I still get the linker error, even with -lstdc++fs. – Exchangeable

I have gcc 8.2 which is meant to support <filesytem>. I have tried adding the flag and the beggining of my compiler output looks like this: g++ -c -pipe -std=c++17 -lstdc++fs -g -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_DISABLE_DEPRECATED_BEFORE=0x060000 -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I.. Still all function calls are apparently undefined... Any thought? – Near

@IronAttorney Please copy and paste the compiler error messages. – Choosey

-lstdc++fs has to go after your .cpp files in the command-line. If using make, add it to your LDLIBS variable. – Hultin

What happens if I add I compile the same program with gcc 8 though? Would it be able to access experimental/filesystem? – Sadiron

@SashankAryal You do not need the -lstdc++fs flag for GCC 8, it now implements <filesystem> with the -std=c++17 flag. – Choosey

I know that, but my question is can Gcc 8 compile <experimental/filesystem> header since it has <filesystem> instead> – Sadiron

@SashankAryal Sorry. No, experimental/ prefix must be removed.As I said GCC 8 now implements <filesystem>. – Choosey

But when I write library code I do not know what compiler will be used to compile it, that's what I'm saying. – Sadiron

@SashankAryal You can use the macro GCC_VERSION, it and other are described in the GCC manual: gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html – Choosey

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.

Tarra

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.

Himes

Answer 4

One not-yet-mentioned essential that eluded me for a few hours was in the default CMakeLists.txt:

target_compile_features(compare_files PUBLIC cxx_std_11)
needs to instead be:
target_compile_features(compare_files PUBLIC cxx_std_17)

Hydrotherapy