Link Errors Using <Filesystem> Members in C++17

Link errors using filesystem members in C++17

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.

experimental::filesystem linker error

The Filesystem TS is nothing to do with C++1z support, it is a completely separate specification not part of the C++1z working draft. GCC's implementation (in GCC 5.3 and later) is even available in C++11 mode.

You just need to link with -lstdc++fs to use it.

(The relevant library, libstdc++fs.a, is a static library, so as with any static library it should come after any objects that depend on it in the linker command.)

Update Nov 2017: as well as the Filesystem TS, GCC 8.x also has an implementation of the C++17 Filesystem library, defined in <filesystem> and in namespace std::filesystem (N.B. no "experimental" in those names) when using -std=gnu++17 or -std=c++17. GCC's C++17 support is not complete or stable yet, and until it's considered ready for prime time use you also need to link to -lstdc++fs for the C++17 Filesystem features.

Update Jan 2019: starting with GCC 9, the C++17 std::filesystem components can be used without -lstdc++fs (but you still need that library for std::experimental::filesystem).

std::filesystem::directory_iterator linker issue (C++17)

Per this reddit post by u/forcecharlie you need to add -lstdc++fs to the compiler options. On Wandbox if we don't add it we get link errors but when we do add it, it compiles successfully.

c++17 `filesystem` is not a namespace-name

GCC 5.4.0 was released in June of 2016; over a year before the C++17 standard was adopted. It and its version of libstdc++ have very limited C++17 support. You can see when GCC added C++17 language features here and when libstdc++ added C++17 standard library features here.

At the time of GCC 5.4's release, the filesystem library was not yet implemented in the std::filesystem namespace. It, along with any other <experimental/...> headers that are included in that version, are in the std::experimental namespace.

std::filesystem example not compiling on c++ 17

There was a defect in C++17 standard that didn't allow operator<< to be called with std::filesystem::directory_entry, reported in LWG 3171. It's now fixed as defect report, but it seems clang only fixed it in version 14: https://godbolt.org/z/3arTcGYvY. gcc seems to have backported the fix to all versions that support std::filesystem (that is, gcc9.1 and up): https://godbolt.org/z/fh7cdMxso

Linking to std++fs - what syntax to choose

Previously, the filesystem part of the standard library wasn't included in libstdc++ but was only available by explicitly linking with libstdc++fs. g++ is still shipped with libstdc++fs so explicitly linking with it isn't a problem even for newer g++ versions.

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_{I was hoping to find a clean cmake way of doing find_package similar to that of when using threads}

find_package(Threads REQUIRED)
target_link_libraries(application PRIVATE Threads::Threads)

_{but unfortunately, I only found how to do that for Boost::filesystem...}

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