CODEWITHSUNDEEP
qtg++c++17std-filesystem
AGK
AGK

Reputation: 95

How to avoid std::filesystem linker errors with Qt?

I would like to use the std::filesystem with Qt 5.12.0 with the g++ version Ubuntu 8.2.0-7ubuntu1, but getting linker errors:

g++ -lstdc++fs -Wl,-rpath,/home/user/Qt/5.12.0/gcc_64/lib -o qf_filesystem_test main.o   -L/home/user/Qt/5.12.0/gcc_64/lib -lQt5Widgets -lQt5Gui -lQt5Core -lGL -lpthread   
/usr/bin/ld: main.o: in function `std::filesystem::exists(std::filesystem::__cxx11::path const&)':
/usr/include/c++/8/bits/fs_ops.h:121: undefined reference to `std::filesystem::status(std::filesystem::__cxx11::path const&)'
/usr/bin/ld: main.o: in function `std::filesystem::__cxx11::path::path<char*, std::filesystem::__cxx11::path>(char* const&, std::filesystem::__cxx11::path::format)':
/usr/include/c++/8/bits/fs_path.h:183: undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
collect2: error: ld returned 1 exit status
make: *** [Makefile:257: qf_filesystem_test] Error 1
22:12:16: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project qf_filesystem_test (kit: Desktop Qt 5.12.0 GCC 64bit)
When executing step "Make"

After some googling, I found that I need to use the linker flag -lstdc++fs. My code builds perfectly with the command g++ main.cpp -std=c++17 -lstdc++fs, but I can't seem to make it work inside Qt Creator. My simple test code is the following:

#include <iostream>
#include <filesystem>

int main(int argc, char *argv[])
{
    if(1 < argc)
    {
        std::cout << argv[1] << " does ";
        if(!std::filesystem::exists(std::filesystem::path(argv[1]))) std::cout << "not ";
        std::cout << "exist!" << std::endl;
    }

    return 0;
}

My .pro file looks like this:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = qf_filesystem_test
TEMPLATE = app

DEFINES += QT_DEPRECATED_WARNINGS

CONFIG += c++17
QMAKE_LFLAGS += -lstdc++fs

SOURCES += main.cpp

After some tests with g++ It seems to me, that the problem is caused by the order of the g++ flags, because Qt puts the -lstdc++fs to the front.

Upvotes: 3

Views: 4539

Answers (1)

jonspaceharper
jonspaceharper

Reputation: 4367

<filesystem> is a separate library for GCC 8 (see this question). Your issue, as you suspected, is in the order of the flags. Poking about a bit in the docs hints that QMAKE_LFLAGS is more for linker flags than library loads, which is why it gets passed early (e.g. -O3).

Using LIBS += -lstdc++fs should work instead.

Credit to this reddit response for this solution.

Upvotes: 4

Related Questions