c++ - 'std::filesystem' has not been declared after including <experimental/filesystem>

Question

I have checked a lot of issues about the link of filesystem under c++17 and I still cannot make the link successfully. My main.cpp file is as the following.

#include <experimental/filesystem>


int main(int argc, char** argv)
{
    std::string imageDirectory = "./image";;
    std::vector<std::string> imagePath;

    for (const auto& entry: std::filesystem::directory_iterator(imageDirectory))
    {
        imagePath.push_back(entry.path());
        std::cout << entry.path() << std::endl;
    }

    return 0;
}

My CMakeLists.txt is as the following.

cmake_minimum_required(VERSION 3.8 FATAL_ERROR)

project(visual_hull LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 17)
add_library(dataIO
        STATIC
            dataIO.hpp
            dataIO.cpp)

find_package(OpenCV REQUIRED core highgui imgproc)

target_link_libraries(dataIO ${OpenCV_LIBS})

add_executable(visual_hull main.cpp)

target_link_libraries(visual_hull PUBLIC dataIO
                                         stdc++fs)

The error is as the following.

/home/SENSETIME/zhangshunkang/Downloads/programming/c++/visual_hull/main.cpp: In function ‘int main(int, char**)’:
/home/SENSETIME/zhangshunkang/Downloads/programming/c++/visual_hull/main.cpp:15:31: error: ‘std::filesystem’ has not been declared
  for (const auto& entry: std::filesystem::directory_iterator(imageDirectory))
                               ^
CMakeFiles/visual_hull.dir/build.make:62: recipe for target 'CMakeFiles/visual_hull.dir/main.cpp.o' failed
make[2]: *** [CMakeFiles/visual_hull.dir/main.cpp.o] Error 1
CMakeFiles/Makefile2:72: recipe for target 'CMakeFiles/visual_hull.dir/all' failed
make[1]: *** [CMakeFiles/visual_hull.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

Answer 1

It seems your C++17 compiler doesn't include the standard filesystem header. One possible way to get around that:

#ifndef __has_include
  static_assert(false, "__has_include not supported");
#else
#  if __cplusplus >= 201703L && __has_include(<filesystem>)
#    include <filesystem>
     namespace fs = std::filesystem;
#  elif __has_include(<experimental/filesystem>)
#    include <experimental/filesystem>
     namespace fs = std::experimental::filesystem;
#  elif __has_include(<boost/filesystem.hpp>)
#    include <boost/filesystem.hpp>
     namespace fs = boost::filesystem;
#  endif
#endif

Then use fs:: instead of std::filesystem:: everywhere.

Checking __cplusplus >= 201703L is just an extra precaution if you want to use filesystem when using C++11/14. In those cases, __has_include(<filesystem>) may be true but including it will not define the std::filesystem namespace.