c++ - Problem adding std::filesystem to CMake Project

Question

I am new to CMake projects and I want to use the file system library in my project. I am running Ubuntu 18.04 with GCC 8.2 and CMake 3.13. In order to achieve this I tried two options:

Option 1

cmake_minimum_required(VERSION 3.13)  
project(TheFsProject)  
set(CMAKE_CXX_STANDARD 17)  
set(CMAKE_CXX_FLAGS "-std=c++17 -lstdc++fs")  

This does not help as the compiler still cannot find the file system library during compile time.

Option 2 (copied from: https://www.scivision.co/cmake-cpp-17-filesystem/)

make_minimum_required(VERSION 3.13)
project(TheFsProject)

set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(CMAKE_REQUIRED_FLAGS -std=c++17)
include(CheckCXXSymbolExists)
CHECK_CXX_SYMBOL_EXISTS(std::filesystem::path::preferred_separator cxx17fs)

if(cxx17fs)
  add_executable(TheFsProject main.cpp)
  set_property(TARGET TheFsProject PROPERTY CXX_STANDARD 17)
endif()

This does not help either as I get a CMake error which I don't understand.

(CHECK_CXX_SYMBOL_EXISTS):  
 CHECK_CXX_SYMBOL_EXISTS Macro invoked with incorrect arguments for macro named: CHECK_CXX_SYMBOL_EXISTS

I feel out of my depth on this topic which is why I came here. I don't mind putting extra work into finding out more but I don't know anymore where to look. Any help would be appreciated!

EDIT 1

Thanks for the replies so far! I made Option 3 based on your feedback:

cmake_minimum_required(VERSION 3.13)
project(TheFsProject)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_executable(TheFsProject main.cpp)
target_link_libraries(TheFsProject stdc++fs)

Sadly it doesn't fix my problem. It still issues an error during compilation that it can't find the compilation header.

EDIT 2

Thanks for all the replies so far. All of these help. I tried Ashkan his answer last (because it seemed intimidating). This one returns

Compiler is missing file system capabilities.

so I'm guessing something is wrong on that end. This is useful in the sense that I now know it's probably not due to my CMake file. I now have to find out why the compiler does support the file system header though...

EDIT 3

Strictly speaking this question is answered because my question is about the CMake file. I am going to mark Ashkan his answer as the solution simply because it produced the next step in my troubleshooting search. If I could I would also mark lubgr his answer because I think that's a really good answer as well. Thanks everyone!

Answer 1

Gcc 8.2. comes with <filesystem>, so there is no need to investigate with regard to the availability. Next, option 1 is sufficient, but needs a fix:

set(CMAKE_CXX_STANDARD 17) # no need to manually adjust the CXXFLAGS

add_executable(yourExecutable yourSourceFile.cpp)

target_link_libraries(yourExecutable stdc++fs)

This should result in compiling the sources with -std=c++17 or -std=gnu++17 and adding -lstdc++fs when linking.

Edit: Note that as @Ashkan has pointed out in the comments, setting CMAKE_CXX_STANDARD_REQUIRED to true results in an immediate error at configure time if C++17 isn't supported by the compiler, instead of a compilation error (due to the missing <filesystem> header) or at link time (due to the missing shared library). This might be desirable.