Build project with "experimental/filesystem" using cmake - c++

Question

I need to add a "experimental/filesystem" header to my project
#include <experimental/filesystem>
int main() {
auto path = std::experimental::filesystem::current_path();
return 0;
}
So I used -lstdc++fs flag and linked with libstdc++fs.a
cmake_minimum_required(VERSION 3.7)
project(testcpp)
set(CMAKE_CXX_FLAGS "-std=c++14 -lstdc++fs" )
set(SOURCE_FILES main.cpp)
target_link_libraries(${PROJECT_NAME} /usr/lib/gcc/x86_64-linux-gnu/7/libstdc++fs.a)
add_executable(testcpp ${SOURCE_FILES})
However, I have next error:
CMake Error at CMakeLists.txt:9 (target_link_libraries): Cannot
specify link libraries for target "testcpp" which is not built by
this project.
But if I compile directly, it`s OK:
g++-7 -std=c++14 -lstdc++fs -c main.cpp -o main.o
g++-7 -o main main.o /usr/lib/gcc/x86_64-linux-gnu/7/libstdc++fs.a
Where is my mistake?

Answer 1

It's just that the target_link_libraries() call has to come after the add_executable() call. Otherwise the testcpp target is not known yet. CMake parses everything sequential.
So just for completeness, here is a working version of your example I've tested:
cmake_minimum_required(VERSION 3.7)
project(testcpp)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
# NOTE: The following would add library with absolute path
# Which is bad for your projects cross-platform capabilities
# Just let the linker search for it
#add_library(stdc++fs UNKNOWN IMPORTED)
#set_property(TARGET stdc++fs PROPERTY IMPORTED_LOCATION "/usr/lib/gcc/x86_64-linux-gnu/7/libstdc++fs.a")
set(SOURCE_FILES main.cpp)
add_executable(testcpp ${SOURCE_FILES})
target_link_libraries(${PROJECT_NAME} stdc++fs)