undefined reference to `std::filesystem

在gcc8.3中使用filesystem，没想到编译加了-std=c++17还是报错。

/home/yuanll/disk2/rk1109/SDK/prebuilts/gcc/linux-x86/arm/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/8.3.0/../../../../arm-linux-gnueabihf/bin/ld: CMakeFiles/aibox.dir/src/app/main.cpp.o: in function `std::filesystem::__cxx11::path::operator/=(std::filesystem::__cxx11::path const&)':
main.cpp:(.text._ZNSt10filesystem7__cxx114pathdVERKS1_[_ZNSt10filesystem7__cxx114pathdVERKS1_]+0x3c): undefined reference to `std::filesystem::__cxx11::path::has_filename() const'
/home/yuanll/disk2/rk1109/SDK/prebuilts/gcc/linux-x86/arm/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/8.3.0/../../../../arm-linux-gnueabihf/bin/ld: main.cpp:(.text._ZNSt10filesystem7__cxx114pathdVERKS1_[_ZNSt10filesystem7__cxx114pathdVERKS1_]+0xa0): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
/home/yuanll/disk2/rk1109/SDK/prebuilts/gcc/linux-x86/arm/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/8.3.0/../../../../arm-linux-gnueabihf/bin/ld: CMakeFiles/aibox.dir/src/app/main.cpp.o: in function `std::filesystem::__cxx11::path::clear()':
main.cpp:(.text._ZNSt10filesystem7__cxx114path5clearEv[_ZNSt10filesystem7__cxx114path5clearEv]+0x20): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
/home/yuanll/disk2/rk1109/SDK/prebuilts/gcc/linux-x86/arm/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/8.3.0/../../../../arm-linux-gnueabihf/bin/ld: CMakeFiles/aibox.dir/src/app/main.cpp.o: in function `std::filesystem::__cxx11::path::is_absolute() const':
main.cpp:(.text._ZNKSt10filesystem7__cxx114path11is_absoluteEv[_ZNKSt10filesystem7__cxx114path11is_absoluteEv]+0x14): undefined reference to `std::filesystem::__cxx11::path::has_root_directory() const'
/home/yuanll/disk2/rk1109/SDK/prebuilts/gcc/linux-x86/arm/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/8.3.0/../../../../arm-linux-gnueabihf/bin/ld: CMakeFiles/aibox.dir/src/app/main.cpp.o: in function `std::filesystem::__cxx11::path::path, std::allocator >, std::filesystem::__cxx11::path>(std::__cxx11::basic_string, std::allocator > const&, std::filesystem::__cxx11::path::format)':
main.cpp:(.text._ZNSt10filesystem7__cxx114pathC2INSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES1_EERKT_NS1_6formatE[_ZNSt10filesystem7__cxx114pathC5INSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES1_EERKT_NS1_6formatE]+0x60): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
collect2: 错误： ld 返回 1

解决办法：-lstdc++fs

具体原理请参考：filesystem太难用了！

c++真是博大精深！！