#include <filesystem>
. It fails afterward.I’m on a macOS 10.15, using clang 11. Output of clang --version
:
Apple clang version 11.0.0 (clang-1100.0.33.17)
Target: x86_64-apple-darwin19.4.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
When I try to compile something simple like:
#include <filesystem>
using namespace std;
filesystem::path dev_dir;
int main() {
dev_dir = "/dev/";
return 0;
}
The compiler does find the filesystem library, but doesn’t recognize the namespace:
$clang trigger_controller.cpp -o trigger_controller
trigger_controller.cpp:##:##: error: use of undeclared identifier 'filesystem'
filesystem::path dev_dir;
This is the file I believe I’m trying to include (which I’ve confirmed exists):
// /usr/local/include/c++/9.2.0/filesystem
#ifndef _GLIBCXX_FILESYSTEM
#define _GLIBCXX_FILESYSTEM 1
#pragma GCC system_header
#if __cplusplus >= 201703L
#include <bits/fs_fwd.h>
#include <bits/fs_path.h>
#include <bits/fs_dir.h>
#include <bits/fs_ops.h>
#define __cpp_lib_filesystem 201703
#endif // C++17
#endif // _GLIBCXX_FILESYSTEM
What am I doing wrong? Are there specific compiler options I need? Is the namespace wrong?
Yes, there are specific compiler options you need. Notice that the entire contents of this header, apart from boilerplate, are wrapped in an #if __cplusplus >= 201703L
... #endif
block. That means the header is effectively empty unless the compiler declares conformance with the 2017 revision of the C++ standard (or later).
For clang and gcc, standards conformance level is controlled with the command line option -std
. GCC's documentation for this option is here; I don't know where to find clang's documentation, but clang generally tries to be command-line compatible with gcc, so the same options should work.
In this case, the exact option you should use is -std=gnu++17
for C++ 2017. As of this writing, support for newer revisions of the C++ standard is still "experimental" and "will almost certainly change in incompatible ways", so I would avoid it. I also recommend you avoid the hyperconformant variant of this mode, -std=c++17
, because it has a good chance of exposing bugs in MacOS system headers.