How to detect <filesystem> usability on OSX?

Question

I wanted to do some cross-platform, optional usage of std::filesystem and used code like the below:

#ifdef __has_include
#if __has_include(<version>)
#include <version>
#endif
#endif

#ifdef __cpp_lib_filesystem
#include <filesystem>
// Use of std::filesystem::path
#endif

Then I can pass -std=c++11 or -std=c++17 and have or have not the support for filesystem.

This works fine almost everywhere but on a recent-ish OSX with no explicit target platform level set. This seems to default to some older OSX and throws a compile error:

error: 'path' is unavailable: introduced in macOS 10.15
...
Applications/Xcode_11.3.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/filesystem:739:24:
note: 'path' has been explicitly marked unavailable here

So how am I supposed to handle this on OSX without relying on configure checks if such code compiles? Wasn't the feature detection macro __cpp_lib_filesystem meant to make such configure steps unnecessary?

Answer 1

Problem is which version of OSX did you setup to support.

On OSX/MacOS C++ standard library is part of the system. So if you set that your program supports some version of MacOS compiler will do checking if this version of system have standard C++ library which supports specific feature.

So for example it doesn't matter you have enabled C++17, but if your program supposed to support OSX 10.13 this program will not compile:

int foo(std::optional<int> x)
{
   return x.value();
}

Since this can throw an exception: std::bad_optional_access which is part of standard library dynamic library shipped with OS.

On other hand this code will compile just fine:

int foo(std::optional<int> x)
{
   return x.value_or(0);
}

since this version doesn't throw and and templeted code will be generated in your executable only.

Exactly same problem is present with filesystem features. That is why compiler complains you need setup your project to support MacOS 10.15 or newer.

If you are using cmake add this to root CMakeLists.txt:

# note this entry must be set before any target or project is created
set(CMAKE_OSX_DEPLOYMENT_TARGET "10.15" CACHE STRING "Minimum OS X deployment version")

If you are using Xcode project:

There is a way do detect a platform and make this 10.15 features available. Here is SO answer describing how to do it. This way you can have application which some feature is available only when run on required OS version.