[C++17] Check whether the output directory exists.

[bangerth]

Fixes #10281 . We can only apply this once we support C++17, but I had this ready so I'll post for whenever the time comes.

[tamiko]

That'll be some fun - For libstdc++ we will have to figure out a sane way to decide whether we have to link against libstdc++fs as well or not.

[bangerth]

Shouldn't -std=c++17 imply that?

[tamiko]

@bangerth gcc version 9 onwards should just require -std=c++17, gcc version 8 and older require manual linking with libstdc++fs. At least if I recall correctly 😸

[marcfehling]

I remember that I had similar issues with gcc8. I solved this by substitution with boost libraries:

#ifdef __cpp_lib_filesystem
#  include <filesystem>
namespace filesystem
{
  using namespace std::filesystem;
}
#else
#  include <boost/filesystem.hpp>
namespace filesystem
{
  using namespace boost::filesystem;
}
#endif

I'm not sure if the __cpp_lib_filesystem flag only exists for gcc.

We could move this code snippet into the std_cxx17 namespace. What do you think?

[bangerth]

The problem is that we don't include the boost filesystem parts in the bundled/ directory :-( There is also the issue that this is a tutorial and I really didn't want to introduce some complicated workaround in tutorials -- I'd rather just leave things as they are if we can't do it right....

[bangerth]

This is now open for merge :-)

[tamiko]

The minimal version we test and thus meaningfully support now is gcc-9 so this can go in as is.

[masterleinad]

The minimal version we test and thus meaningfully support now is gcc-9 so this can go in as is.

tester-ng still uses gcc-7.5, though, see https://cdash.dealii.org/viewSite.php?siteid=3&project=1&currenttime=1688518800.