Set file creation time with standard library only

Question

C++17 includes a new and portable API for working with filesystems (<filesystem>).

My program unpacks an archive. This archive contains files, directories and creatation time for each file. With the new API I have a portable solotution for creating directories (std::filesystem::create_directories), and I am using std::ofstream for creating files.

Can I use the new API for setting file createtion (or modification) time?

EDIT: Time, who providing by archive - it is seconds, start at 1970.01.01 00:00:00(UNIX Epoch time)

Answer 1

The answer to your question is... complicated. Because C++ gotta be C++.

In the most technical sense, the answer is yes: std::filesystem::last_write_time can be used to set the last time of writing for the file.

The reason why the answer is not true in a more realistic sense is because last_write_time takes a std::filesystem::file_time_type as its time. And that's problematic.

See, the epoch for this time point (what constitutes 0 time) is... implementation-defined. It might be UNIX time (Zero is Jan 1, 1970 at 00:00:00). Or it might be something else.

Because it is implementation-defined, there is no way to portably turn a count of seconds/minutes/days/etc relative to some known epoch (like, for example, the time in your archive) into a std::filesystem::file_time_type, simply because you don't know how to speak file_time_type. The only thing you can really do is use existing file_time_types values and adjust them by any arbitrary count of time (or set it to the current time via file_time_type::clock::now()). But you can't set a file's time to some known, absolute date/time.

C++20 fortunately plugs this hole. system_clock will be required to run on UNIX time, and file_time_type will be required to be able to be std::chrono::clock_casted from/to any type that can be converted to the system_clock time.

But until then, last_write_time is not nearly as useful as it ought to be...