std::filesystem::file_time_type from time_t – how?

I want to create a std::filesystem::file_time_type from a std::time_t and I can’t figure out how.

Example:

time_t t = 1337;
std::filesystem::file_time_type ft = ...; //how?

Ideally I would like this to work with c++17 but I’ll take a c++20 solution too.

>Solution :

You can do it in two steps:

1. time_t to system_clock::time_point via std::chrono::system_clock::from_time_t
2. system_clock::time_point to file_clock::time_point via std::chrono::clock_cast

Altogether:

auto from_time_t_to_file_clock(std::time_t tt) {
    auto st = std::chrono::system_clock::from_time_t(tt);
    return std::chrono::clock_cast<std::chrono::file_clock>(st);
}

There is an optional function std::chrono::file_clock::from_sys that you can use instead for step 2, but it’s optional (there might be a from_utc instead), and the clock_cast will just do the right thing anyway, so might as well just use it.