How to convert std::filesystem::file_time_type to time_t?

Question

I wrote a solution for windows using MSVC2015 where the follow code converts the std::filesystem::last_write_time result time_t:

time_t ftime = std::file_time_type::clock::to_time_t(fs::last_write_time("/Path/filename"))

It works well. Then when I tried to port the solution to Linux using gcc 9.3 (-std=C++2a) I've got the follow error:

Error: 'to_time_t' is not member of 'std::chrono::time_point::clock' {aka 'std::filesystem::__file_clock'}

I searched for a solution, but what I found is based on solution included on example of std::filesystem::last_write_time at cplusplus.com. The solution is shown bellow:

auto ftime = fs::last_write_time(p);
std::time_t cftime = decltype(ftime)::clock::to_time_t(ftime);

Unfortunately it doesn't work to me. Actually, the example has a comment that say it won't work at MSVC(worked at MSVC2015) or GCC 9; C++20 will allow portable output.

Now, I'm stuck... How can I make this conversion using gcc?

Answer 1

As already said, there is no perfect way to do that in C++17. Depending on the actual use-case it might be good enough to use a portable approximation. Based on my answer to "How to convert std::filesystem::file_time_type to a string using GCC 9", I want to suggest the helper function used there:

template <typename TP>
std::time_t to_time_t(TP tp)
{
    using namespace std::chrono;
    auto sctp = time_point_cast<system_clock::duration>(tp - TP::clock::now()
              + system_clock::now());
    return system_clock::to_time_t(sctp);
}

Be aware, that it uses a call to now() on each clock, so it is not an exact, round-trip-guaranteed solution, but it might be usable for you, until the gap in the library is closed. It is based on the fact that difference between time points of the same clock is easy and there is an operator+ for duration and time_point of different sources.

For ways to lower the risk of a relevant error even more, I want to point to the conversion between C++11 clocks where some statistical analysis was made with ideas to mitigate possible errors, but when acceptable, I just use the code above.