Ad-hoc std::ostream support for <chrono>

chrono_io.hpp

#include <ctime>
#include <chrono>
#include <iostream>
#include <iomanip>

namespace std {

// https://timsong-cpp.github.io/cppwp/n4868/time.cal.ymd.nonmembers#14
// "yyyy-mm-dd"
inline ostream& operator<<(ostream& os, const chrono::year_month_day& ymd) {
  char prev = os.fill('0');
  os << std::setw(4) << (int)ymd.year() << '-'
     << std::setw(2) << (unsigned)ymd.month() << '-'
     << std::setw(2) << (unsigned)ymd.day();
  os.fill(prev);
  return os;
}

// https://timsong-cpp.github.io/cppwp/n4868/time.clock.system.nonmembers#2
// "yyyy-mm-dd HH:MM:SS<.n>" (unspecified precision)
inline ostream& operator<<(ostream& os, const chrono::system_clock::time_point& tp) {
  std::time_t t = chrono::system_clock::to_time_t(tp);
  return os << std::put_time(std::gmtime(&t), "%F %T");
}

// https://timsong-cpp.github.io/cppwp/n4868/time.clock.system.nonmembers#4
// "yyyy-mm-dd"
inline ostream& operator<<(ostream& os, const chrono::sys_days& dp) {
  return os << chrono::year_month_day{dp};
}

} // namespace std