postgresql timestamp to std::chrono value

With help from boost:

s.creationtime = makeTimePoint(r[0]["creationtime"].as<string>());

makeTimePoint:

std::chrono::system_clock::time_point makeTimePoint(const std::string& s)
{
  using namespace boost::posix_time;
  using namespace std::chrono;

  const ptime ts = time_from_string(s);
  auto seconds = to_time_t(ts);
  time_duration td = ts.time_of_day();
  auto microseconds = td.fractional_seconds();
  auto d = std::chrono::seconds{seconds} + std::chrono::microseconds{microseconds};
  system_clock::time_point tp{duration_cast<system_clock::duration>(d)};
  return tp;
}

The C++20 spec introduces a family of chrono::time_points called local_time:

// [time.clock.local], local time
struct local_t {};
template<class Duration>
  using local_time  = time_point<local_t, Duration>;
using local_seconds = local_time<seconds>;
using local_days    = local_time<days>;

These time_points represent a "timestamp without a timezone".

There exists a free, open-source preview of this C++20 library here:

https://github.com/HowardHinnant/date

which is currently in use by other projects around the globe. This library has a few minor changes from the C++20 spec, such as putting everything in namespace date instead of namespace std::chrono.

Example program using this library:

#include "date/date.h"
#include <iostream>

int
main()
{
    using namespace date;
    using namespace std::chrono;
    int y = 2019;
    int m = 8;
    int d = 28;
    int H = 14;
    int M = 42;
    int S = 16;
    int US = 500'000;
    local_time<microseconds> lt = local_days{year{y}/m/d} + hours{H} +
                                  minutes{M} + seconds{S} + microseconds{US};
    std::cout << lt << '\n';
}

Output:

2019-08-28 14:42:16.500000