How do I convert a string back into a time_point? - c++

Question

I am taking a system_clock time_point, converting it to a string, and then saving it to a configuration file.
Now I want to read that config file and turn the string back into a time point so that I can calculate the difference in time between two time points.
void SaveLastShuffleTime() {
m_lastShuffleTime = std::chrono::system_clock::now();
auto m_lastShuffleTimeTimeT = std::chrono::system_clock::to_time_t(m_lastShuffleTimeTimepoint);
stringstream m_lastShuffeTimeSS;
m_lastShuffeTimeSS << std::put_time(std::localtime(&m_lastShuffleTimeTimeT), "%Y-%m-%d %X");
m_deviceStateSettings.UpdateDeviceStateSettings(LAST_SHUFFLE_TIME, m_lastShuffeTimeSS.str());
}
void CompareLastShuffleTime() {
m _currentShuffleTime = std::chrono::system_clock::now();
/* READ CONFIG FILE AND CONVERT BACK TO TIME POINT */
int timeSinceLastShuffle = (duration_cast<minutes>(m_currentShuffleTime - m_oldShuffleTime)).count();
}
Please let me know if this is viable. The alternative is to save the timepoint as an integer but I would prefer not to do that.
Thanks

Answer 1

I recommend outputting UTC as opposed to local time so that the difference between timestamps isn't altered by UTC offset jumps (e.g. daylight saving).
C++20 makes this very easy, and allows timestamps with subsecond precision:
#include <cassert>
#include <chrono>
#include <iostream>
#include <sstream>
int
main()
{
using namespace std;
using namespace std::chrono;
stringstream s;
auto tp = system_clock::now();
auto tp_save = tp;
s << tp << '\n'; // Write it out
tp = {}; // Zero out the timestamp
s >> parse("%F %T", tp); // Parse it back in
assert(tp == tp_save); // Make sure it is the same
std::cout << s.str(); // This is what was formatted/parsed
}
Example output:
2021-06-17 16:10:10.562738
Vendors are still working on getting this out. But you can use this syntax today with C++11/14/17 and a free, open-source, header-only preview of this part of C++20.1
Just add:
#include "date/date.h"
using namespace date;
and the above works with the preview library.
1 Full disclosure: I am the lead author of this library. I am not pursuing any financial gain from this effort. But sometimes people get grumpy if I don't fully disclose this information.

Answer 2

You could use cctz library. It provides convenient functions for formatting timepoint cctz::format to a string and parsing cctz::parse from a string. And also for working with timezones. Example:
#include <chrono>
#include <iostream>
#include <string>
#include "cctz/time_zone.h"
int main() {
const std::chrono::system_clock::time_point now =
std::chrono::system_clock::now();
std::string now_str =
cctz::format("%Y-%m-%d %H:%M:%S%z", now, cctz::utc_time_zone());
std::cout << now_str << std::endl;
std::chrono::system_clock::time_point tp;
const bool ok =
cctz::parse("%Y-%m-%d %H:%M:%S%z", now_str, cctz::utc_time_zone(), &tp);
if (!ok)
return -1;
}

Answer 3

std::istringstream ss(str);
tm t;
ss >> std::get_time(&t, "%Y-%m-%dT%H:%M:%S");
std::time_t tt = std::mktime(&t);
/// If you don't need UTC time, just comment out the line below.
tt = std::mktime(std::gmtime(&tt));
return std::chrono::system_clock::from_time_t(tt);