C++ chrono: How do I convert an integer into a time point

Question

I managed to convert a time point into an integer and write it into a file using code that looks like the following code:

std::ofstream outputf("data");
std::chrono::time_point<std::chrono::system_clock> dateTime;

dateTime = std::chrono::system_clock::now();

auto dateTimeSeconds = std::chrono::time_point_cast<std::chrono::seconds>(toSerialize->dateTime);
unsigned long long int serializeDateTime = toSerialize->dateTime.time_since_epoch().count();
outputf << serializeDateTime << "\n";

Now I'm trying to read that integer from the file, convert it into a time_point, and print it. Right now, my code looks something like this:

std::ifstream inputf("data");

unsigned long long int epochDateTime;
inputf >> epochDateTime;
std::chrono::seconds durationDateTime(epochDateTime);
std::chrono::time_point<std::chrono::system_clock> dateTime2(durationDateTime);

std::time_t tt = std::chrono::system_clock::to_time_t(dateTime2);
char timeString[30];
ctime_s(timeString, sizeof(timeString), &tt);
std::cout << timeString;

However, it doesn't print anything. Does anyone know where I went wrong?

Answer 1

You have some strange conversions and assign to a variable that you don't use. If you want to store system_clock::time_points as std::time_ts and restore the time_points from those, don't involve other types and use the functions made for this: to_time_t and from_time_t. Also, check that opening the file and that extraction from the file works.

Example:

#include <chrono>
#include <ctime>
#include <fstream>
#include <iostream>

int main() {
    {   // save a time_point as a time_t
        std::ofstream outputf("data");
        if(outputf) {
            std::chrono::time_point<std::chrono::system_clock> dateTime;
            dateTime = std::chrono::system_clock::now();
            outputf << std::chrono::system_clock::to_time_t(dateTime) << '\n';
        }
    }

    {   // restore the time_point from a time_t
        std::ifstream inputf("data");
        if(inputf) {
            std::time_t epochDateTime;
            if(inputf >> epochDateTime) {
                // use epochDateTime with ctime-like functions if you want:
                std::cout << std::ctime(&epochDateTime) << '\n';

                // get the time_point back (usually rounded to whole seconds):
                auto dateTime = std::chrono::system_clock::from_time_t(epochDateTime);

                // ...
            }
        }
    }
}