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

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);
// ...
}
}
}
}

Answer 2

Putting aside the possibility of the wrong date value, the problem here is with sizeof(timeString). It appears that you think it is 30, but it in fact is the size of the char*, likely 8 (or maybe 4).
According to ctime_s:
the following errors are detected at runtime and call the currently installed constraint handler function:
buf or timer is a null pointer
bufsz is less than 26 or greater than RSIZE_MAX