How to use chrono::system_clock from a text file?

Question

I have following situation: I'm writing two programs, the first one starts with windows and writes the current system time in a text document. The code for this looks like this:

auto start = std::chrono::system_clock::now();
std::time_t start_time = std::chrono::system_clock::to_time_t(start);

ofstream Write ("Time.txt");
Write << std::ctime(&start_time);

The date format looks like this: Thu Jul 16 14:19:53 2020.

My second program autostarts with the shutdown. It should build the difference between the current system time and the starting time and write the difference in a different text document.

#include <iostream>
#include <ctime>
#include <string>
#include <cmath>
#include <fstream>
#include <chrono>
#include <windows.h>

using namespace std;

double k; //Get leftover time from txt doc//
int s;    //Seconds//
int m;    //Minutes//   
int h;    //Hours//
int d;    //Days//
long int z;    //Var containing new leftover time//


int main()
{

    ifstream Time ("Time.txt");
        Time >> start;
        //Get starting time//


    ifstream Check ("Test.txt");
        Check >> d; //Reading days from txt doc to d//
        Check >> h; //Reading hours from txt doc to h//
        Check >> m; //Reading minutes from txt doc to m//
        Check >> s; //Reading seconds from txt doc to s//

    k = (s + 60*m + 3600*h + 24*3600*d);

    auto end = std::chrono::system_clock::now();
    //Get current time ending the program//

    std::chrono::duration<double> elapsed_seconds = end-start; 
    //Calculate difference between start and end//

    z = (k-round(elapsed_seconds.count())); 
    //Calculate time difference in seconds//


    //Converting seconds into days, hours, minutes, seconds//
    d = z / (24*3600);
    h = (z / 3600) % 24;
    m = (z / 60) % 60;
    s = z % 60;
    //End of converting//

    ofstream Write ("Test.txt");
        Write << d << "\n" << h << "\n" << m << "\n" << s;  
        //Write all values back to txt doc//

    }

The problem I have is that I don't have an idea on how to get the starting time from the Time.txt in a format I can use in this chrono operation std::chrono::duration<double> elapsed_seconds = end-start.

I've tried to set it up like in the startup program

std::time_t start_time = std::chrono::system_clock::to_time_t(start);
ifstream Time ("Time.txt");
Time >> start;

The result: a ridiculous negative number in the Test.txt which is obviously wrong considering I didn't mess with the system time.

A string variable didn't help me much either. It gets the job done copying the txt line to the string variable but afterwards I didn't manage to transform it into the right chrono variable.

I hope someone has an idea on how to do this. I'm thanking you already for your answers.

Answer 1

Why do you write the time as string into the file? std::time_t is an arithmetic type. You could write and read it directly

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

using std::literals::chrono_literals::operator""s;

int main() {
    {
        auto start = std::chrono::system_clock::now();
        std::time_t start_time = std::chrono::system_clock::to_time_t(start);
        std::ofstream file("file.txt");
        file << start_time;
        std::cout << start_time << '\n';
    }
    std::this_thread::sleep_for(3s);
    {
        std::time_t start_time;
        std::ifstream file("file.txt");
        file >> start_time;
        std::cout << start_time << '\n';
        auto start = std::chrono::system_clock::from_time_t(start_time);
        auto end = std::chrono::system_clock::now();
        std::cout << std::chrono::duration_cast<std::chrono::seconds>(end-start).count();
    }
}