I'm searching for a method to subtract two dates (ISO 8601 format) which are passed by the user as argument vector. Currently I have this pseudocode:
#include <iostream>
#include <string>
#include <chrono>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <cmath>
int main(int argc, char**argv){
std::istringstream ssStart{argv[1]};
std::istringstream ssEnd(argv[2]);
std::chrono::year_month_day tStart, tEnd;
ssStart >> std::chrono::parse("%Y-%m-%d", tStart);
ssEnd >> std::chrono::parse("%Y-%m-%d", tEnd);
auto tDays = std::chrono::duration_cast<std::chrono::days>(tEnd.day()-tStart.day());
std::cout << ssStart.str() << "," << ssEnd.str() << "," << tDays.count() << std::endl;
return 0;
}
Which works if you stay within the same month eg. ./main 2024-01-01 2024-01-31 -> 30, but once you change months ./main 2024-01-01 2024-02-01 the result will be 0. I could sum the diff between year, month and day, but I'm looking for a better solution (I'm skipping error checking for now since this is only a proof of concept).
I've read that year_month_day is not the right structure to use, so please assist me in choosing the correct one.
As stated in the title I use the C++20 standard (g++-14), and no external libraries.
year_month_day is an appropriate type to parse from a string input, but it doesn't support arithmetic. There is however a conversion to std::chrono::sys_days, which is a specialisation of std::chrono::time_point.
auto tDays = std::chrono::sys_days{ tEnd } - std::chrono::sys_days{ tStart };
tDays is a std::chrono::days value already.