C++ - compare two dates - c++

Question

I am writing a application which needs the possibility to compare two dates. This is what I have so far:
struct entry {
string text;
string date; // format: dd.mm.yyyy
bool finished;
};
string addNulls(int number, int cols) {
string num = to_string(number);
if (num.size() < cols) {
int count = cols - num.size();
for (int i = 0; i < count; i++) {
num = "0" + num;
}
}
return num;
}
// [...]
entry e = {"here is some text", "21.03.2019", false};
int day2 = atoi(e.date.substr(0, 2).c_str());
int month2 = atoi(e.date.substr(3, 2).c_str());
int year2 = atoi(e.date.substr(6, 4).c_str());
time_t t = time(0);
struct tm * now = localtime(&t);
string date1 = e.date.substr(6, 4) + "-" + e.date.substr(3, 2) + "-" + e.date.substr(0, 2) + " 00:00:00";
string date2 = addNulls(now->tm_year, 4) + "-" + addNulls(now->tm_mon, 2) + "-" + addNulls(now->tm_mday, 2) + " 00:00:00";
if(date2 > date1) {
// do something
}
the code gets an "entry" struct which contains a date. Than the code compares the date with the actual time. The problem is, it does not work! I run some tests with some example content, but the result (date2 > date1) returns false.
Why?
I read this: C++ compare to string dates

Answer 1

I'm not actually answering your question. However I am offering you a solution. Have you considered a date/time library? Boost datetime is very popular.
If you are compiling in C++11 or later, I recommend this date time library, as it is header-only (eliminating the need to link to a library such as boost), and in my opinion, it has cleaner syntax (that is a very subjective and biased viewpoint).
This latter library builds on the C++11 <chrono> library. Here is your example code using this library:
#include "date.h"
#include <iostream>
#include <string>
struct entry {
std::string text;
date::year_month_day date;
bool finished;
};
int
main()
{
entry e = {"here is some text", date::day(21)/3/2019, false};
auto day2 = e.date.day();
auto month2 = e.date.month();
auto year2 = e.date.year();
auto t = std::chrono::system_clock::now();
auto date1 = date::sys_days{e.date};
auto date2 = t;
if (date2 > date1)
std::cout << "It is past " << e.date << '\n';
else
std::cout << "It is not past " << e.date << '\n';
}
Which currently outputs:
It is not past 2019-03-21
In C++14, the chrono literals make specifying literal times very compact:
using namespace std::literals;
auto date1 = date::sys_days{e.date} + 0h + 0min + 0s;
Also on the subject of literals, you can make the construction of entry slightly more compact if you drop in a using namespace date;:
entry e = {"here is some text", 21_d/3/2019, false};
Reusing a date or datetime class, or even creating your own, is easier than trying to use a string to hold a date. Additionally you get the type-safety of not accidentally adding a string to a date, when you meant to add a time duration to a time point.

Answer 2

Why don't you use strptime to parse your date strings, convert them to epoch times and then compare?
#include <time.h>
char *
strptime(const char *restrict buf, const char *restrict format,
struct tm *restrict tm);