Operator Function within a class to subtract 2 dates

[Shishykish]

Hello,

I have to subtract 2 dates from each other in long int form and then I have to display the new date. Unfortunately, when I do so I get a ridiculous number for the month!

Could someone see where there maybe an error?

  #include <iostream>
#include <iomanip>
using namespace std;
// class declaration section
class Date
{
private:
    int month, day, year;
public:
    Date(int = 7, int = 4, int = 2005); // constructor
    Date(long); // type conversion constructor
    Date operator+(int); // overload the + operator
    Date operator-(int);
    void showDate();
};

// class implementation section
// constructor
Date::Date(int mm, int dd, int yyyy)
{
month = mm;
day = dd;
year = yyyy;
}

// type conversion constructor from long to Date
Date::Date(long findate)
{
    year = int(findate/10000.0);
    month = int((findate - year * 10000.0)/100.0);
    day = int(findate - year * 10000.0 - month * 100.0);
}

Date Date::operator+(int days)
{
    Date temp; // a temporary date to store the result
    temp.day = day + days; // add the days
    temp.month = month;
    temp.year = year;

    while (temp.day > 30) // now adjust the months
    {
        temp.month++;
        temp.day -= 30;
    }

    while (temp.month > 12) // adjust the years
    {
        temp.year++;
        temp.month -= 12;
    }

return temp; // the values in temp are returned
}

Date Date::operator-(int days)
{
    Date temp; // a temporary date to store the result
    temp.day = day - days; // subtract the days
    temp.month = month;
    temp.year = year;

    while (temp.day > 30) // now adjust the months
    {
        temp.month++;
        temp.day -= 30;
    }

    while (temp.month > 12) // adjust the years
    {
        temp.year++;
        temp.month -= 12;
    }

return temp; // the values in temp are returned
}
// member function to display a date
void Date::showDate()
{
    cout << setfill('0')
        << setw(2) << month << '/'
        << setw(2) << day << '/'
        << setw(2) << year % 100;

    return;
}

int main()
{
    Date a, b(20061225L), c(4,1,2007); // declare 3 objects and
// initialize 2 of them

    cout << "Dates a, b, and c are ";
    a.showDate();

    cout << ", ";
    b.showDate();

    cout << ", and ";
    c.showDate();

    cout << ".\n";
    a = Date(20080103L); // cast a long to a Date

    cout << "Date a is now ";
    a.showDate();

    cout << "The initial date is ";
    a.showDate();

    cout << endl;
    b = a + 284; // add in 284 days = 9 months and 14 days

    cout << "The new date is ";
    b.showDate();

    cout << endl;

    cout << ".\n";
    a = Date(20080103L); // cast a long to a Date

    cout << "Date a is now ";
    a.showDate();

    cout << "The initial date is ";
    a.showDate();

    cout << endl;
    b = a - 284; // add in 284 days = 9 months and 14 days

    cout << "The new date is ";
    b.showDate();

    cout << endl;

    cout << ".\n";

return 0;
}

[Enoizat]

In your overloaded operator-() you perform nearly the same calculation than in the overloaded operator+()...
Maybe you want it to be more similar to this:

Date Date::operator-(int days)
{
    Date temp; // a temporary date to store the result
    temp.day = day - days; // subtract the days
    temp.month = month;
    temp.year = year;

    while (temp.day < 1) // now adjust the months
    {
        temp.month--;
        temp.day += 30;
    }

    while (temp.month < 1) // adjust the years
    {
        temp.year--;
        temp.month += 12;
    }

return temp; // the values in temp are returned
}

To be honest, I haven't check the result if it's correct, but I'm quite sure it isn't: you don't consider neither 31 days long months nor February, let alone leap years...

[gunnerfunner]

Set up the Date class as a wrapper around std::chrono::system_clock::time_point so that you can use the in-build standard library time and date facilities to overload the operators instead of trying to rebuild from scratch:

# include <iostream>
# include <chrono>
# include <ctime>
# include <string>

struct Date
{
    int m_year;
    int m_month;
    int m_day;
    std::chrono::system_clock::time_point m_timePoint;

    Date(int year, int month, int day)
    {
        struct std::tm t;
        t.tm_sec = 0;        // second of minute (0 .. 59 and 60 for leap seconds)
        t.tm_min = 0;        // minute of hour (0 .. 59)
        t.tm_hour = 0;      // hour of day (0 .. 23)
        t.tm_mday = day;       // day of month (1 .. 31)
        t.tm_mon = month-1;      // month of year (0 .. 11)
        t.tm_year = year-1900; // year since 1900
        t.tm_isdst = -1;       // determine whether daylight saving time
        std::time_t tt = std::mktime(&t);
        if (tt == -1)
        {
            throw "no valid system time";
        }
        m_timePoint = std::chrono::system_clock::from_time_t(tt);
    }
    std::chrono::system_clock::time_point operator + (const Date& rhs)
    //returns the greater timepoint + duration b/w the 2 timepoints
    {
        return ((m_timePoint >= rhs.m_timePoint) ?
                    m_timePoint + (m_timePoint - rhs.m_timePoint)
                    : rhs.m_timePoint + (rhs.m_timePoint - m_timePoint));
    }
    double operator - (const Date& rhs)
    //returns absolute duration in hours, can also extend to days, minutes on similar lines
    {
        return((m_timePoint >= rhs.m_timePoint) ?
            std::chrono::duration_cast<std::chrono::hours>(m_timePoint - rhs.m_timePoint).count()
            : std::chrono::duration_cast<std::chrono::hours>(rhs.m_timePoint - m_timePoint).count());
    }
};
inline
std::string asString (const std::chrono::system_clock::time_point& tp)
{
    // convert to system time:
    std::time_t t = std::chrono::system_clock::to_time_t(tp);
    std::string ts = ctime(&t);
    // convert to calendar time, taking local time zone into a/c
    //std::string ts = asctime(gmtime(&t));
    //// convert to calendar time, using universal time UTC
    ts.resize(ts.size()-1);        // skip trailing newline
    return ts;
}

int main()
{
    Date d(2017, 5, 20);
    std::cout << asString((d + Date(2017, 6, 11))) << "\n";

    auto diff = d - Date(2017, 6, 11);
    std::cout << diff << " hours \n";
}

you can look up any unknown functions individually but for a comprehensive treatment of clocks and timers see 'The C++ Standard Library' (2nd ed), Nicolai Josuttis, section 5.7

[Shishykish]

Hello Enoizat!

Thanks very much, I actually tried that, but must've missed something along the way. Works a treat.

Gunnerfunner, I'm just working through some exercises in a textbook from one of my old University courses. I'm not aware of one of that library. So I'll have to look it up. I do usually use standard libraries where I can. No point in re-inventing the wheel, just use someone else's.