Question

I have this class:

class Date
{
public:
    Date();                                         // Sets a date of January 1, 2000
    Date(int mm, int dd, int yyyy);                 // Sets a date with the passed arguments; automatically converts two-digit years as being AFTER the year 2000
    Date after_period_of ( int days ) const;        // Elapses an arbitrary amount of time, and returns a new date
    string mmddyy () const;                         // Returns date in MM/DD/YY format (1/1/00)
    string full_date () const;                      // Returns date in full format (January 1, 2000)
    string month_name () const;                     // Returns the name of the month as a string;
private:
    int days_in_february(int yr);
    int month;
    int day;
    int year;
};

When I try to pass private variable year as an argument into days_in_february, I get the following error message:

passing ‘const Date’ as ‘this’ argument of ‘int Date::days_in_february(int)’ discards qualifiers

days_in_february is called in after_period_of, like this:

Date Date::after_period_of (int days_elapsed) const
{
    int new_month;
    int new_year = year;    // tried copying 'year' to get around this issue, but it did not help
    int days_into_new_month;
    int max_days_in_month[12] =  { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 } ;
    max_days_in_month[1] = days_in_february(new_year);

and again in the same function:

if (new_month == 13)
            {
                new_month = 1;
                new_year += 1;
                max_days_in_month[1] = days_in_february(new_year);
            }

days_in_february simply returns the number 28 or 29 based on year that is passed into it. It does not attempt to manipulate anything outside of its own block.

I have even tried to pass a non-programmed variable into it (days_in_february(2000)) and I get the same error.I’ve tried moving that function into the public domain, but that didn’t fix the issue either.

Why is this happening?

Why am I not allowed to do this?

Answer 1

Inside after_period_of, you cannot access non-constant functions; in particular, you cannot access days_in_february. You can fix this by also declaring the latter function const.