Changing date for certain amount of days function - c++

Question

In class where we learn C++, I was given an assignment to make a class "date". Now, there is a specific function that I am required to make but I really do not have idea how to approach this. Members of class are day, month and year. Function takes some integer that represents days, and is supposed to set a new date, that comes after that many days. For example, if date is (DD-MM-YY) 20.01.2015, and we pass as an argument 15, new date is 04.02.2015, the problem is that I have to consider how many days each month has(considering February has 28 days), and if argument is too big, that we pass into next year, create an exception that prints how many days are there until next year(considering year has 365 days). That is, if the date is 20.12.2010, and argument is greater than 11, it should print 11.
My attempt was using while, where I declared at the beginning that int k=0; and where argument of function is a, than I used while(k!=a), but body of the function got really confusing because I used just too many if conditions. Another thing that I tried is to overlap operator++ and that surely gives me simpler function, because than there is only one for loop inside it, but I am not solving the problem, because in that overlapped operator function I am still using many if conditions.
Is there some elegant way to do this? Code and explanation would be great! Thank you!

Answer 1

Simply identify what you need and then create it, like
static unsigned int Date::days_of_month(const Month month, const unsigned int year);
void Date::operator+=(const unsigned int days);
+= could look like
void Date::operator+=(const unsigned int days){
increase current day by days
check if current day too large
if so, increase month (and maybe year)
repeat until day legal
}
Concept is clear? Haven't build in your output on purpose to give you something to do, after all, it's an exercise. Also wrote this in pseudo-code on purpose for the same reason. Try to create some code and if you really can't proceed, I'll add actual code.

Answer 2

const int m_daysInMonth[] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 }; // first for index offset (no "zero" month)
class date {
public:
int days=20, month=11, year=2012;
void addDays(int daysNum)
{
int n_days = days, n_month = month; //new result. not saved, if there is an exception
while (daysNum > 0)
{
int daysToNextMonth = m_daysInMonth[n_month] - n_days + 1; //days before needs to change motn index
if (daysToNextMonth < daysNum) // change month
{
daysNum -= daysToNextMonth;
n_month++;
if (n_month > 12)
{
int daysLeft = m_daysInMonth[month] - days ;
n_month = month + 1;
while (n_month <= 12)
{
daysLeft += m_daysInMonth[n_month];
n_month++;
}
throw daysLeft;
}
n_days = 1; // start of the month
}
else // set day in the month
{
n_days += daysNum;
daysNum = 0;
}
}
days = n_days;
month = n_month;
}
};