Question

C++ : Suppose I have a user-defined type Date, which has member variables int y, int m, and int d. I also have a vector dates of type Date. I want to use sort(dates.begin(), dates.end)) to sort the vector from the earliest date to the latest date, but I need to define first how sort() will do its job in this case. How do I write the overloading function inside the Date class (i.e. how do I write Date's member function bool operator < ... etc.)?

Answer 1

if you're using a structure

struct Date {
int y;
int m;
int d;
};

then you can define a function to compare the Dates

bool DateComparer (const Date& date1, const Date& date2) {
if(date1.y < date2.y) {
return true;
} else if(date1.y == date2.y && date1.m < date2.m) {
return true;
} else if(date1.y == date2.y && date1.m == date2.m && date1.d < date2.d) {
return true;
}
return false;
}

#include <algorithm>

... namespace std ... etc

vector<Date> dates;

... insert your dates

sort(dates.begin(), dates.end(), DateComparer);

but if you want to overload the < operator in a class...

class Date {
// data members ...
public:

bool operator < (const Date& date) const {
if(this->y < date.y) {
return true;
} else if(this->y == date.y && this->m < date.m) {
return true;
} else if(this->y == date.y && this->m == date.m && this->d < date.d) {
return true;
}
}
};

then

sort(dates.begin(), dates.end());

should do the trick, hope this helps =)