What is the role of greater <int> in Set?

Question

While declaring a set,

set <int, greater <int> > gquiz1;

why do we use greater <int>? What purpose does it serve?

Answer 1

std::set is a container that contains an ordered set of objects.

The ordering of the objects is determined by the second argument. By default, it is std::less<Key>. See the definition of std::set for additional details. However, you can override the default argument by using your own Compare type as the second argument, as you have done in your posted code.

E.g.

std::set<int> set1; // Use default compare class, std::less<int>
set1.insert(10);
set1.insert(5);
set1.insert(7);

The order of the objects in the above container will be 5, 7, and 10. The objects in the container are sorted in increasing orer.

If you use

std::set<int, std::greater<int>> set2;
set2.insert(10);
set2.insert(5);
set2.insert(7);

The order of the objects in the above container will be 10, 7, and 5. The objects in the container are sorted in decreasing orer.

Answer 2

The declaration of std::set looks like this:

template<
    class Key,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<Key>
> class set;

The entry goes on to say:

std::set is an associative container that contains a sorted set of unique objects of type Key. Sorting is done using the key comparison function Compare.

By default, the key comparison is done using std::less, meaning that the incoming entry is inserted at the first position where it is less than the item in the set that it's being compared against.

In your example, insertion is being performed using the std::greater comparison, which will result in a set where the entries are sorted in an order opposite to the default case.