In C++ multimap is a container to store key-value pairs allowing duplicate keys which is not allowed in a map container. By default, the multimap container uses the less than ‘<‘ operator to compare the keys for ordering the entries but also allows the use of the custom comparator. In this article, we will learn how to use a multimap with a custom comparator in C++.

Custom Comparator for Multimap in C++

To use a std::multimap with a custom comparator, we can define a comparison function or a functor (a class that overloads the operator()) and then use it as a template parameter when declaring the multimap so that the elements of the multimap will be ordered automatically based on the conditions mentioned in the custom comparator.

C++ Program to Use a Multimap with a Custom Comparator

The below example demonstrates how we can use a multimap with a custom comparator in C++.

Multimap using custom comparator
kiwi: 2
apple: 5
apple: 10
banana: 3
orange: 4

Time Complexity: O(N log N), here N is the number of elements in a multimap.
Auxiliary Space: O(N)