C++ Library - <multiset>

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Introduction

A multiset is an associative container which contains a sorted collection of objects of type Key. Unlike set, multiple elements can have equivalent values, which means duplicates are allowed.

The value of the elements in a multiset cannot be modified in the container, i.e., the elements are always const. But they can be inserted or removed from the container.

The multiset containers are generally slower than unordered_multiset containers in accessing individual elements by their key, but they allow the direct iteration on subsets based on their order.

Characteristics of Multiset

Following are the key Characteristics of the multiset:

• In multiset, the elements get automatically sorted. By default, it is sorted in ascending order.
• The elements in multiset are always const.
• Multiset is generally implemented as Red-Black tree(balanced binary search tree).
• Multiset allows duplicate values.

Key difference from set: The main difference between a set and a multiset is that multiset accepts duplicate elements while set contains only unique elements.

Definition

Below is the definition of std::multiset from <set> header file

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

Parameters

• Key: It represents the type of element contained. The key may be substituted by any other data type including user-defined type.

Member types

Following member types can be used as parameters or return type by member functions.

Functions from <multiset>

Below is list of all methods of multiset from <set> header.

Default Member Functions

Iterators

Capacity

Modifiers

Lookup

Observers

Allocator

Example Usage

Here is an example to insert elements in the multiset and counting the occurrences of an element:

#include <iostream>
#include <set>

int main() {
    // Create a multiset
    std::multiset<int> ms;
    
    // Insert elements (duplicates allowed)
    ms.insert(10);
    ms.insert(20);
    ms.insert(10);  // Duplicate
    ms.insert(30);
    ms.insert(20);  // Duplicate
    
    // Display elements
    std::cout << "Multiset elements: ";
    for (auto it = ms.begin(); it != ms.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;
    
    // Count occurrences
    std::cout << "Count of 10: " << ms.count(10) << std::endl;
    
    return 0;
}

The output of the above code is as follows −

Multiset elements: 10 10 20 20 30
Count of 10: 2