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Unordered Multiset in C++ STL

Last Updated : 23 Jul, 2025

In C++, unordered multiset is an unordered associative container that works similarly to an unordered set, but it can store multiple copies of the same value. It provides fast insert, delete and search operations using hashing, but the elements are not in any particular order.

Example:

#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
    
    // Creating unordered multiset of
    // integers
    unordered_multiset<int> ums =
                          {5, 1, 3, 4, 1};

    for (auto x : ums)
        cout << x << " ";
    return 0;
}

Explanation: In the above program, we create an unordered multiset ums with values: 5, 1, 3, 4 and 1. We can see from the output that there is no order of the elements and we were able to store 1 two times.

Unordered multiset is defined as the std::unordered_multiset class template inside the <unordered_multiset> header file.

unordered_multiset<T> ums;

where,

• T: Type of elements in the unordered multiset.
• ums: Name assigned to the unordered multiset.

We can declare and initialize an unordered multiset in different ways as shown in the below example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    
    // Create an empty unordered multiset
    unordered_multiset<int> ums1;
    
    // Create and initialize elements
    // using initialzer list
    unordered_multiset<int> ums2 =
                          {5, 3, 4, 1, 1};
    
    for(auto x : ums2)
        cout << x << " ";;
    return 0;
}

Explanation: In this example, we initialize an unordered multiset in three ways:

• Statement unordered_multiset<int> ums1 creates an empty unordered multiset.
• Statement unordered_multiset<int> ums2 = {2, 5, 3, 4, 1, 1} initialize the unordered multiset using initializer list.

The basic operations of unordered multiset are shown below:

Elements can be inserted into an unordered multiset using the insert() method. The position of the element is determined by the hashing function so we cannot specify any position while inserting.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    unordered_multiset<int> ums;
    
    // Insert elements using insert()
    ums.insert(5);
    ums.insert(1);
    ums.insert(3);
    ums.insert(1);
    ums.insert(2);
    ums.insert(4);
    
    for (auto x : ums) cout << x << ' ';
    return 0;
}

An unordered multiset does not allow direct access to elements by index as accessing elements is not the primary operation of this container. However, we have to increment or decrement iterator obtained from begin() or end() methods respectively to access the element by position. This can also be done with the help of next() or advance() function.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    unordered_multiset<int> ums =
                    {5, 1, 3, 2, 4, 1};

    // Accessing element using iterator
    auto it = next(ums.begin(), 2);
    cout << *it;
    return 0;
}

Explanation: In the above program, iterator it accesses the third element by advancing begin() by 2 positions using next().

Unordered multiset does allow changing the value of the accessed elements as it may disrupt the container functions.

It is the primary operation for which this container is optimized for. The find() method provides fast search of any element by value. This function returns iterator to the element if found, otherwise returns iterator to the end().

#include <bits/stdc++.h>
using namespace std;

int main() {
    unordered_multiset<int> ums =
                     {5, 1, 3, 4, 1};
    
    // Finding 3
    auto it = ums.find(3);
    
    if (it != ums.end()) cout << *it;
    else cout << "Element not Found!";
    return 0;
}

We can traverse unordered multiset either using range-based for loop or using begin() and end() iterator.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    unordered_multiset<int> ums =
                    {5, 1, 3, 4, 1};
    
    // Using range-based for loop
    for(auto x : ums)
        cout << x << " ";
    cout << "\n";
    
    return 0;
}

We can use erase() method to delete elements from unordered multiset. It deletes all occurrences of an elements if we provide the value of the element. If we provide iterator of the element, it deletes only that value from the unordered multiset.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    unordered_multiset<int> ums =
                       {5, 1, 3, 2, 4, 1};

    // Delete all occurrences of element 1
    ums.erase(1);
    
    // Delete 2 using iterator
    ums.erase(ums.find(2));
    
   for (auto x: ums) cout << x << " ";
    return 0;
}

Explanation: In the above program,

• ums.erase(1) deletes all occurrences of the element 1 from the set.
• ums.erase(ums.find(2)) deletes one occurrence of the element 2 by using an iterator returned from find(2).

The below table lists the time complexity of the above operations on unordered multiset:

Unordered multiset is used in many situations for different purposes. The following examples’ aim is to help you master unordered multiset beyond the basics:

• Check if Unordered Multiset is Empty
• Find the Size of Unordered Multiset
• Check Existence of an Element
• Find Frequency of an Element
• Reverse Unordered Multiset
• Swap Two Unordered Multiset
• Create Unordered Multiset of User Defined Datatype

In C++, unordered multiset container provides the built-in implementation of a hash table data structure. Each element is hashed to a bucket and elements with the same hash value are stored in the same bucket. These buckets are generally implemented as linked lists. This allows for fast access and insertion, basically with constant time complexity (O(1)) for operations like insert(), erase(), and find(). However, due to hash collisions, the time complexity can degrade to O(n) in the worst case.

Primary difference between unordered multiset and multiset is shown below:

• Unordered multiset allows multiple occurrences of the same element and does not maintain the order of the elements. Elements are stored based on their hash values.
• Multiset allows multiple occurrences of the same element and also elements store in sorted way either ascending or descending.

Following is the list of all member functions of std::unordered_multiset class in C++:

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