C++ unordered_multiset – Store Unordered Duplicates with Fast Lookup

Introduction – Why Use std::unordered_multiset in C++

In some applications, you need a data structure that allows duplicate values and offers fast average-time lookup. That’s where std::unordered_multiset comes in. Unlike set, it allows multiple identical elements and uses a hash table for O(1) average complexity on insertions and lookups.

In this guide, you’ll learn:

• What std::unordered_multiset is and how it works
• How to insert, count, and remove elements
• Comparisons with set, multiset, and unordered_set
• Best practices and real-world use cases

What Is a C++ Unordered Multiset?

A std::unordered_multiset is an unordered associative container that stores duplicate elements. It does not maintain order, and elements are grouped by hash values. All values are keys.

Include the header:

#include <unordered_set>

Declare an unordered multiset:

unordered_multiset<int> ums;

Code Examples – With Output

Insert and Traverse

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

int main() {
    unordered_multiset<int> ums;
    ums.insert(10);
    ums.insert(20);
    ums.insert(10);

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

Output (order may vary):

10 10 20

Count and Erase

cout << "Count of 10: " << ums.count(10) << endl;
ums.erase(10); // Removes all instances of 10

unordered_multiset Member Functions

unordered_multiset vs Other Containers

Best Practices & Tips

Use count() before erasing to verify existence
Prefer unordered_multiset when duplicates are common and order doesn’t matter
Do not rely on element order—it’s implementation-specific
Avoid using it when frequent range queries or sorted output are needed

Use Cases for Unordered Multisets

Frequency Tracking – Count repeated values efficiently
Fast Lookups – Duplicate data with average O(1) access
Histogram Generation – Multiple entries for the same key
Inventory Systems – Stackable items with identical types
Data Ingestion – Handle noisy input with duplicates

Summary – Recap & Next Steps

Key Takeaways:

• std::unordered_multiset stores duplicate elements without maintaining order
• Offers fast average-time operations using hash tables
• Suitable for large, unsorted collections with repeated values

Real-World Relevance:
Used in big data pipelines, histogram analyzers, search indexes, frequency maps, and load balancers where order isn’t important but duplicates and fast access are.

Next Steps:
Explore C++ Iterators to traverse and manipulate STL containers like unordered_multiset in a generic way.

FAQ – C++ unordered_multiset

Can unordered_multiset have duplicates?
Yes. It’s specifically designed to support them.

Is order guaranteed in unordered_multiset?
No. The order is non-deterministic and based on hash buckets.

Can I access elements by index?
No. Use iterators to traverse through elements.

What’s the difference between unordered_set and unordered_multiset?
unordered_set stores only unique values; unordered_multiset allows duplicates.

Is it faster than multiset?
Yes, on average. But worst-case performance can be worse with hash collisions.