Multiset in C++ STL (original) (raw)

Last Updated : 1 Jun, 2026

Multiset is an associative container similar to a set, but it can store multiple elements with the same value. It is sorted in increasing order by default, but it can be changed to any desired order using a custom comparator.

Allows duplicate elements, unlike a set.
Supports standard set operations like insert(), erase(), count(), and find().
It uses a self-balancing Binary Search Tree, ensuring O(log n) time complexity for operations. C++ `

#include #include using namespace std;

int main() { // Creating an empty multiset of integers multiset ms1;

// Initialize with initializer list
multiset<int> ms2 = {5, 3, 3, 1};

// Print elements (automatically sorted)
for (auto i : ms2)
    cout << i << " ";

return 0;

}

Explanation

ms1 creates an empty multiset of integers.
ms2 is initialized with four elements, including the duplicate value 3.
The multiset automatically stores elements in sorted order.
Unlike set, both occurrences of 3 are preserved and displayed.

Syntax

multiset ms;

**Where:

T -> data type of elements
ms -> name of the multiset

Basic Operations on Multiset

A multiset provides several built-in member functions to efficiently store, access, search, and remove elements while maintaining sorted order.

Inserting Elements

We can insert elements into a multiset by using insert() method. The multiset will automatically keep the elements sorted.

C++ `

#include #include using namespace std;

int main() { // Creating a multiset of integers multiset ms;

// Inserting elements (duplicates allowed)
ms.insert(5);
ms.insert(3);
ms.insert(3);
ms.insert(1);

// Print elements (automatically sorted)
for (auto i : ms)
    cout << i << " ";

return 0;

}

Accessing Elements

Elements cannot be accessed by index; use an iterator from begin() and advance it. The first element is *begin(), and the last is one step back from end().

C++ `

#include #include #include using namespace std;

int main() { multiset ms = {5, 3, 3, 1};

// Access first element
auto it1 = ms.begin();
cout << *it1 << " ";

// Access third element
auto it2 = next(it1, 2);
cout << *it2;

return 0;

}

Finding Elements

Multiset provides fast search by value operation using the **find() member function. This function returns iterator the element if found, otherwise returns **end() iterator.

C++ `

#include #include using namespace std;

int main() { multiset ms = {5, 3, 3, 1};

// Finding an element (3)
auto it = ms.find(3);

if (it != ms.end())
    cout << *it;
else
    cout << "Not Found!";

return 0;

}

Traversing

Use a range-based for loop or begin()/end() iterators to traverse a multiset. And It use equal_range() to traverse all elements with the same value.

C++ `

#include #include using namespace std;

int main() { multiset ms = {5, 3, 3, 1};

// Traversing using range-based for loop
for (auto i : ms)
    cout << i << " ";

return 0;

}

Deleting Elements

The erase() method removes elements by value (all occurrences) or by iterator (specific element).

C++ `

#include #include using namespace std;

int main() { multiset ms = {5, 3, 3, 1};

// Delete first element
ms.erase(ms.begin());

// Delete all occurrences of 3
ms.erase(3);

// Print remaining elements
for (auto x : ms)
    cout << x << " ";

return 0;

}

**Note: Elements in a multiset cannot be updated or modified once inserted.