Multiset in C++ STL

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.

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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<T> 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.

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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().

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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.

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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.

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Deleting Elements

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

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