Topic 64 of 91 9. C++ Standard Template Library (STL) Interactive Tutorial

std::multiset in C++: Sorted Collections That Allow Duplicates

6 mins read Last updated: 26 Sep 2026 16,768 views

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Direct Answer & Core Concept

std::multiset is an associative container that stores sorted elements where duplicate values are permitted. Calling erase(value) removes ALL occurrences of that value; to remove only a single occurrence, you must pass an iterator to erase(iterator).

Detailed Explanation & Mechanics

1. Duplicate Key Storage:

Architecture Diagram

MULTISET ORDERED SEQUENCE:
[ 10 ] ──► [ 20 ] ──► [ 20 ] ──► [ 20 ] ──► [ 30 ] ──► [ 40 ]

2. Erase Behavior Caution:

ms.erase(20): Deletes ALL three 20s!
ms.erase(ms.find(20)): Deletes ONLY the first 20 found.

Runnable Code Example: std::multiset in C++: Sorted Collections

C++20 / C++23

main.cpp

Run Online

#include <iostream>
#include <set>

int main() {
    std::multiset<int> scores = {90, 75, 90, 85, 90, 60};

    std::cout << "All Scores (Multiset): ";
    for (int s : scores) std::cout << s << " ";
    std::cout << "\n";

    std::cout << "Count of 90s: " << scores.count(90) << "\n";

    // 1. Remove ONLY ONE occurrence of 90
    auto it = scores.find(90);
    if (it != scores.end()) {
        scores.erase(it); // Erase by iterator removes 1 item
    }

    std::cout << "After erasing single 90: ";
    for (int s : scores) std::cout << s << " ";
    std::cout << "\n";

    // 2. Remove ALL occurrences of remaining 90s
    scores.erase(90); // Erase by value removes all
    std::cout << "After erasing all 90s: ";
    for (int s : scores) std::cout << s << " ";
    std::cout << "\n";
    return 0;
}

Common C++ Technical Interview Questions & Answers

Interview Prep

Q: What is the difference between `multiset::erase(val)` and `multiset::erase(iterator)`?

A: `erase(val)` removes every matching element with that value and returns the count of removed elements. `erase(iterator)` removes only the specific single node referenced by the iterator.

Q: How is `std::multiset` implemented?

A: Like `std::set`, it is typically implemented as a self-balancing Red-Black Tree where equivalent keys are linked together in the tree structure.

Key Takeaways, Modern Best Practices & Idioms

C++ Best Practices & Production Rules

• Be careful when erasing by value: use ms.erase(ms.find(val)) if your intention is to remove only a single instance.
• Use std::multiset for tracking dynamic sliding-window medians and frequency counters.
• Use .equal_range(val) to iterate over all duplicates of a specific value efficiently.

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