C++ Sets – Store Sorted Unique Elements with std::set

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

In C++ applications where duplicates are not allowed and fast search, insertion, and deletion are required, std::set is the ideal choice. It stores elements in a sorted order and provides logarithmic time complexity for all operations, thanks to its balanced tree structure.

In this guide, you’ll learn:

• What std::set is and how it works
• Key operations like insert(), find(), erase()
• How to use iterators and custom sort orders
• Best practices and real-world use cases

What Is a C++ Set?

A std::set is an ordered associative container that contains only unique keys. It stores the keys in ascending order by default using a Red-Black Tree (balanced BST).

Include the header:

#include <set>

Declare a set:

set<int> s;

Code Examples – With Output

Insert and Traverse

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

int main() {
    set<int> s;
    s.insert(30);
    s.insert(10);
    s.insert(20);
    s.insert(10); // Duplicate ignored

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

Output:

10 20 30

Search and Erase

if (s.find(20) != s.end())
    s.erase(20);

Set Member Functions

Custom Sort Order with Comparator

struct Desc {
    bool operator()(int a, int b) const {
        return a > b; // Descending
    }
};

set<int, Desc> descSet;

Iterating a Set

for (set<int>::iterator it = s.begin(); it != s.end(); ++it)
    cout << *it << " ";

Set vs Multiset vs Unordered Set

Best Practices & Tips

Use count() if you only need to check for existence
For heavy insert/delete workloads, consider unordered_set
Avoid modifying set elements via iterators—sets reorder elements automatically
Use a custom comparator for descending order or complex types

Use Cases for Sets

Unique Element Tracking – Email lists, usernames, identifiers
Sorted Data Retrieval – Maintain live sorted datasets
Efficient Searching – Membership tests and quick lookups
Event Logs – Avoid duplicate event IDs
Mathematical Sets – Implement union, intersection, difference

Summary – Recap & Next Steps

Key Takeaways:

• std::set stores unique, automatically sorted elements
• Efficient for searching, insertion, and deletion (O(log n))
• Supports iterators and custom comparators

Real-World Relevance:
C++ sets are used in simulations, indexes, validation systems, constraint checkers, and anywhere uniqueness and order are critical.

Next Steps:
Learn about C++ Maps to associate keys with values in sorted order.

FAQ – C++ Sets

Are duplicates allowed in std::set?
No. Use multiset if duplicates are needed.

Can I change an element in a set directly?
No. You must remove and re-insert to modify value.

How is set sorted?
By default in ascending order using < operator.

Can I sort a set in descending order?
Yes. Use a custom comparator.

Is find() faster than looping through the set?
Yes. find() uses binary search (O(log n)).