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

Last Updated : 9 Apr, 2026

A Set is a container which stores unique elements in some sorted order. It is an implementation of a Self-Balancing Binary Search Tree, specifically a Red-Black Tree which ensures,

Search, insert, and delete in O(log n) time.
Does not allow duplicates.
Elements are always sorted in ascending order by default. You can also choose your own way of ordering them using a custom rule (comparator).
Compared to unordered_set, the time taken to search, insert, and delete an higher, but we get elements in sorted order and also support additional functions like upper_bound() and lower_bound().

Let us take a look at an example to create a set and traverse it.

C++ `

#include #include using namespace std;

int main() {

// Creating an empty set
set<int> s1;

// Initialize set with list 
set<int> s2 = {1, 2, 3, 2, 1}; 

// Traversing the set
for (auto& x : s2)
    cout << x << " ";
cout << endl;

return 0;

}

Syntax

The set container is defined as **std::set class template inside ****** header file.

set s;

where,

**T: Data type of elements in the set.
**s: Name assigned to the set.

Basic Operations

Basic operations on set containers are shown below:

1. Inserting Elements

The insert() operation adds a new element to the set only if it not already present.
If the element already exists, insert() does nothing (since duplicates are not allowed in set). C++ `

#include #include using namespace std;

int main() { // Initialize set with values set s = {2, 3};

// Inserting an element
s.insert(1);

// Traversing the set
for (auto x : s)
    cout << x << endl;

return 0;

}

2. Searching Elements

The find() function is used to check whether an element exists. It returns an iterator to the element if found, else returns end() if the element is not found.
The count() function can also be used to check existence, returns 1 if the element is present, 0 otherwise. C++ `

#include #include using namespace std;

int main() { set s = {1, 2, 3};

// Accessing elements using find()
auto it = s.find(1);
if (it != s.end())
    cout << "Element found: " << *it << endl;

// Accessing elements using count()
if (s.count(2))
    cout << "2 exists in the set" << endl;

// Accessing all elements by traversal
cout << "All elements: ";
for (auto x : s)
    cout << x << " ";
cout << endl;

return 0;

}

Output

Element found: 1 2 exists in the set All elements: 1 2 3

3. Traversing

Loops (like range-based for loop or iterators) can be used to traverse all elements in a set.
The traversal visits elements in sorted order (by default ascending, or according to a custom comparator). C++ `

#include #include using namespace std;

int main() { set s = {1, 2, 3};

// Traversing using iterators
for (auto it = s.begin(); it != s.end(); ++it)
    cout << *it << endl;  

return 0;

}

4. Deleting Elements

To delete an element from a set, use erase(), it removes the element if it exists, else does nothing.

C++ `

#include #include using namespace std;

int main() { set s = {1, 2, 3, 4};

// Deleting by value
s.erase(2);

// Deleting by iterator
s.erase(s.begin());

// Traversing the set
for (auto i : s)
    cout << i << " ";
cout << endl;

return 0;

}