Set begin() function in C++ STL

This tutorial will discuss about the set begin() function in C++ stl.

Table Of Contents

In the C++ Standard Template Library (STL), the set is a container that stores unique elements in a specific order. One of its fundamental member functions is begin(), which provides an iterator pointing to the first element in the set.

Syntax of set::begin()

iterator begin() noexcept;
const_iterator begin() const noexcept;

This function returns an iterator that points to the first element in the set. Since the returned iterator is mutable, the position it points to can be modified, but not the element’s value (as set elements are constant).

Parameters of set::begin()

- None

Return Value of set::begin()

  • Iterator pointing to the first element in the set.

Example of set::begin()

Let’s see the complete example,

#include <iostream>
#include <set>

int main()
    std::set<int> numbers = {5, 15, 25, 35, 45};

    std::set<int>::iterator it = numbers.begin();

    std::cout << "First element in the set: " << *it << "n";
    return 0;


First element in the set: 5

Scenarios for Exceptions and Undefined Behavior

  1. Iterator Invalidation:

    • If the set is modified in such a way that the structure changes (like after an set::erase() operation), previously acquired iterators through begin() function or any other function, might get invalidated. Using such iterators can result in undefined behavior.
  2. Dereferencing End Iterators:

    • If set is empty then the begin() function would return an iterator equivalent to end(). Dereferencing such an iterator is undefined behavior.
  3. Modifying Elements:

    • The value of elements in a set is immutable. If you try to modify an element’s value through a (mutable) iterator acquired via begin(), it will result in a compile-time error.
  4. Thread Safety:

    • Concurrent access by multiple threads can lead to race conditions. I

Using begin() with end()

The begin() function of the set container is fundamental for iterative access, often used in combination with end() to iterate over the entire set. While using it, be mindful of the state of your set (like if it’s empty) and remember that the actual elements in a set are immutable.


Today, we learned about set begin() function in C++ stl.

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