Iterators and Their Use in the C++ STL

The C++ Standard Library (STL) relies on a powerful abstraction called the iterator, which enables traversal over data structures. Iterators are the core of STL containers (vector, list, map, set…) and allow moving from one element to another in a unified and consistent way. This article explains iterator types, common usage patterns, and how they integrate with STL algorithms.

1) What Is an Iterator?

An iterator is an object used to access elements of a container. It behaves similarly to a pointer but is fully abstracted by the STL.

Main purposes of iterators:

• Traversing a container
• Reading or modifying elements
• Interacting with STL algorithms (sort, find, count…)

Example

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

int main() {
    vector<int> v = {10, 20, 30};

    vector<int>::iterator it = v.begin();

    cout << *it; // 10
}

2) Types of Iterators

Different STL containers support different iterator categories.

• Input Iterator – read only
• Output Iterator – write only
• Forward Iterator – forward movement
• Bidirectional Iterator – forward + backward
• Random Access Iterator – pointer-like, allows jumps

Examples: vector → random access list → bidirectional map/set → bidirectional

3) begin() and end() Functions

All STL containers provide these two essential functions:

• begin() → points to the first element
• end() → points to the position after the last element

vector<int> v = {1, 2, 3};

for (auto it = v.begin(); it != v.end(); ++it) {
    cout << *it << " ";
}

4) Const Iterator (const_iterator)

A const iterator is used for read-only access; it cannot modify elements.

vector<int> v = {10, 20, 30};

vector<int>::const_iterator it = v.begin();

// *it = 50; // ERROR: cannot modify through const_iterator

cout << *it;

5) Reverse Iterators (rbegin & rend)

Reverse iterators allow traversing a container backward.

vector<int> v = {1, 2, 3};

for (auto it = v.rbegin(); it != v.rend(); ++it) {
    cout << *it << " ";
}
// Output: 3 2 1

6) Modifying Elements with Iterators

vector<int> v = {10, 20, 30};

for (auto it = v.begin(); it != v.end(); ++it) {
    *it *= 2;
}

for (int x : v)
    cout << x << " ";
// Output: 20 40 60

7) Iterators and STL Algorithms

All STL algorithms operate using iterators.

Using sort

#include <algorithm>

vector<int> v = {50, 20, 10};

sort(v.begin(), v.end());

Using find

auto it = find(v.begin(), v.end(), 20);

Using count

int c = count(v.begin(), v.end(), 10);

8) Iterating over map and set

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

int main() {
    map<string, int> ages = {
        {"John", 25},
        {"Emily", 30},
        {"Michael", 28}
    };

    for (auto it = ages.begin(); it != ages.end(); ++it) {
        cout << it->first << " : " << it->second << endl;
    }
}

9) Deleting Elements Using Iterators

Iterators help remove specific elements efficiently.

vector<int> v = {10, 20, 30, 40};

auto it = find(v.begin(), v.end(), 20);

if (it != v.end())
    v.erase(it);

10) Advantages of Using Iterators

• Container-independent traversal
• Cleaner and more readable code
• Full compatibility with STL algorithms
• Efficient memory access

11) TL;DR

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