Introduction to Iterators in C++

An iterator is an object like a pointer that points to an element inside the container. We can use iterators to move through the contents of the container. They can be visualized as something similar to a pointer pointing to some location and we can access the content at that particular location using them.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v = {1,2,3,4,5};
    
    // Get the iterator of first element of the vector
    vector<int>::iterator it = v.begin();
    
    // Print the content of location which is 
    // pointed by iterator (it)
    cout << *it;
    return 0;
}

1

Explanation: The above example demonstrates accessing the first element of a vector container by first obtaining its iterator (it). Then dereference this iterator with the asterisk (*) operator, which outputs the value 1.

Syntax

cType::iterator iteratorName;

where,

• cType: Container type.
• iteratorName: Named assign to the iterator.

We can use autokeyword to skip to write container type because auto keyword automatically detected container type itself.

auto iteratorName;

Types of Iterators

All iterators do not have similar functionality, depending upon the functionality of iterators they can be classified into five categories:

1. Input Iterator
2. Output Iterator
3. Forward Iterator
4. Bidirectional Iterator
5. Random-Access Iterator

Not all of these iterators are supported by all containers in STL. Different containers support different iterators, like vectors support random-access iterators, while lists support bidirectional iterators. The whole list is as given below:

The following diagram shows the difference in their functionality with respect to various operations that they can perform.

To know more, refer to the article - Iterators in C++

Iterators vs Pointers

The main differences between Iterators and Pointers are given in the below table:

Advantages of Iterators

There are following advantages of iterators listed below:

• Safe and Simple Traversal: Enable sequential access to container elements without managing indices or container boundaries.
• STL Algorithm Compatibility: Work seamlessly with standard algorithms such as sort, find, count, and accumulate, independent of container type.
• Multiple Iterator Categories: Provide different capabilities through iterator types such as input, forward, bidirectional, and random-access iterators.
• Support for Reverse Traversal: Allow backward traversal of containers using reverse iterators without modifying container structure.