CppHow-ToBeginner · 3 min read

How to Use Iterator in C++: Syntax and Examples

In C++, an iterator is an object that points to elements in a container like vector or list. You use iterators to traverse or modify container elements by incrementing them with ++ and dereferencing with *.

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Syntax

An iterator is declared using the container's iterator type. You get an iterator to the start of the container with begin() and to the end with end(). Use ++it to move to the next element and *it to access the element.

Container::iterator it = container.begin(); — declares an iterator pointing to the first element.
it != container.end() — loop condition to stop at the end.
++it — moves iterator forward.
*it — accesses the element the iterator points to.

cpp

std::vector<int>::iterator it = container.begin();
while(it != container.end()) {
    // use *it to access element
    ++it; // move to next
}

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Example

This example shows how to use an iterator to print all elements of a std::vector.

cpp

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {10, 20, 30, 40, 50};
    for(std::vector<int>::iterator it = numbers.begin(); it != numbers.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;
    return 0;
}

Output

10 20 30 40 50

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Common Pitfalls

Common mistakes when using iterators include:

Dereferencing an iterator that equals end() causes undefined behavior.
Modifying the container (like adding or removing elements) invalidates existing iterators.
Using the wrong iterator type (e.g., const_iterator vs iterator) when modifying elements.

cpp

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {1, 2, 3};
    auto it = v.end();
    // Wrong: dereferencing end() iterator
    // std::cout << *it << std::endl; // undefined behavior

    // Correct way:
    it = v.begin();
    std::cout << *it << std::endl; // prints 1
    return 0;
}

Output

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Quick Reference

Operation	Syntax	Description
Declare iterator	Container::iterator it;	Creates an iterator for the container
Get start	it = container.begin();	Points to first element
Get end	it = container.end();	Points past last element
Access element	*it	Dereferences iterator to get element
Move forward	++it	Moves iterator to next element
Move backward (if supported)	--it	Moves iterator to previous element
Check end	it != container.end()	Loop until end reached

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Key Takeaways

Use begin() and end() to get iterators for container traversal.

Increment iterators with ++ and access elements with *.

Never dereference an iterator equal to end() as it is invalid.

Modifying containers can invalidate iterators, so be careful when changing containers during iteration.

Use the correct iterator type depending on whether you need to modify elements or not.