What are iterators?
- C++ course
- Intermediate
- Lesson 59/116
- 5 min
Learn what iterators are and their relation to containers.
What are iterators?
An iterator is a pointer-like object that gives access to elements in a container. Like pointers it is dereferenced with *, advanced to the next element with ++ and compared with !=.
Example: for std::vector<int> v = {10, 20, 30}; the call auto it = v.begin(); returns an iterator to the first element. *it reads the value 10. ++it moves to the next element. Iterators let you write one loop that works regardless of the container type.
Dereference and advance
An iterator has two core operations. *it reads or writes the element it points at. ++it moves it to the next element. These two form the loop body.
Example loop: auto it = v.begin(); while (it != v.end()) { std::cout << *it << " "; ++it; }. Here it != v.end() keeps the loop going while the iterator has not moved past the last element. *it prints the element each step and ++it advances to the next. This way every element of the container is printed in turn.
Practice Print a vector using an iterator
For the vector v = {10, 20, 30}, take an iterator with v.begin() and in a while loop print each element with *it. Add a space after each element.
Starter code
#include <iostream>
#include <vector>
int main() {
std::vector<int> v = {10, 20, 30};
// Take an iterator and print with a while loop
return 0;
}Sample solution
#include <iostream>
#include <vector>
int main() {
std::vector<int> v = {10, 20, 30};
auto it = v.begin();
while (it != v.end()) {
std::cout << *it << " ";
++it;
}
return 0;
}Expected output
10 20 30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which operation is NOT valid on every iterator?
auto it = v.begin();- Take an offset with it + 2 Correct answer
- Advance with ++it
- Access the element with *it
- Compare with it != v.end()
Explanation: it + 2 is valid only on random access iterators.
2 Print the element in the loop body
auto it = v.begin();
while (it != v.end()) {
std::cout << ____;
++it;
}auto it = v.begin();
while (it != v.end()) {
std::cout << *it;
++it;
}Explanation: *it accesses the element the iterator points at.
3 Match each operation to its role
| Term | Meaning |
|---|---|
auto it = v.begin() |
Iterator to first |
*it |
Access element |
++it |
Move to next |
it != v.end() |
Loop condition |