Iterators

Iterators

An iterator is an object that stands at one element of a container and can move to the next one. It gives one uniform way to walk through a vector, a list, or a map, no matter how those containers store data inside.

begin() and end()

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

auto it = v.begin();   // points at the first element

• v.begin() returns an iterator to the first element.
• v.end() returns the marker one past the last element. It is the stop line for loops.

Read with *, move with ++

std::cout << *it;   // 10, the value it points to
++it;               // advance to the next element
std::cout << *it;   // 20

The star dereferences the iterator to get the element. The ++ operator steps it to the next element.

The classic loop

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

It starts at begin(), keeps going while it != end(), and advances each trip with ++.

Why not just indexes?

For a basic vector you could count with 0, 1, 2. But a map has no index - the whole idea is walking from key to key. And every algorithm in the library accepts the same [begin, end) pair. Learn the iterator once and one pattern works everywhere.

TL;DR

• An iterator points at one element and can step to the next.
• begin() and end() fence the collection; end() is one past the last.
• *it reads the value; ++it walks on.
• Loop while it != v.end() and you will never run past the end.
• Range-loops and <algorithm> both build on iterators.