Iterator

Provides a way to sequentially access elements of a collection without exposing its underlying representation.

The Problem

A collection has a complex internal structure (tree, graph, ring buffer). You want to iterate over it without exposing how the data is stored.

How It Works

›Separates traversal logic from the collection class
›Supports multiple simultaneous independent traversals of the same collection
›Enables a unified for-in / for-of interface across all collections
›Lazy iterators (Kotlin Sequences, Python generators) compute elements on demand
›Internal iterator hides the loop; external iterator gives the caller control

When to Use

✓Custom collections that need to support for-in/for-of loops

✓When you need multiple traversal strategies for the same collection

✓Hiding complex traversal logic (tree traversal, pagination)

✓Lazy sequences where computing all elements upfront is expensive

Real-World Examples

🍎

iOS

IteratorProtocol + Sequence, IndexingIterator, for-in on any Collection type

🤖

Android

Iterable<T>, Kotlin Sequences for lazy evaluation, Cursor for database rows

🌐

Web

Symbol.iterator, Generator functions (function*), built-in Array/Map/Set iterators

Code Examples

Swift

struct Countdown: Sequence {
    let from: Int

    func makeIterator() -> CountdownIterator {
        CountdownIterator(current: from)
    }

    struct CountdownIterator: IteratorProtocol {
        var current: Int
        mutating func next() -> Int? {
            guard current >= 0 else { return nil }
            defer { current -= 1 }
            return current
        }
    }
}

let countdown = Countdown(from: 5)
for n in countdown { print(n) }  // 5, 4, 3, 2, 1, 0

// Works with all Sequence methods
print(countdown.contains(3))      // true
print(Array(countdown))           // [5, 4, 3, 2, 1, 0]