# IndexingIterator A type that iterates over a collection using its indices. ``` @frozen struct IndexingIterator where Elements : Collection ``` ## Overview The `IndexingIterator` type is the default iterator for any collection that doesn’t declare its own. It acts as an iterator by using a collection’s indices to step over each value in the collection. Most collections in the standard library use `IndexingIterator` as their iterator. By default, any custom collection type you create will inherit a `makeIterator()` method that returns an `IndexingIterator` instance, making it unnecessary to declare your own. When creating a custom collection type, add the minimal requirements of the `Collection` protocol: starting and ending indices and a subscript for accessing elements. With those elements defined, the inherited `makeIterator()` method satisfies the requirements of the `Sequence` protocol. Here’s an example of a type that declares the minimal requirements for a collection. The `CollectionOfTwo` structure is a fixed-size collection that always holds two elements of a specific type. ``` struct CollectionOfTwo: Collection { let elements: (Element, Element) init(_ first: Element, _ second: Element) { self.elements = (first, second) } var startIndex: Int { return 0 } var endIndex: Int { return 2 } subscript(index: Int) -> Element { switch index { case 0: return elements.0 case 1: return elements.1 default: fatalError("Index out of bounds.") } } func index(after i: Int) -> Int { precondition(i < endIndex, "Can't advance beyond endIndex") return i + 1 } } ``` Because `CollectionOfTwo` doesn’t define its own `makeIterator()` method or `Iterator` associated type, it uses the default iterator type, `IndexingIterator`. This example shows how a `CollectionOfTwo` instance can be created holding the values of a point, and then iterated over using a `for`-`in` loop. ``` let point = CollectionOfTwo(15.0, 20.0) for element in point { print(element) } // Prints "15.0" // Prints "20.0" ``` ## Relationships ### Conforms To [`Sendable`](/documentation/Swift/Sendable) [`Sequence`](/documentation/Swift/Sequence) [`Copyable`](/documentation/Swift/Copyable) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`Escapable`](/documentation/Swift/Escapable) [`IteratorProtocol`](/documentation/Swift/IteratorProtocol) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)