---
title: IndexingIterator
framework: swift
role: symbol
role_heading: Structure
path: swift/indexingiterator
---

# IndexingIterator

A type that iterates over a collection using its indices.

## Declaration

```swift
@frozen struct IndexingIterator<Elements> where Elements : Collection
```

## Overview

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<Element>: 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"

## Topics

### Type Aliases

- [IndexingIterator.SubSequence](swift/indexingiterator/subsequence.md)

### Default Implementations

- [IteratorProtocol Implementations](swift/indexingiterator/iteratorprotocol-implementations.md)
- [Sequence Implementations](swift/indexingiterator/sequence-implementations.md)

## Relationships

### Conforms To

- [Copyable](swift/copyable.md)
- [Escapable](swift/escapable.md)
- [IteratorProtocol](swift/iteratorprotocol.md)
- [Sendable](swift/sendable.md)
- [SendableMetatype](swift/sendablemetatype.md)
- [Sequence](swift/sequence.md)

## See Also

### Indices and Iterators

- [IteratorSequence](swift/iteratorsequence.md)
- [EnumeratedIterator](swift/enumeratediterator.md)
- [SetIterator](swift/setiterator.md)
- [StrideThroughIterator](swift/stridethroughiterator.md)
- [StrideToIterator](swift/stridetoiterator.md)