NEW: My new book Pro SwiftUI is out now – level up your SwiftUI skills today! >>

What are indirect enums?

Swift version: 5.6

Paul Hudson    @twostraws   

Indirect enums are enums that need to reference themselves somehow, and are called “indirect” because they modify the way Swift stores them so they can grow to any size. Without the indirection, any enum that referenced itself could potentially become infinitely sized: it could contain itself again and again, which wouldn’t be possible.

As an example, here’s an indirect enum that defines a node in a linked list:

indirect enum LinkedListItem<T> {
    case endPoint(value: T)
    case linkNode(value: T, next: LinkedListItem)
}

Because that references itself – because one of the associated values is itself a linked list item – we need to mark the enum as being indirect.

Apart from the special way they store their values internally, indirect enums work identically to regular enums. So, we could make a linked list using that enum and loop over it, like this:

let third = LinkedListItem.endPoint(value: "Third")
let second = LinkedListItem.linkNode(value: "Second", next: third)
let first = LinkedListItem.linkNode(value: "First", next: second)

var currentNode = first

listLoop: while true {
    switch currentNode {
    case .endPoint(let value):
        print(value)
        break listLoop
    case .linkNode(let value, let next):
        print(value)
        currentNode = next
    }
}
Hacking with Swift is sponsored by Play

SPONSORED Play is the first native iOS design tool created for designers and engineers. You can install Play for iOS and iPad today and sign up to check out the Beta of our macOS app with SwiftUI code export. We're also hiring engineers!

Click to learn more about Play!

Sponsor Hacking with Swift and reach the world's largest Swift community!

Available from iOS 8.0

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

BUY OUR BOOKS
Buy Pro Swift Buy Pro SwiftUI Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.5/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.