NEW: Nominations are now open for the 2019 Swift Community Awards! >>

What are class and subtype existentials?

Swift version: 5.1

Paul Hudson    @twostraws   

You should already know that Swift lets you specify a concrete type as a function parameter, meaning that you can say “this parameter must be a string.” Swift also lets you specify protocols types for function parameters, meaning that you can say “this parameter must conform to Codable.”

Those two are straightforward, but sometimes you need more power. Fortunately, Swift lets us represent existentials of classes and subtypes that conform to protocols, which is dramatic way of saying that you can now refer to types in a more complex way.

As an example, consider this class hierarchy:

class Animal { }

class Dog: Animal, Codable { }
class Cat: Animal, Codable { }

There’s a main Animal class, and two subclasses called Dog and Cat. Both of the subclasses also conform to Codable. Swift lets us combine both Animal (the class) and Codable (the protocol) to write functions that require a parameter to be both at the same time:

func encodeAnimal(with animal: Animal & Codable) {


let animal = Dog()
encodeAnimal(with: animal)

Without the Codable part that function would accept instances of the main Animal class even though it wouldn’t work inside, and without the Animal part that function would accept anything that’s Codable – only the combination of the two makes sense.

SAVE 20% ON iOS CONF SG The largest iOS conference in Southeast Asia is back in Singapore for the 5th time in January 2020, now with two days of workshops plus two days of talks on SwiftUI, Combine, GraphQL, and more! Save a massive 20% on your tickets by clicking on this link.

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 Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift 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: 3.7/5