How to conform to the Comparable protocol

Swift version: 5.1

Paul Hudson    @twostraws    May 28th 2019

The Comparable protocol allows use to use the <, >, <=, and >= operators with conforming data types, which in turn means that Swift knows how to sort arrays of those types. Most of Swift’s built-in types support Comparable out of the box, but if you want your own type to conform to them then you need to implement < – from that Swift can provide default implementations of the other three operators.

The < function needs to accept two instances of your type, one of the left-hand side and one on the right, and return true if the left-hand object should be ordered before the right-hand object.

As an example, consider this simple Person struct:

struct Person {
    var name: String
}

That has one property called name, and we’re going to make Person conform to the Comparable protocol based on that property. This means writing a static method called < that takes two instances of Person and internally compares the name properties of each of them:

struct Person: Comparable {
    var name: String

    static func <(lhs: Person, rhs: Person) -> Bool {
        return lhs.name < rhs.name
    }
}

With that in place you can now use < to compare two instances of Person like this:

let taylor = Person(name: "Taylor Swift")
let justin = Person(name: "Justin Bieber")
print(taylor < justin)

Available from iOS 8.0

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