NEW: Master Swift design patterns with my latest book! >>

How to conform to the Comparable protocol

Written by Paul Hudson    @twostraws

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

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

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

Want to build macOS apps?

Hacking with macOS delivers 18 awesome projects that teach you macOS development in no time – take control of your desktop today!

Click here to visit the Hacking with Swift store >>