Hashable

The Hashable protocol is a fundamental protocol in Swift that allows types to be hashed into integer values, facilitating their use as keys in hash-based collections like dictionaries and sets.

In Swift, the following types conforms to the Hashable protocol by default: Int, UInt, Float, Double, Bool, String, Character, Tuples, Optionals, Enums.

Syntax

protocol Hashable {
    func hash(into myhasher: inout Hasher)
}

Conforming to the protocol automatically generates the hash() method. This method accepts an inout Hasher parameter (myhasher), which is responsible for combining the hash values of the properties of the type.

Example Using a struct

In this example, the Person struct conforms to the Hashable protocol by implementing hash(). The hash value is calculated by combining the name and age properties using the myhasher.combine() method. This allows instances of the Person struct to be stored in a set (personSet) and efficiently looked up with their hashed values.

struct Person: Hashable {
    var id: Int
    var name: String
    var age: Int

    func hash(into myhasher: inout Hasher) {
        // Using id to uniquely identify each person.
        myhasher.combine(id)
    }
}

var personSet: Set<Person> = []
let person1 = Person(id: 1, name: "Alice", age: 30)
let person2 = Person(id: 2, name: "Bob", age: 25)

personSet.insert(person1)
personSet.insert(person2)

print("Is Alice in the set? \(personSet.contains(person1) ? "Yes" : "No")")

This example results in the following output:

Is Alice in the set? Yes

Example Using an enum

In this example, an enum named Color contains three cases: red, green, and blue. Since enum cases are unique by definition, they are automatically hashable, and can be used in data structures like sets or dictionaries without any additional implementation of the Hashable protocol.

enum Color: Hashable {
    case red
    case green
    case blue
}

let colorSet: Set<Color> = [.red, .green, .blue]

for color in colorSet {
    print(color)
}

This example results in the following output:

red
green
blue