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What is an enum?

Swift version: 5.2

Paul Hudson    @twostraws   

“Enum” is short for “enumeration”, and it’s a way of letting you use fixed names for special values rather than relying on strings or integers.

For example, if we wanted to track how happy a user was, you could use a number scale where -1 meant unhappy, +1 meant happy, and 0 meant they were in between, but then the onus is on you to remember what those numbers mean. A better idea is to use an enum like this one:

enum Satisfaction {
    case unhappy
    case meh
    case happy
}

Those cases can now be referenced as Satisfaction.happy, so it’s clear what you mean – and internally it’s treated no different from an integer, so it has no performance impact.

We can create a Person struct using that new enum, like this:

struct Person {
    var name: String
    var satisfaction: Satisfaction
}

Because Swift knows the satisfaction property must be a value from the Satisfaction enum we can just specify the case we want to use when creating a value:

let person = Person(name: "Taylor", satisfaction: .happy)
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