What is a protocol associated type?

Swift version: 5.6

Paul Hudson    @twostraws    May 28th 2019

Associated types are a powerful way of making protocols generic, but they can be a bit confusing at first. In essence, they mark holes in protocols that must be filled by whatever types conform to those protocols.

Let’s start with a simple example: an ItemStoring protocol that can store items in an array. What type those items are depends on whatever conforms to the protocol, but we can still use them inside the protocol and any extensions.

Here’s the basic protocol:

protocol ItemStoring {
    associatedtype DataType

    var items: [DataType] { get set}
    mutating func add(item: DataType)
}

As you can see, it requires that conforming types provide an items array that holds an array of whatever is used to fill the DataType hole, and also a mutating method to add items of that type.

That mutating method is probably going to be the same for all conforming types, so we can write a protocol extension that provides a default implementation:

extension ItemStoring {
    mutating func add(item: DataType) {
        items.append(item)
    }
}

Finally we can create a NameDatabase struct that conforms to the ItemStoring protocol like this:

struct NameDatabase: ItemStoring {
    var items = [String]()
}

Swift is smart enough to realize that String is being used to fill the hole in the associated type, because the items array must be whatever DataType is.

That’s all the code written, so you can go ahead and use NameDatabase:

var names = NameDatabase()
names.add(item: "James")
names.add(item: "Jess")

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