Kodebits Day 13: Associated Types

What does this print?

protocol Box {
  associatedtype Item
  var item: Item { get }
}
struct IntBox: Box {
  let item: Int
}
let b = IntBox(item: 9)
print(b.item * 2)

Try it in the online Swift Playground →

[spoiler title="Solution"]

Answer:

18

Explanation:

The Box protocol uses an associated type Item as a placeholder. IntBox conforms by setting Item = Int, making item a concrete Int property.

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Further Reading

• Generics, Protocols & Functional Programming in Swift
• Use Protocol Associated Types in Swift