Mastering Swift 2

Associated types

An associated type declares a placeholder name that can be used instead of a type within a protocol. The actual type to be used is not specified until the protocol is adopted. While creating generic functions and types, we used a very similar syntax. Defining associated types for a protocol, however, is very different. We specify an associated type using the typealias keyword.

Let's see how to use associated types when we define a protocol. In this example, we will define the QueueProtocol protocol that will define the capabilities that need to be implemented by the queue that implements it:

protocol QueueProtocol { typealias QueueType mutating func addItem(item: QueueType) mutating func getItem() -> QueueType? func count() -> Int ...