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 itself is adopted. When creating generic functions and types, we used a very similar syntax to what we have seen throughout this chapter. Defining associated types for a protocol, however, is a little different. We specify an associated type using the associatedtype keyword.

Let's see how to use associated types when we define a protocol. For this example, we will create a simple protocol named MyProtocol:

protocol MyProtocol { 
    associatedtype E 
    var items: [E] {get set} 
    mutating func add(item: E) 
}

In this protocol, we declare an associated type named E. We then use that associated type as the type for the items array, as well as the parameter type for the add(item:) method.

We...