When defining a protocol, there are times when it is useful to define one or more associated types. An associated type gives us a placeholder name that we can use within the protocol in place of a type. The actual type to use for the associated type is not defined until the protocol is adopted. The associated type basically says: We do not know the exact type to use; therefore, when a type adopts this protocol it will define it. As an example, if we were to define a protocol for a queue, we would want the type that adopts the protocol to define the instance types that the queue contains rather than the protocol.

To define an associated type, we use the associatedtype keyword. Let's see how to use associated types within a protocol. In this example, we will illustrate the Queue protocol that will define the requirements needed to implement a queue:

protocol Queue {  
  associatedtype QueueType 
  mutating func addItem(item: QueueType)  
  mutating func getItem() -> QueueType?  
  func count() -> Int 
} 

In this protocol, we define one associated type named QueueType. We then use this associated type twice within the protocol. We use it first as the parameter type for the addItem() method. We then use it again when we define the return type of the getItem() method as an optional type.

Any type that implements the Queue protocol must specify the type to use for the QueueType placeholder, and must also ensure that only items of that type are used where the protocol requires the QueueType placeholder.

Let's look at how to implement Queue in a non-generic class called IntQueue. This class will implement the Queue protocol using the integer type:

struct IntQueue: Queue {  
  var items = [Int]() 
  mutating func addItem(item: Int) {  
    items.append(item) 
  } 
  mutating func getItem() -> Int? {  
    if items.count > 0 { 
      return items.remove(at: 0) 
    } 
    else { 
      return nil 
    } 
  } 
  func count() -> Int {  
    return items.count 
  } 
} 

As we can see in the IntQueue structure, we use the integer type for both the parameter type of the addItem() method and the return type of the getItem() method. In this example, we implemented the Queue protocol in a non-generic way. Generics in Swift allow us to define the type to use at runtime rather than compile time. We will show how to use associated types with generics in Chapter 4, Generics.

Now that we have explored protocols in some detail, let's look at how we can use them in the real world. In the next section, we will see how to use protocols to implement the delegation design pattern.