Patterns for Working With Associated Types

A Box Type

A Box Type

The next step is the most difficult to understand part of type erasure, which means that after this, it'll be easy. We will introduce another private type. This will be the actual box that houses our original type (the XBox360 or the PowerMac G5). Let's start by having a look at the code:

private class AnyComputerBox<ConcreteComputer: Computer>: 
        AnyComputerBase<ConcreteComputer.ProcessorType> 
{
    private let internalComputer: ConcreteComputer
    override var processor: ConcreteComputer.ProcessorType {
        return internalComputer.processor
    }
    override var processorCount: Int {
        return internalComputer.processorCount
    }
    init(_ computer: ConcreteComputer) {
        internalComputer = computer
    }
}

The most important concept here can be found in the very first line:

private class AnyComputerBox<ConcreteComputer: Computer>: 
        AnyComputerBase<ConcreteComputer.ProcessorType>

Here, we define a new type AnyComputerBox which is generic over any computer (ConcreteComputer). This new type, then, is a subclass of our earlier abstract class AnyComputerBase. Remember that AnyComputerBase made the original ProcessorType of the Computer protocol generic by adding it as a generic parameter CPU. Now, our new box has a different generic type (Computer) and provides only its associated type ProcessorType to the abstract superclass. In a simpler explanation, this is what happens (in a mock language):

  1. Computer<CPU>
  2. AnyComputerBase<Processor: CPU>: Computer<CPU> where Computer.CPU = Processor
  3. AnyComputerBox<ConcreteComputer: Computer>: AnyComputerBase<ConcreteComputer.ProcessorType>

So the box (AnyComputerBox) subclasses the abstract class and forwards in the Processor type via its own generic Computer type which also has a ProcessorType.

Why do we do this? It makes the box generic over any computer so that any computer can be boxed into it.

The rest of the class is simple. There's an internal computer internalComputer which is the actual type conforming to the Computer protocol. We're also overriding the two classes that are required by the protocol and forwarding the implementations of the internalComputer. Finally we have an initializer with a new ConcreteComputer (i.e. the Computer protocol).