The abstract base class

The abstract base class is very similar to the AnyAnimal public wrapper due to the following:

• It conforms to our base protocol.
• It's an abstract class so provides a top implementation for the private box.
• It defines a generic parameter, T or F, bound to the associated type.

The convention begs to call this class _Any#MY_PROTOCOL#Base<>. As the design pattern requires the implementation of this class as an abstract class, we implement it with fatalError():

private class _AnyAnimalBase<F>: Animal where F: Food {
    var preferredFood: F? {
        get { fatalError() }
        set { fatalError() }
    }
    var name: String { fatalError() }
    func eat(food: F) { fatalError() }
}