Boxing-based type erasure

The boxing-based type erasure pattern requires three distinct objects: 

• An abstract base class
• A private box, which will inherit the abstract base class
• A public wrapper

As for the public wrapper, we can reuse the one that we wrote above AnyAnimal<T>.

In order to make the exercise a bit more interesting, let's change the Animal protocol to introduce some complexities such as the animal's preferred food and name:

protocol Animal {
    associatedtype FoodType: Food
    var preferredFood: FoodType? { get set }
    var name: String { get }
    func eat(food: FoodType)
}

Reflecting the changes in the Animal protocol, we need to update AnyAnimal. Let's not bother too much with the initializers now, as we'll implement them as we go:

final class AnyAnimal<T>: Animal where T: Food {
    typealias FoodType = T
    var preferredFood: T?
    let name: String = ""
    func eat(food: T) {}
}

Let's now go over the two new required components that will allow us to achieve the type erasure pattern fully.