Creating a data repository with generics and protocols

Now, we want to create a repository that provides us with entities so that we can apply the functional programming features included in Swift to retrieve and process data from these entities. First, we will create an EntityProtocol protocol that defines the requirements for an entity. We want any class that conforms to this protocol to have a read-only id property of the Int type to provide a unique identifier for the entity:

public protocol EntityProtocol {
    var id: Int { get }
}

The next lines create a Repository<T> generic class that specifies that T must conform to the recently created EntityProtocol protocol in the generic type constraint. The class declares a getAll method that we will override in the subclasses:

public class Repository<T: EntityProtocol> {
    public func getAll() -> [T] {
        return [T]()
    }
}

The next lines create the Entity class, which is the base class for all the entities. The class...