Protocols, inheritance, default implementations, and optional methods

Protocols.swift

import Foundation

protocol P {
    var required: String { get }
}

extension P {
    var required: String { "Default implementation of `required`" }
    var nonRequired: String { "Default implementation of `nonRequired`" }
}

class C: P {
    var required: String { "Superclass implementation of `required`" }
    var nonRequired: String { "Superclass implementation of `nonRequired`" }
}

class D: C {
    override var required: String { "Subclass implementation of `required`" }
    override var nonRequired: String { "Subclass implementation of `nonRequired`" }
}

for p in [C(), D()] as [P] {
    print("\(p) as P: \(p.required)")
    print("\(p) as P: \(p.nonRequired)")
}

for c in [C(), D()] as [C] {
    print("\(c) as C: \(c.required)")
    print("\(c) as C: \(c.nonRequired)")
}