A demonstration of a use case where inheritance is preferred to composition.

Composition.swift

protocol Mammal {
    var age: Int { get set }
    var offspring: Int { get set }

    func grow()
    func reproduce() -> Self
}

protocol Person: Mammal {
    var name: String { get }
    // ...

    init(named name: String)

    func growOld()
    func speak()
}

extension Person {
    func speak() {
        print("My name is \(name) and I'm \(age) year(s) old")
    }

    func growOld() {
        for _ in 0..<50 { self.grow() }
    }
}

class Employee: Person {
    enum Job {
        case programmer
        case businessOwner
        case stayAtHomeParent
    }

    var offspring: Int = 0
    var age: Int = 1
    var name: String
    var occupation: Job? = nil

    required init(named name: String) {
        self.name = name
    }

    convenience init(named name: String, job: Job) {
        self.init(named: name)
        self.occupation = job
    }

    func grow() {
        self.age += 1
        if self.age.isMultiple(of: 10) {
            self.speak()
        }
    }

    func reproduce() -> Self {
        self.offspring += 1
        return type(of: self).init(named: self.name + " Jr.")
    }
}

let e = Employee(named: "Steve", job: .businessOwner)
e.growOld()

Inheritance.swift

class Mammal {
    var age: Int = 1
    var offspring: Int = 0

    func grow() {
        self.age += 1
    }

    func reproduce() -> Self {
        fatalError("Subclasses must override")
    }
}

class Person: Mammal {
    let name: String
    // ...

    required init(named name: String) {
        self.name = name
    }

    func speak() {
        print("My name is \(name) and I'm \(age) year(s) old")
    }

    override func grow() {
        super.grow()
        if self.age.isMultiple(of: 10) {
            self.speak()
        }
    }

    func growOld() {
        for _ in 0..<50 { self.grow() }
    }

    override func reproduce() -> Self {
        self.offspring += 1
        return type(of: self).init(named: self.name + " Jr.")
    }
}

class Employee: Person {
    enum Job {
        case programmer
        case businessOwner
        case stayAtHomeParent
    }

    var occupation: Job? = nil

    convenience init(named name: String, job: Job) {
        self.init(named: name)
        self.occupation = job
    }
}

let e = Employee(named: "Steve", job: .businessOwner)
e.growOld()

// Prints:
//    My name is Steve and I'm 10 year(s) old
//    My name is Steve and I'm 20 year(s) old
//    My name is Steve and I'm 30 year(s) old
//    My name is Steve and I'm 40 year(s) old
//    My name is Steve and I'm 50 year(s) old