Subclasses

Part of the Object Oriented Programming section of Coddy's Swift journey. Lesson 22 of 57.

Inheritance lets a class build on another one. class Dog: Animal makes Dog a subclass of the superclass Animal: a dog has every property and method of an animal, plus what Dog adds:

class Animal {
    var name: String
    init(name: String) { self.name = name }
    func describe() -> String { "\(name) is an animal" }
}

class Dog: Animal {
    func fetch() -> String { "\(name) fetches the ball" }
}

let rex = Dog(name: "Rex")
print(rex.describe())
print(rex.fetch())

Output:

Rex is an animal
Rex fetches the ball

A subclass that adds no stored properties inherits the superclass initializers, which is why Dog(name:) works without an init in Dog. Inheritance models an is-a relationship: every dog is an animal, so a Dog can be used wherever an Animal is expected:

class Animal {
    var name: String
    init(name: String) { self.name = name }
}

class Cat: Animal {}
class Dog: Animal {}

let pets: [Animal] = [Cat(name: "Tom"), Dog(name: "Rex")]
for pet in pets { print(pet.name) }

Output:

Tom
Rex

Only classes support inheritance; structs and enums cannot inherit. A class has at most one superclass, but a superclass can have many subclasses, forming a hierarchy:

class Vehicle { var wheels = 0 }
class Car: Vehicle { }        // Car is a Vehicle
class Bike: Vehicle { }       // Bike is a Vehicle
// struct Truck: Vehicle { }  // error: structs cannot inherit

Put what all subclasses share in the superclass, and only the differences in each subclass. When one type is not really a kind of the other, inheritance is the wrong tool; a property holding the other type often fits better.

class Shape {
    var name: String
    init(name: String) { self.name = name }
    func label() -> String { "a \(name)" }
}

class Square: Shape {
    var side = 1.0
    func area() -> Double { side * side }
}

let s = Square(name: "square")
s.side = 3
print(s.label())
print(s.area())

Output:

a square
9.0

Challenge

Easy

Employee stores a name and a salary and has a method raise(by:) that increases the salary by a percentage (rounded down). Declare a subclass Manager that adds a stored property reports (an array of names, empty at first) and a method addReport(_:), and a method teamSize() that returns the number of reports.

The supplied code creates a manager from the first line (name,salary), adds each following name as a report, gives a 10% raise, and prints Ada earns 5500 and leads 2.

Your code goes in Employee.swift and Manager.swift. main.swift holds the supplied input/output code and cannot be edited.

Try it yourself

// Supplied input/output code: keep it as it is
var input: [String] = []
while let line = readLine() { input.append(line) }
let head = input[0].split(separator: ",").map(String.init)
let boss = Manager(name: head[0], salary: Int(head[1])!)
for name in input.dropFirst() { boss.addReport(name) }
boss.raise(by: 10)
print("\(boss.name) earns \(boss.salary) and leads \(boss.teamSize())")