Aaron Brager Aaron Brager - 5 months ago 24
Swift Question

Implementing NSCopying in Swift with subclasses

Consider two classes. The first is

, an
subclass that conforms to

class Vehicle : NSObject, NSCopying {

var wheels = 4

func copyWithZone(zone: NSZone) -> AnyObject {
let vehicle = self.dynamicType()
vehicle.wheels = self.wheels
return vehicle

The second class,
, inherits from

class Starship : Vehicle {

var photonTorpedos = 6
var antiGravity = true

override func copyWithZone(zone: NSZone) -> AnyObject {
let starship = super.copyWithZone(zone) as Starship

starship.photonTorpedos = self.photonTorpedos
starship.antiGravity = self.antiGravity
return starship

This code doesn't compile because:

Constructing an object of class type 'Vehicle' with a metatype value must use a 'required' initializer.

So I go ahead and add a required initializer:

required override init () {

And now the app compiles, and
objects respond to

Two questions:

  1. Why does constructing an object with a metatype need a
    initializer? (It appears the initializer I wrote does nothing.)

  2. Is there anything I wrote incorrectly, or should add to the initializer? Is there a case I'm not considering?


The Short Answer

You cannot use self.dynamicType() without marking init() as required because there's no guarantee subclasses of Vehicle will also implement init().

Exploring The Problem

Taking a look at The Swift Programming Language: Initialization, it's mentioned how

subclasses do not inherit their superclass initializers by default

The situations in which a subclass will inherit its superclass' initialisers are:

Assuming that you provide default values for any new properties you introduce in a subclass, the following two rules apply:

Rule 1

If your subclass doesn’t define any designated initializers, it automatically inherits all of its superclass designated initializers.

Rule 2

If your subclass provides an implementation of all of its superclass designated initializers—either by inheriting them as per rule 1, or by providing a custom implementation as part of its definition—then it automatically inherits all of the superclass convenience initialisers.

Take a look at the example:

class MySuperclass {
    let num = 0

    // MySuperclass is given `init()` as its default initialiser
    // because I gave `num` a default value.

class MySubclass : MySuperclass {
    let otherNum: Int

    init(otherNum: Int) {
        self.otherNum = otherNum

According to the information above, since MySubclass defined the property otherNum without an initial value, it doesn't automatically inherit init() from MySuperclass.

Now suppose I want to add the following method to MySuperclass:

func myMethod() {

You'll get the error you described because there is no guarantee subclasses of MySuperclass will implement init() (and in this example they don't).

To solve this problem you therefore need to mark init() as required, to ensure all subclasses of MySuperclass implement init(), and so calling self.dynamicType() is a valid thing to do. It's the same problem as in your question: Swift knows Vehicle implements init(), however it doesn't know any subclasses will implement init() and so you need to make it required.

An alternative solution, which isn't suitable in your example, is to mark Vehicle as final, meaning Vehicle can't be subclassed. Then you'll be able to use self.dynamicType(); but you might as well just use Vehicle() in that case.