Why can't I assign an Array of Dog(s) to an array of Animal(s) in Swift?

Question

Ok, let me better explain what I mean.

Array and Classes

Let's say I have these 2 classes:

class Animal { }
class Dog: Animal { }

I CAN now write the following code and the compiler is totally happy.

var animals = [Animal]()
let dogs = [Dog]()
animals = dogs

This is possible because in Swift Array is a Struct, so it's a value type and therefore the dogs array is copied (at least at conceptual level untill a change is made to animals or dogs).

So there is no risk that writing this animals.append(Animal()) I put an Animal inside the dogs Array. Infact animals and dogs are 2 distinct values. This would not be possible if Array were an Object like in Java.

Let's try with protocols!

Let's now define Animal and Dog like protocols

protocol Animal { }
protocol Dog: Animal { }

Now this code makes the compiler pretty unhappy

var animals = [Animal]()
let dogs = [Dog]()
animals = dogs
^ error: cannot assign value of type '[Dog]' to type '[Animal]'

Can you explain me why this is NOT possible?

I am using Swift 2.2.

I had same issue a while back. This answer helped me http://stackoverflow.com/questions/32237753/conform-to-protocol-mkannotation-via-custom-protocol — DogCoffee, Apr 15 '16 at 11:39
We've both discussed this a lot already recently, but check out this Q&A: http://stackoverflow.com/questions/33090626/swift-upcasting-array-of-protocol-to-array-of-super-protocol-causes-error — Luka Jacobowitz, Apr 15 '16 at 11:40
you to convert it dog into animal before assign it as a animal animals = dogs as! Animal — Tyson Vignesh, Apr 15 '16 at 11:43
@TysonVignesh: your code does not compile. You cannot force cast `dogs` to `Animal`. — Luca Angeletti, Apr 15 '16 at 11:45
@LukaJacobowitz: your link perfectly explains the reason why this is not possibile. Thanks. — Luca Angeletti, Apr 15 '16 at 11:50

Why can't I assign an Array of Dog(s) to an array of Animal(s) in Swift?

Array and Classes

Let's try with protocols!

0 Answers0