Generic class as Parameter

Question

I was trying to use generics to get away with a lot of casting. I will provide a simple example:

import UIKit

class List<T> {
    let items: [T]

    init(items: [T]) {
        self.items = items
    }
}

class ListsViewController: UIViewController {
     var lists: [List<AnyObject>] = []
}

let viewController = ListsViewController()
let lists = [List(items: ["Fruits"])]
viewController.lists = lists
                       ^ error "String" is not identical to "AnyObject"

I've read that i could do

class ListsViewController<T>: UIViewController

but than i would have the problem further down the chain where i have to save ListsViewController instances...so that doesn't help.

What i don't get here why does the example work with the default Swift Arrays:

import UIKit

class ListsViewController: UIViewController {
     var lists: [AnyObject] = []
}

let viewController = ListsViewController()
let lists = ["Fruits", "Meat"]
viewController.lists = lists

No error although lists is of type [String] not [AnyObject]. Can i get the same behaviour for my generic class as well?

Thanks so much for your help in advance!

Answer 1

Update 8/22/2016: As @DeFrenZ pointed out in the comments, my example relies on collections being reference types. However, in Swift, collections are value types. So if you could compile my example, I don't think there would be any issues. The variables lists and lists2 are separate collections. Mutating one will not affect the other.

Original post continues below:

In addition to my comments above and the link I shared http://nomothetis.svbtle.com/type-variance-in-swift (by Alexandros Salazar) it's worth pointing out why covariance of collections (with no constraints) is generally not type safe.

Here's an example:

var lists: [List<AnyObject>] = []
var lists2: [List<String>] = List(items: ["Fruits"])
lists = lists2            // lists is now an alias for lists2 and is a reference to a list of strings
lists.append(MyObject())  // Now I just put a MyObject into a list of strings
var str:String = lists2[1] // assuming the previous lines compiled and ran ok, this would be a problem because lists2[1] is a MyObject. 

(I'm writing this syntax from memory and don't recall Swift's actual semantics when assigning collections to variables, but hopefully this points out the issue.)