Arrays of Generics in Swift

There is a way – sort of – to do what you want – kind of. There is a way, with protocols, to eliminate the type restriction and still get the result that you want, kind of, but it isn’t always pretty. Here is what I came up with as a protocol in your situation:

protocol MyProtocol {
    func getValue() -> Self 
}

extension Int: MyProtocol {
    func getValue() -> Int {
        return self
    }
}

extension Double: MyProtocol {
    func getValue() -> Double {
        return self
    }
}

Note that the value property that you originally put in your protocol declaration has been changed to a method that returns the object.

That’s not very interesting.

But now, because you’ve gotten rid of the value property in the protocol, MyProtocol can be used as a type, not just as a type constraint. Your Container class doesn’t even need to be generic anymore. You can declare it like this:

class Container {
    var values: [MyProtocol]

    init(_ values: MyProtocol...) {
        self.values = values
    }

    func myMethod() -> [MyProtocol] {
        return values
    }
}

And because Container is no longer generic, you can create an Array of Containers and iterate through them, printing the results of the myMethod() method:

var containers = [Container]()

containers.append(Container(1, 4, 6, 2, 6))
containers.append(Container(1.2, 3.5))

for container in containers {
    println(container.myMethod())
}

//  Output: [1, 4, 6, 2, 6]
//          [1.2, 3.5]

The trick is to construct a protocol that only includes generic functions and places no other requirements on a conforming type. If you can get away with doing that, then you can use the protocol as a type, and not just as a type constraint.

And as a bonus (if you want to call it that), your array of MyProtocol values can even mix different types that conform to MyProtocol. So if you give String a MyProtocol extension like this:

extension String: MyProtocol {
    func getValue() -> String {
        return self
    }
}

You can actually initialize a Container with mixed types:

let container = Container(1, 4.2, "no kidding, this works")

[Warning – I am testing this in one of the online playgrounds. I haven’t been able to test it in Xcode yet…]

Edit:

If you still want Container to be generic and only hold one type of object, you can accomplish that by making it conform to its own protocol:

protocol ContainerProtocol {
    func myMethod() -> [MyProtocol]
}

class Container<T: MyProtocol>: ContainerProtocol {
    var values: [T] = []

    init(_ values: T...) {
        self.values = values
    } 

    func myMethod() -> [MyProtocol] {
        return values.map { $0 as MyProtocol }
    }
}

Now you can still have an array of [ContainerProtocol] objects and iterate through them invoking myMethod():

let containers: [ContainerProtocol] = [Container(5, 3, 7), Container(1.2, 4,5)]

for container in containers {
    println(container.myMethod())
}

Maybe that still doesn’t work for you, but now Container is restricted to a single type, and yet you can still iterate through an array of ContainterProtocol objects.