Swift generics not preserving type

This code works as expected.

class BaseClass {

    required init() {} // <-- ADDED THIS

    func printme() -> Void {
        println("I am BaseClass")
    }
}

class DerivedClass : BaseClass {
    override func printme() -> Void {
        println("I am DerivedClass")
    }
}

class Util<T: BaseClass> {
    func doSomething() {
        var instance = T()
        instance.printme()
    }
}

var util = Util<DerivedClass>()
util.doSomething()

Code base are stolen from @GregoryHigley answer 🙂

Marking init() {} as required did the thing.
This guarantees init() is the designated initializer of ANY derived class from BaseClass.

Without it, one can make illegal subclass like:

class IllegalDerivedClass : BaseClass {
    var name:String

    init(name:String) {
        self.name = name
        super.init()
    }

    override func printme() -> Void {
        println("I am DerivedClass")
    }
}

var util = Util<IllegalDerivedClass>()
util.doSomething()

You know this doesn’t work because IllegalDerivedClass doesn’t inherit init() initializer.

I think, that is the reason of your problem.

Anyway, whose fault is that?

• Compiler should warn about ambiguousness.
• Runtime should try to initialize DerivedClass() as specified with T.
• Debugger should show instance is a instance of BaseClass as it actually is.

---

ADDED:

As of Xcode 6.1 GM 2, It seems, you need more work. (in addition to required init() {})

class Util<T: BaseClass> {
    let theClass = T.self // store type itself to variable

    func doSomething() {
        var instance = theClass() // then initialize
        instance.printme()
    }
}

I have absolutely no idea why we need this, what’s going on X(

ADDED:2014/10/18

I found this also works:

    func doSomething() {
        var instance = (T.self as T.Type)()
        instance.printme()
    }

---

ADDED: 2015/02/10

As of Xcode Version 6.3 (6D520o) / Swift 1.2

We no longer need (T.self as T.Type)() hack. Just T() works as long as T has required init() initializer.

class Util<T: BaseClass> {
    func doSomething() {
        var instance = T()
        instance.printme()
    }
}