20

I am implementing a class Foo in Swift, that is supposed to instantiate objects of a given subclass of SuperBar, e.g. Bar: SuperBar. I really like generics in Swift, so I tried implementing it this way:

class Foo<T: SuperBar> {

    func instantiateObject() -> T {
        return T()
    }

}

class SuperBar {

}

class Bar: SuperBar {

}

let foo = Foo<Bar>()

let obj = foo.instantiateObject()

You can run the code snippet in an Xcode Playground and observe that obj is of type SuperBar instead of Bar, although it says Bar when I Alt-click on the constant name.

Any Ideas? :)

knl
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    This may be a shortcoming of the current version of Swift; consider [filing a bug](http://bugreport.apple.com). It also seems not to work if you pass the type as a parameter, like `init(_ t: T.Type) { ... t() }` – jtbandes Aug 17 '14 at 06:39

4 Answers4

8

Mark the class init as required and then call init :

class SuperBar {
    required init() {
    }
}


class Foo<T: SuperBar> {

    func instantiateObject() -> T {
        return T.init()
    }

}
Oliver Pearmain
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Morteza Soleimani
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5

If you add required init() { } to SuperBar, it works correctly. That requirement should always be required anyway because you are creating an instance from an variable class.

The fact that it allows you to create it using an initializer that is not required is a bug.

newacct
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2
  • used protocol to define the blueprint of init method;
  • other classes adopted this Initializable prototol have to implement init function
  • use function with generic type to create instance of a type
protocol Initializable {
    init()
}

class SuperBar : Initializable {
    required init() {
        print("init SuperBar")
    }
}

class Bar : SuperBar {
    required init() {
        print("init Bar")
    }
}

func createInstance<T>(type:T.Type) -> T where T:Initializable {
    return type.init()
}

test:

let a = createInstance(type: SuperBar.self)
print("SuperBar: \(a.self)")
let b = createInstance(type: Bar.self)
print("Bar: \(b.self)")

result:

init SuperBar
SuperBar: __lldb_expr_22.SuperBar
init Bar
init SuperBar
Bar: __lldb_expr_22.Bar
RY_ Zheng
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1

You can force cast it if you know exactly the type of the generic.

let emptyObject:T = NSObject() as! T

Obviously this defeats the purpose of the generic class, but it can be useful for some default values or base class functionality as was in my case.

This is also dangerous and will crash if type T is not NSObject

teradyl
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