Assigning `nil` value to a generically typed variable in Swift

Question

I think var value: T = nil is causing error below because XCode can't convert nil value to the generic type T.

class Node<T> {
    var value: T = nil
    var next: Node

    init(value: T) {
        self.value = value
        self.next = Node()
    }

    init() {
        self.next = Node()
    }
}

The error message reads

Could not find an overload for '_coversion' that accepts the supplied arguments

Is there a way to assign nil value to a variable in Swift?

MagerValp · Accepted Answer · 2014-06-05 19:07:55Z

23

You need to declare the variable as optional:

var value: T? = nil

Unfortunately this currently seems to trigger an unimplemented compiler feature:

error: unimplemented IR generation feature non-fixed class layout

You can work around it by declaring T with a type constraint of NSObject:

class Node<T:NSObject> {
    var value: T? = nil
    var next: Node

    init(value: T) {
        self.value = value
        self.next = Node()
    }

    init() {
        self.next = Node()
    }
}

answered Jun 5, 2014 at 19:07

MagerValp

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2 Comments

Sulthan Over a year ago

Oh, ANOTHER limitation?

Kokodoko Over a year ago

In the playground, this does not generate an error. Perhaps it's updated: var value: T? = nil

nanothread · Accepted Answer · 2014-06-05 19:07:27Z

0

Try using

var value: T? = nil

The question mark makes the variable an optional, meaning that it can either have a value or be nil.

answered Jun 5, 2014 at 19:07

nanothread

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