[swift-evolution] [swift-evolution-announce] [Returned for revision] SE-0117: Default classes to be non-subclassable publicly

Matthew Johnson matthew at anandabits.com
Sat Jul 16 11:32:28 CDT 2016

Sent from my iPhone

> On Jul 16, 2016, at 10:59 AM, T.J. Usiyan via swift-evolution <swift-evolution at swift.org> wrote:
> Yes, sorry, my point was that this consideration isn't spelled out. 
> Another question is whether or not making a subclass of an open class public by default is what we want. I see why it would be, I just think that it is a wrinkle to default to internal otherwise but not here.

I can't think of any good reason to assume a specific class should be public just because it is a subclass of an open class.  The internal default would still be the right default in this case.

>> On Sat, Jul 16, 2016 at 10:32 AM, Karl <razielim at gmail.com> wrote:
>> > On 16 Jul 2016, at 16:10, T.J. Usiyan via swift-evolution <swift-evolution at swift.org> wrote:
>> >
>> > What happens if I want an `internal` subclass of an `open` class?
>> That should be allowable. You may want some optimised implementations, similar to how Apple used class-clusters in Obj-C. I don’t think that same pattern is exactly possible in Swift (I don’t think a class can set ‘self’ in its initialiser, or at least it couldn’t in Swift 1). But the same principle applies - you may want a public class which you don’t allow others to subclass, but you might have a static method or other function which returns an internal optimised implementation.
>> If you used a protocol rather than a concrete type in that case, theoretically others could conform to it and throw their own objects back at your code, which goes against the point of this proposal.
>> We might think about creating ‘sealed’ protocols, too.
>> Karl
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