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

[Karl]

> 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