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

[T.J. Usiyan]

"open is invalid on declarations that are not also public (see the
Alternatives discussion for rationale)."
+

"If an open class inherits an open method from a superclass, that method
remains open. If it overrides an open method from a superclass, the
override is implicitly open if it is not final."

I understand that the intent is probably not to say that subclasses are
public by default. My point is that those two statements, without an
explicit spelling out of the implicit access level, could lead me to
believe that subclasses are implicitly public by default. It is open to
interpretation. Neither the prose nor the code examples address it.


On Sat, Jul 16, 2016 at 1:35 PM, John McCall <rjmccall at apple.com> wrote:

> On Jul 16, 2016, at 9:32 AM, Matthew Johnson via swift-evolution <
> swift-evolution at swift.org> wrote:
> 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.
>
>
> Right, there's no new restriction here.  Of course you can make a private
> or internal subclass of a public open class — otherwise, you'd have to
> publicize every subclass of (say) UIViewController.
>
> John.
>
>
>
> 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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