[swift-users] Cannot subclass a class with objc_subclassing_restricted attribute

Rod Brown rodney.brown6 at icloud.com
Mon Apr 17 09:03:12 CDT 2017

> On 18 Apr 2017, at 12:01 am, Rod Brown via swift-users <swift-users at swift.org> wrote:
> Hi Rick,
> Unfortunately, that is the case. You cannot subclass a Swift class (even if it is a subclass of NSObject and available to the Objective-C runtime) because of deliberate limitations baked into Obj-C to block subclassing Swift classes in Obj-C code.

Sorry, correction here: You cannot subclass a Swift class *in Objective C*.

> I believe the reason for this limitation is that Swift includes features that cannot be utilised in Obj-C and therefore subclasses would be restricted and would get undefined behaviour when implementing methods that cannot cross into Obj-C.
> If Apple were to allow Obj-C -> Swift -> Obj-C subclassing, then it would be on a limited basis. Some methods wouldn’t do the same thing in Swift as they would in Obj-C selectors, and you could theoretically declare conflicting methods in your subclass that would have different actions depending on whether you were addressing the class in Swift and Obj-C. Additionally, the Swift Compiler couldn’t see beyond the Swift barrier and therefore may make optimisations that would break your Obj-C subclasses.
> While I understand the frustration behind this, both in personal projects and philosophically, I think that this is unfortunately the better of the two options.
> - Rod

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