[swift-evolution] [Proposal] Sealed classes by default

[Karl Wagner]

Yeah I know - that's why I said it would only work if we are okay with saying non-open != final, as a long-term decision. So the compiler won't devirtualize those calls.
  

  
As I understand it, that is how it works today - calls to non-open, non-final classes are still dynamically dispatched. There was a suggestion to make non-open == final, but that was expressly rejected.
  

  
Karl
  
  
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>   
> On Aug 15, 2016 at 9:50 am,  <Jean-Daniel Dupas (mailto:mailing at xenonium.com)>  wrote:
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>   
>   
>   
> >  Le 14 août 2016 à 20:43, Karl via swift-evolution  <swift-evolution at swift.org (mailto:swift-evolution at swift.org)>  a écrit :  
> >   
> >   
> >>  On 14 Aug 2016, at 11:17, John Holdsworth via swift-evolution  <swift-evolution at swift.org (mailto:swift-evolution at swift.org)>  wrote:  
> >>   
> >>  Hi folks,  
> >>   
> >>  I see from building the latest Swift-3.0 branch that I’m a little behind the times and this proposal has been accepted :(  
> >>   
> >>   https://github.com/apple/swift-evolution/blob/master/proposals/0117-non-public-subclassable-by-default.md   
> >>   
> >>  Is there no way we can revisit this decision? 60 emails on a mail group not read by the whole community  
> >>  doesn’t quite seem enough validation for such a significant change to me. Of those that expressed an  
> >>  opinion on the thread many were against sealing classes outside the module.  
> >>   
> >>   https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160627/022364.html   
> >>   
> >>  I personally feel it's a huge mistake for the following reasons:  
> >>   
> >>  1) it “breaks open source” reducing the reusability of third party software by default.  
> >>   
> >>  2) Contrary to arguments about the likely performance benefits of de-virtualisation of method dispatch it is  
> >>  likely to make Swift programs launch more slowly due to the dynamic linker having to slide large numbers  
> >>  of function pointers for most method calls (itself an expensive operation) whether a call is even made.  
> >>   
> >>  On the first point it's an established technique in OOP to be able to subclass and override to adapt a class  
> >>  for a role perhaps it’s author never considered. Call this “monkey-patching” if you like but it is a part of being  
> >>  able to use open source libraries without having to make a fork as would be the case if sealed by default.  
> >>  Are we expecting developers to remember to leave their classes (and methods?) "open”? If they do feel  
> >>  that others shouldn’t subclass or override there was always “final". Distinctions about whether this is possible  
> >>  inside or outside a module seem a mute point to me.  
> >>   
> >>  The second point is more of an engineering one. “Virtual” dispatch on Swift is not a significant overhead  
> >>  compared to fully dynamic dispatch on Objective-C which even then was seldom a problem. There is a  
> >>  cost however to de-virtualisation and resolving to a direct call to a method address in that the dynamic  
> >>  linker has to resolve and slide the pointer on load. This type of concern is a bad argument to use in  
> >>  designing a language anyway even if it is called Swift.  
> >>   
> >>  Well, I know the boat has left on this one and I’ll likely have to live with it but this is the proposal I  
> >>  most disagree with on evolution this far. Programmers have morals and just because you shouldn’t  
> >>  do something doesn’t mean the language needs to turn that into can’t by default when the need arises.  
> >>  If this change does go ahead please try to get it into the next Xcode beta as it is a very incompatible one.  
> >>   
> >>  Cheers,  
> >>   
> >>  John  
> >>   
> >>   
> >>  _______________________________________________  
> >>  swift-evolution mailing list  
> >>   swift-evolution at swift.org (mailto:swift-evolution at swift.org)   
> >>   https://lists.swift.org/mailman/listinfo/swift-evolution   
> >   
> >  It we are happy with non-open != final, I would be in favour of some kind of monkey-patching import attribute, similar to @testable, which allows you to subclass non-open classes at your own risk.  
> >   
>
> The non subclassable across module boundary is not just a compiler enforced limitation. Once you compile a module with classes that are ‘final’ in the module, the compiler may devirtualize all call sites in the module, or even inlining some calls, making subclassing unpredictable and pointless.  
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