[swift-evolution] Final by default for classes and methods

[Joe Groff]

I think a lot of people in this thread are conflating "final by default" or "sealed by default" with "sealed everywhere". No matter what the default is, the frameworks aren't going to suddenly rewrite themselves in Swift with final everything; Objective-C will still be what it is. Furthermore, we're only talking about language defaults; we're not taking away the ability for frameworks to make their classes publicly subclassable or dynamically overrideable. That's a policy decision for framework authors to make. The goal of "sealed by default" is to make sure the language doesn't make promises on the developer's behalf that they weren't ready to keep. ObjC's flexibility is valuable, and Apple definitely takes advantage of it internally all over place; Apple also has an army of compatibility engineers to make sure framework changes work well with existing software. Not every developer can afford that maintenance burden/flexibility tradeoff, though, so that flexibility is something you ought to opt in to. You can always safely add public subclassability and dynamic overrideability in new framework versions, but you can never take them back.

-Joe

> On Dec 7, 2015, at 11:12 AM, Javier Soto via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This was brought up in a different thread about private by default. Creating a new thread for this. Quoting Mathew from the other thread with a short summary about the motivation behind this:
> 
> "It is not uncommon to have a need for a reference type without a need for inheritance.  Superclasses should be intentionally designed to be subclasses and the author required to opt-in to subclassing and member overrides where that is required by the design."
> -- 
> Javier Soto  _______________________________________________
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> swift-evolution at swift.org
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