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

Matthew Johnson matthew at anandabits.com
Mon Dec 21 11:12:46 CST 2015

> On Dec 21, 2015, at 10:56 AM, Tino Heth <2th at gmx.de> wrote:
> oh, cool - discussion ;-)

Yes, discussion is good!  I really want to see this seriously considered from all angles and the best decision made as an outcome.

>> However, any Swift application is necessarily going to have a lot of classes as the Cocoa frameworks require apps to subclass framework classes in many instances.  Even when we are not required to subclass a framework class they often require our types to be a class, for example when we are providing a delegate to a Cocoa class.  These requirements of the frameworks result in a lot of classes in Swift applications that are not ever intended to be subclassed.
> So you'd vote for making non-final the default again as soon as the "Cocoa-legacy" is gone?

Probably not.  I think it is less clear and less safe.  I would only consider a change in my opinion if superclasses became extremely common in application level code.  I don’t expect to see that happen.  

We are learning to use techniques such as protocols and composition which are more robust than inheritance.  I believe the use cases where inheritance is the best solution will continue to decline as our languages become more powerful and we learn how to best use the new features.

>> You ask why I don’t think `inheritable` should be the default.  I believe I have addressed that by making a case that it is less clear and less safe as a default.
> Sorry if he overlooked something, but I thought the arguments were against the current default…

The current default is to allow inheritance by default.  This is the same as `inheritable`, we just don’t have a keyword for it as it isn’t necessary.  So yes, the argument is against the current default, which is the same as `inheritable` by default.

> What exactly compromises safety when a subclass is allowed to monitor usage of its inherited methods?
> True, when a method is overwritten, the base class might lose something it relies on; but when this can't happen, can you point out an example where this behavior causes danger (damn, I actually know one, but you have to tell first ;-).

This discussion is around making classes final by default.  I also believe methods should be final by default but that is a separate discussion.  Let’s stay focused on the discussion about classes in this thread.

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