[swift-evolution] [Review] SE-0026 Abstract classes and methods

[Evan Maloney]

> On Mar 3, 2016, at 4:21 PM, Matthew Judge <matthew.judge at gmail.com> wrote:
> 
> Not necessarily commenting for or against abstract classes, but the following logic confuses me:
> 
> "ONLY an abstract class, ON ITS OWN, can force concrete subclassers to provide an implementation for a given thing.
> 
> Protocols can't do this, because if you forget to declare conformance with the protocol, the compiler can't enforce anything. This pushes what could be caught at compile-time to a runtime (potentially crashing) problem."
> 
> What is the difference between forgetting to declare conformance to a protocol and forgetting to subclass an abstract class? An abstract class, ON ITS OWN, cannot force a class to do anything unless that class declares itself a subclass of the abstract class.

It's the difference between forgetting to do two things and forgetting to do just one.

Let's say I have a framework that requires me to supply a subclass of Foo to do something useful.

With abstract classes, I just need to subclass Foo, and the compiler tells me what implementations I need to provide.

With the protocol-based "solution" being proposed, I need to subclass Foo *and* I need to remember to declare conformance to FooProtocol. Then and only then can the compiler enforce anything.

2 != 1