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

[Joseph Lord]

> On Feb 26, 2016, at 10:21 PM, David Scrève via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 	Actually, protocols and extensions cannot handle methods that requires attributes and data storage.

You don't implement the required code in the protocol/extension you provide a concrete instance conforming to the protocol. 

> 	So, by design, protocols and extensions can only be used to implement stateless interface where abstract classes are true classes with a state supported with attributes and properties.

It is safer to share state only through the specific interface of the protocol.

> 	Abstract class can also enforce methods to be implemented and avoid misuse of default method implementation.

Require the object conforming to the protocol in the initializer. 

> 	For example, NSOperation is a good candidat to abstract class because NSOperation itself is useless as a standalone class and should not be used as-it.  -(void)main should be made abstract because I don’t think NSOperation does not have any internal property and has default behavior.

As mentioned it could just be initialised with a closure (or a simple delegate). With the delegate approach you could still subclass and just return self as the delegate if you really wanted to of some reason. 

Joseph