What's the difference between a protocol extended from AnyObject and a class-only protocol?

Question

Both this declaration

protocol SomeProtocol : AnyObject {
}

and this declaration

protocol SomeProtocol : class {
}
<         


        

Answer 1

If you open the help (alt-click) in Xcode 9 for class in a line such as protocol P: class {}, you will get typealias AnyObject.

Thus, the code compiled (in Swift 4) will be the same whether you constrain the protocol to class or AnyObject.

That said, there is also the question of style and future options — a future Swift version might want to treat class and AnyObject differently in some subtle way, even if that is not the case right now.