Trying to understand something with Swift protocols

Question

I have recently inherited a quite undocumented, spaghetti-esque and extremely buggy project written in Swift.

Am tidying up some things here and there, and came across this on every single protocol declaration:

protocol SomeProtocol: class { ...

as in literally : class - that is not a placeholder for something else.

My question is: What does the : class achieve or declare?

Personally have never put the : class afterwards, I usually reserve that for inheriting from other protocols. I removed a couple without result, but figured I should check the actual purpose (if any) before I continue.

Best regards,

Frankie

that means that your protocol must be implemented by classes only — Reinier Melian, Dec 02 '17 at 09:19
See https://stackoverflow.com/questions/30176814/whats-the-difference-between-a-protocol-extended-from-anyobject-and-a-class-onl — Martin R, Dec 02 '17 at 09:22
Thank you (and for link), I tried finding similar questions but my wording was off. Most of the classes in this projects are going to be converted to structs, so will be able to refactor accordingly now. — Frank Jones, Dec 02 '17 at 09:25

Sweeper · Accepted Answer · 2017-12-02T09:45:08.060

1

: class means that this protocol can only be conformed to by a class.

One use case of this is delegates. delegate properties are usually declared as weak to avoid retain cycles. For example:

class MyCoolClass {
    weak var delegate: MyCoolClassDelegate?
}

If MyCoolClassDelegate is declared like this:

protocol MyCoolClassDelegate { }

Then structs can conform to it as well. But struct types can't be declared weak! Therefore, this error occurs:

'weak' may only be applied to class and class-bound protocol types, not 'MyCoolClassDelegate'

This is why you need to declare it as : class.

edited Dec 02 '17 at 09:45

answered Dec 02 '17 at 09:30

Sweeper

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Nitpicking: *"... by a reference type, namely, a class."* – Reference type and class are not synonyms, a closure is a reference type as well. A class protocol can only be adopted by a class. – Martin R Dec 02 '17 at 09:44

Trying to understand something with Swift protocols

1 Answers1