What exactly does `: class` do in a protocol declaration?

This SO post explains pretty well how to solve the issue of creating a `delegate` that is `weak`. Essentially, there are 2 approaches: Using the `@objc` keyword: ``` @objc protocol MyClassDelegate { } class MyClass { weak var delegate: MyClassDelegate? } ``` Using the `:class` keyword: ``` protocol MyClassDelegate: class { } class MyClass { weak var delegate: MyClassDelegate? } ``` I am trying to do some research to understand what exactly the differences between the two approaches are. The docs are pretty clear about using `@objc`: To be accessible and usable in Objective-C, a Swift class must be a descendant of an Objective-C class or it must be marked `@objc`. However, nowhere I found some information about what `:class` actually does. Considering the whole notion in detail, it actually doesn't make a lot of sense to. My understanding is that `class` is a keyword in Swift to declare classes. So, here it seems like we are using the keyword `class` itself as a protocol (as we're appending it after the `:` after the protocol declaration). So, why does this even work syntactically and what exactly does it do?