Metatype of Metatype

Question

In Swift we are able to write following construction:

class SomeClass {} let metaMetatype: SomeClass.Type.Type = SomeClass.Type.self 

Here metaMetatype does not conform to type AnyObject (SomeClass.Type does). Construction can be even longer, as long as we wish:

let uberMetatype: SomeClass.Type.Type.Type.Type.Type.Type.Type.Type.Type.Type = SomeClass.Type.Type.Type.Type.Type.Type.Type.Type.Type.self 

Are this constructions have any sense? If SomeClass.Type.Type not an object, what is this, and why we able to declare it?

Answer 1

If SomeClass.Type.Type not an object, what is this and why we able to declare it?

I will try to dissect what you're asking.

SomeClass.Type.Type is a Metatype of a Metatype. Metatypes exist in Swift because Swift has types that are not classes. This is most similar to the Metaclass concept in Objective-C.

Lexicon.rst in the Swift Open Source Repo has a pretty good explanation:

metatype

The type of a value representing a type. Greg Parker has a good explanation of Objective-C's "metaclasses" because Swift has types that are not classes, a more general term is used.

We also sometimes refer to a value representing a type as a "metatype object" or just "metatype", usually within low-level contexts like IRGen and LLDB. This is technically incorrect (it's just a "type object"), but the malapropism happened early in the project and has stuck around.

Why are we able to declare a type of a type of a type... and so on? Because it's a feature of the language called type metadata:

type metadata

The runtime representation of a type, and everything you can do with it. Like a Class in Objective-C, but for any type.

Note that you can't do something like NSObject().class in Swift because class is a reserved keyword for the creation of a class. This is how you would get the type (or class in this case) of an NSObject in Swift:

let nsObj = NSObject() nsObj.classForCoder // NSObject.Type nsObj.classForKeyedArchiver // NSObject.Type nsObj.dynamicType // NSObject.Type 

Note that nsObj and nsObj.self are identical and represent the instance of that NSObject.

I don't see where in the Swift module or open source repo where types allow for .Type, but I'm still looking. It might have to do with the inheritance from SwiftObject, the Objective-C object all Swift classes inherit from (at least on Mac).