[swift-dev] Swift root meta-class

[John McCall]

> On May 8, 2017, at 6:46 PM, Giacomo Leopizzi <gi.le at icloud.com> wrote:
> Our starting point was that in ObjC at the beginning of the execution, the root meta-class (most of the time NSObject) instantiates all the other meta-classes, that create the class as object for the software.

This is somewhat like how a Smalltalk VM is bootstrapped, but there isn't any real sense in which root classes control how classes are created or loaded in ObjC.

> In Swift is it the same? If so, which is the name of the root meta-class? 

The root class of a Swift class is a private implementation detail; it's not guaranteed in any way.

If you're just curious about the current implementation, you can look in stdlib/public/runtime/SwiftObject.mm, but again, nothing in there is something that you should rely on as a Swift programmer.

John.

> Thanks for the fast reply. 
> 
> Giacomo 
> 
>> On 9 May 2017, at 00:30, John McCall <rjmccall at apple.com> wrote:
>> 
>>> On May 8, 2017, at 6:21 PM, Giacomo Leopizzi via swift-dev <swift-dev at swift.org> wrote:
>>> Hello everyone! 
>>> I was discussing with a friend about metaclasses in Objective-C. In Obj-C the root meta-class was the NSObject's one.
>> 
>> ObjC does not have a single root class.  Most ObjC classes inherit from NSObject, but that is not guaranteed, and in fact there are other common root classes including NSProxy.
>> 
>>> When in a swift class you create a subclass of NSObject, the root metaclass should be the same. What happen when you delcare a class without NSObject dependence? There is an hidden root-class? Where can we read more about this topic? 
>> 
>> When ObjC interop is enabled, Swift classes that do not inherit from NSObject use a private root class.  However, that is a private implementation detail and we don't promise much about it.
>> 
>> Do you have any specific questions?
> 
>> 
>> John.
>