[swift-dev] Swift root meta-class
rjmccall at apple.com
Mon May 8 18:00:51 CDT 2017
> 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.
> Thanks for the fast reply.
>> 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?
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