12.7: The Metaclass of Metaclass Is an Instance of Metaclass
- Page ID
- 39231
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The final question to be answered is: what is the class of Metaclass class?
The answer is simple: it is a metaclass, so it must be an instance of Metaclass, just like all the other metaclasses in the system (see Figure \(\PageIndex{1}\)).
The figure shows how all metaclasses are instances of Metaclass, including the metaclass of Metaclass itself. If you compare Figures 12.6.1 and \(\PageIndex{1}\) you will see how the metaclass hierarchy perfectly mirrors the class hierarchy, all the way up to Object class.
The following examples show us how we can query the class hierarchy to demonstrate that Figure \(\PageIndex{1}\) is correct. (Actually, you will see that we told a white lie — Object class superclass → ProtoObject class, not Class. In Squeak, we must go one superclass higher to reach Class.)
Code \(\PageIndex{1}\) (Squeak): The Class Hierarchy
TranslucentColor superclass → Color Color superclass → Object
Code \(\PageIndex{2}\) (Squeak): The Parallel Metaclass Hierarchy
TranslucentColor class superclass → Color class Color class superclass → Object class Object class superclass superclass → Class "NB: skip ProtoObject class" Class superclass → ClassDescription ClassDescription superclass → Behavior Behavior superclass → Object
Code \(\PageIndex{3}\) (Squeak): Instances of Metaclass
TranslucentColor class class → Metaclass Color class class → Metaclass Object class class → Metaclass Behavior class class → Metaclass
Code \(\PageIndex{4}\) (Squeak): Metaclass Class Is a Metaclass
Metaclass class class → Metaclass Metaclass superclass → ClassDescription