CS 6140: Smalltalk and The Truth About Metaclasses

Creating new Smalltalk classes
Smalltalk metaclasses

Object subclass: #Account
                  instanceVariableNames: 'balance'
                  classVariableNames: ''
                  poolDictionaries: ''
                  category: nil !
!Account class methodsFor: 'instance creation'!
  new 
      | r |
      r := super new.
      r init.
      ^r
! !
!Account methodsFor: 'instance initialization'!
  init 
      balance := 0
! !

• 1. Account inherits from Object, and calls Object's new.
• 2. This returns an instance of type Account, with storage for balance.
• 3. But Object doesn't really have a new.
• 4. The new method is implemented inside of a Behavior class, a subclass of
  Object, a subclass of the abstract class Class, a subclass of abstract
  ClassDescription, a subclass of concrete Behavior, a subclass of Object, ...
• 5. Behavior instances are light-weight classes, they do not have separate
  metaclasses, instead share Behavior as their metaclass.
• 6. The Class class is the abstract superclass of all metaclasses.
• 7. Object is the superclass of all instances as well as all metaclasses.
• 8. Metaclass is an instance of an instance of itself.
• 9. Metaclass is a subclass of ClassDescription.
• 10. Class methods are a lie, they're simply instance methods that are understood
  by instances of metaclasses.
• 11. Parallel hierarchy between classes and metaclasses.
• 12. Classes are objects.
• 13. When you create a class, Smalltalk creates a metaclass.
• 14. A class describes how methods for its instances work.
• 15. A metaclass describes how class methods for that same class work.
• 16. Metaclass implements creation and initialization of its instances, which are
  classes (subclasses of Class).
• 17. Implements a clean, elegant and (with some contemplation) understandable
  facility for self-definition of classes. Allows classes to talk about
  themselves, posing the foundation for the creation of browsers.
• 18. Java has reflection.