Squeak Tutorial

Class Class

Inheritance:

Object
  Behavior
    ClassDescription
      Class

Class Class is one of the classes that implement the code management features of a Smalltalk system. An instance of Class is used to describe

the kind of a class (normal class, variable class, variable byte class, weak class)
the memory layout of instances of a class
the instance protocol of a class.

Class Metaclass is used to describe the class protocol and some other properties of classes.

Instance Variables

superclass - Class
The immediate superclass of the class. This variable is nil for classes that do not have a superclass.
methodDict - MethodDictionary
The keys of this dictionary are selector names (that is, Symbols), the associated values are compiled methods.
The virtual machine uses this dictionary to retrieve compiled methods.
format - SmallInteger
This integer encodes the number of named instance variables and the kind of class (normal, variable, variable byte, weak)
instanceVariables - Array
The elements of this array are strings; the names of the instance variables that the class defines. Inherited instance variables are stored in the superclasses.
organization - ClassOrganizer
This object stores the categrization of the instance protocol and the class comment.
subclasses - UndefinedObject or Array
nil for a class that has no subclasses. For a class with subclasses, this instance variable contains an array with all direct subclasses.
The following assertion states that the elements of subclasses are classes:
```
subclasses isNil
  or:
    [subclasses allSatisfy:
          [:item |item isKindOf: Class]
    ].
```
The next assertion says that the collection subclasses contains only the direct subclasses of a class:
```
[:class |
  class subclasses isNil
     or:
      [class subclasses
         allSatisfy:
           [:item |
             item superclass == class
           ]
      ]
].
```
name - Symbol
The name of the class. This name is used to retrieve the class in the SystemDictionary.
classPool - Dictionary or nil
nil or a dictionary of all class variables that follow the classVariableNames: keyword of the class definition.
sharedPools - OrderedCollection or nil
nil or a collection of all pool dictionaries that follow the poolDictionaries: keyword of the class definition.
environment - SystemDictionary
category - or nil
This instance variable is currently not used.

For a better understanding of this class, you can inspect various classes and compare their data items with the definitions that a class browser shows.

First, try

OrderedCollection inspect

and compare with the definition and the instance protocol of OrderedCollection.

You see that, in spite of its demanding name, an instance of Class is essentially a symbol table: It keeps information that is gathered by the compiler and used by the compiler as well as by inspectors and the debugger. To keep things simple, an instance of Class stores only instance-related information. Instances of Metaclass are used to store class-related information.

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