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1.4) What Is A Meta-Class? (Object-Oriented Technology)




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This article is from the Object-Oriented Technology FAQ, by Bob Hathaway rjh@geodesic.com with numerous contributions by others.

1.4) What Is A Meta-Class? (Object-Oriented Technology)

[See also section 1.6]

A Meta-Class is a class' class. If a class is an object, then that object
must have a class (in classical OO anyway). Compilers provide an easy way to
picture Meta-Classes. Classes must be implemented in some way; perhaps with
dictionaries for methods, instances, and parents and methods to perform all
the work of being a class. This can be declared in a class named "Meta-Class".
The Meta-Class can also provide services to application programs, such as
returning a set of all methods, instances or parents for review (or even
modification). [Booch 91, p 119] provides another example in Smalltalk with
timers. In Smalltalk, the situation is more complex. To make this easy, refer
to the following listing, which is based on the number of levels of distinct
instantiations:

1 Level System
All objects can be viewed as classes and all classes can be viewed as
objects (as in Self). There is no need for Meta-Classes because objects
describe themselves. Also called "single-hierarchy" systems.
There is only 1 kind of object.
2 Level System
All Objects are instances of a Class but Classes are not accessible to
programs (no Meta-Class except for in the compiler and perhaps for type-safe
linkage, as in C++).
There are 2 kinds of distinct objects: objects and classes.
3 Level System
All objects are instances of a class and all classes are instances of
Meta-Class. The Meta-Class is a class and is therefore an instance of
itself (really making this a 3 1/2 Level System). This allows classes to
be first class objects and therefore classes are available to programs.
There are 2 kinds of distinct objects (objects and classes), with a
distinguished class, the metaclass.
5 Level System
What Smalltalk provides. Like a 3 Level System, but there is an extra level
of specialized Meta-Classes for classes. There is still a Meta-Class as in
a 3 Level System, but as a class it also has a specialized Meta-Class, the
"Meta-Class class" and this results in a 5 Level System:
object
class
class class (Smalltalk's Meta-Classes)
Meta-Class
Meta-Class class

The "class class"es handle messages to classes, such as constructors and
"new", and also "class variables" (a term from Smalltalk), which are
variables shared between all instances of a class (static member data in
C++). There are 3 distinct kinds of objects (objects, classes, and
metaclasses).

 

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