Metaclasses play an important
role when creating new instances of a class - this is done by sending the
message new.
Problem:
If there is only one metaclass
in the system, then all classes are forced to reply the the same set of
class messages - i.e. they have the same "class protocol".
Solution:
Many metaclasses were
introduced (initially there was only one) - actually for each class
in the system there is a corresponding metaclass.
Each metaclass has only
one instance - the class of which it is the metaclass.
As "normal" classes, metaclasses
also inherit from their superclasses.
There is a parallel metaclass
hirerchy which corresponds (almost) 1:1 to the hierarchy of "normal" classes.
Another imortant task
of metaclasses is initializing the values of class variables. This
is typically done with the message initialize.
Metaclass Class
is the abstract superclass of all intances of metaclass. Class
describes the general layot/structure of classes. Each metaclass
adds specific behaviour for its single instance, i.e. for its corresponding
"normal" class - e.g.: creation of new instances.
Examples of class methods:
Time now.
"Answers an instnace
of Time representing the time right now."
Date today.
"Answer an instance
of me representing the day and year right now."
Point x:50 y:50.
"Answer an instance
of me with coordinates (x,y)."
SomeStream on: aSequenceableCollection.
"Creates an instance of the
receiver.
Elements to be processd must
be part of a sequenceable collection."
ReadStream on: 'Time is #'.
WriteStream on: String new.
2 Hierarchy of Smalltalk
Classes
Hierarchy of Smalltalk
classes since Smalltalk-80 (simplified, intentionally incorrect):
Hierarchy of Smalltalk
classes before Smalltalk-80
Hierarchy of Smalltalk
classes since Smalltalk-80 (incomplete, almost correct)
Hierarchy of Smalltalk
classes since Smalltalk-80 - another view
3 Instances, Classes &
Metaclasses
3.1 Instances
Instances are logically
made up of:
Data
Class Pointer
Data points to other objects
Data is user defined
Class Pointer points to
a class object
Holds code for instance
One class object for all
instances of that class
3.2 Class
Also an instance
Data part holds code for
instances (+ other stuff)
Has pointer to its
class
Class of a class is called
a metaclass
A metaclass holds code
for class
One metaclass for each
class
Class differs from other
instances (ST/ V)
Hash for a class is set
Instance variable (format)
known behind wall
Responds to new (implemented
by primitive)
3.3 Metaclasses
Also an instance
Data part:
Holds code for its instance
(a class)
+ other stuff
Has class pointer to its
class
Class of a metaclass is
Metaclass
Metaclass
holds code for all metaclasses
Metaclass
is just an ordinary class
Has a metaclass
4 Behavior
Superclass of both Class
and Metaclass.
Gathers all the behavior
for class-like objects.
Default method new
is an instance method of Behavior
- if none of the Metaclasses implements a new
method, inheritance up the metaclasses will arrive at Behavior
and execute the default new.
Besides new
and allInstances,
other instance methods of Behavior
are very useful: allSubclasses,
allSuperclasses,
instVarNames,
methodDictionary,
...
Remark: Some Smalltalk
dialects have yet another class (ClassDescription)
which is mostly there for administrative purposes.
5 Summary
Smalltalk is pure OO -
everything is an object.
Objects like class or
metaclass are also instances of some class.
Metaclass inheritance
parallels class inheritance.
Behavior
centralizes most of the OO-behavior of Smalltalk.
Code browsers lie
Objects do not have two
kinds of methods
Classes have instance
variables
Objects are either:
Classes: can have instances
Non-classes: cannot have
instances
Code for all objects is
in the class of that object
Regularity
All classes are instances
of their metaclass
All metaclasses are instances
of class Metaclass
Parallel inheritance tree
A class inherits from
its superclass
A metaclass inherits from
the metaclass of the same superclass
Just three exceptions
Superclass of Object
is nil
Superclass of Object
class is Class
(not Class class)
Metaclass loop: Metaclass
is an instance of one of its own instances
