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Metaclasses 4 ◆ Dynamic Binding Implies That Classes Must Have Some ◆ Meta-: Beyond; Transcending; More Comprehensive; E.G., As Memory Representation

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Metaclasses 4 ◆ Dynamic Binding Implies That Classes Must Have Some ◆ Meta-: Beyond; Transcending; More Comprehensive; E.G., As Memory Representation ® 1 ® Class Representation in Memory? 2 Case Study: Meta Classes ◆ A class is an abstract entity, so why should it be represented in the runtime environment? ◆ Answer: Dynamic Binding! ◆ Class representation in memory ● The actual method bound to a message is determined at run time based on the type (class) of an object. ◆ Class variables and class methods ● There must be a link between the object and its ◆ Meta Classes methods. ● Since these links are the same for all objects of the ◆ 3 Level System same class, it makes sense to share the representation. ◆ ◆ 4 Level System Schematic diagram, common to all OO languages: Method ◆ 5 Level System 1 Method ◆ 1 Level System Class Rep 2 ◆ Msg Object Methods Infinite Levels System Table Methodn oop Õ Ö oop Õ Ö ® A Class as an Object? 3 ® Metaclasses 4 ◆ Dynamic binding implies that classes must have some ◆ Meta-: Beyond; transcending; more comprehensive; e.g., as memory representation. in metalinguistics. ◆ Why not as an object? ◆ Class: generates instances. ◆ Answer: Let's go for it! ◆ Metaclass: a class whose instances are classes. ● Each object o has a pointer to its class object c which is the ◆ Terminal Instance: cannot be instantiated further. object representing the class to which o belongs. Objects Class Classes Terminal Instances Novel Play Instantiable objects Non instantiable objects Macbeth Othello Metaclasses Instances are classes 1984 War & Peace As you like Main Text Book Reference: G.Masini et al. Oliver Twist Object-Oriented Languages, APIC series, No.34 oop Õ Ö oop Õ Ö ® 5 ® 6 Taxonomy of Metaclass Systems The 3 Level System ◆ 1-Level System: Objects only Object ● Each object describes itself INSTANCE _OF ● No need for classes: objects are ìinstantiatedî or ìinheritedî from SUBCLASS _OF other objects Class Example: Self VARIABLES instance_of ,167$1&(B2) ◆ 2-Level System: Objects, Classes METHODS ... 68%&/$66B2) subclass_of ● Objects: described by classes 9$5,$%/(6 variables ● Classes: need no description--do not exist in run-time. Not first- methods class objects: print 0(7+2'6 Not instances of classes ... Novel Not created by constructors new Cannot receive messages INSTANCE _OF add sub- Examples: C++, Eiffel Oliver Twist SUBCLASS _OF set_author author class INSTANCE _OF ◆ 3-Level System: Objects, Classes, Metaclass VARIABLES get_author year ... AUTHOR : D ICKENS ● Objects: described by classes METHODS set_year ... Other instance ● Classes: described by the metaclass get_year variables ● The Metaclass: describes itself ... Examples: Smalltalk-76, Little-Smalltalk oop Õ Ö oop Õ Ö ® 7 ® 8 One Metaclass Structure The 1 Level System ◆ Simple and elegant model: courtesy of Smalltalk-76, the Object Traits first language to introduce the metaclass concept. method for P RINT printing ● Later adopted by Little-Smalltalk objects ◆ The &ODVV metaclass: Novel Traits ● The only metaclass in the system PARENETPARENT method for ● STORE storing in Instance of itself library Avoids the infinite regress of the instantiation relationship Oliver Twist ● Inherits from 2EMHFW PARENETPARENT Oliver ● Holds behaviour common to all class objects: TITLE Twist How to add methods Novel Prototype 1984 TITLE: How to instantiate a class PARENETPARENT PARENETPARENT AUTHOR D ICKENS ● Holds structure common to all class objects: TITLE ??? TITLE 1984 AUTHOR : Super class TITLE: TITLE: Instance variables AUTHOR ??? AUTHOR Orwell List of methods AUTHOR : AUTHOR : ● Uniform behaviour and structure of all classes: Different classes cannot have different QHZ methods Clone QHZ &ODVV QHZ The defined in class calls defined in the objectís Clone class oop Õ Ö oop Õ Ö ® Singleton Classes 9 ® Class Variables and Class Methods 10 ◆ In a 1-level system, each object describes itself. ◆ If a class is an object, then it makes sense to ask ● A bit awkward for classes with many similar objects what are the variables and methods of that object. ● Very elegant for Singleton Classes ◆ Simple answer (Objective-C approach): ◆ Examples: Classes with only one instance ● Class Methods: ● True: true Constructor! Send a message to the class, requesting it ● UndefinedObject: nil to create a new instance. ● ● Smalltalk: smalltalk Class Variables: ● TApplication: application The mould: ● Simulation: ï Data structure defining the structure of instances. ï Data structure determining the behaviour of instances. Clock Other shared variables. Simulator ◆ The more sophisticated answer: ◆ In class based OOP, double check that the appropriate abstraction was used. ● Smalltalk-80 (a 5 level system): ● Use a module? Class specific variables ● Use a global function/procedure? Class specific methods ● C++: Static data and function members. oop Õ Ö oop Õ Ö ® 11 ® 12 A 4 Level System Properties of the 4 Level System Object ◆ Each object: is an instance of a class INSTANCE_OF ◆ Each class: is an instance of a metaclass SUBCLASS_OF Meta Class ● May have its own singleton metaclass VARIABLES instance_of INSTANCE_OF ● May share a metaclass with other classes METHODS Meta Novel subclass_of ... ● SUBCLASS_OF May use the default metaclass Meta Object variables INSTANCE_OF VARIABLES ◆ Each metaclass: is an instance of a meta-metaclass. methods SUBCLASS_OF METHODS ◆ One meta-metaclass: VARIABLES print ● Novel new Instance of itself METHODS ... ● Inherits from Object (or Class) INSTANCE_OF ... SUBCLASS_OF ◆ Examples: (with minor variations) Oliver Twist VARIABLES ● LOOPS: Lisp Object and data Oriented Programming author n_novels INSTANCE_OF System METHODS add_novel set_author year ... ● ObjVlisp: OBJect Virtual extension of LISP AUTHOR: D ICKENS N_NOVELS: 1 get_author del_novel ... A model more than a real language Other instance Other class ... variables variables ... oop Õ Ö oop Õ Ö ® 13 ® 14 Infinite Object-Class Regression ... A 5 Level System Three Kinds of Entities: ◆ 3-KRS: an infinite number of levels system ● Objects Metaclass ● Objects: instances of ìmetaobjectsî (aka classes). ● Classes: each class X has an object X describing it. x class ● Classes: instances of metaclasses. Integer Object ● Metaclasses: instances of meta-metaclasses. Class: the class of all class objects ● Meta-metaclasses: instances of meta-meta- Metaclass: the class of all metaclass objects metaclasses. x Metaclass ● Metaclasses: the object X describing class X ● ... is an instance of a singleton metaclass: ìX class” describing it. ◆ For each object X, there is a meta-object X Integer class: the class of ìIntegerî ● x Integer Object Lazy creation of meta-objects enables the system Class class: the class of ìClassî to work in a finite world. class class Metaclass Class: the class of ìMetaclassî x The metaclass ìNovel classî is also the result of evaluating the Smalltalk Integer Object expression: x 1RYHOFODVV 12 13 oop Õ Ö oop Õ Ö ® 15 Inheritance in the 5 Level System A metaclass object ìX classî: Object ● is an instance of ìMetaclassî ● inherits from ìClassî Integer Class: Metaclass Class ● an abstract class ● parent of all ìX classî Class objects Hierarchy Metaclasses hierarchy: ● Instance Of X inherits from Y Object ìX classî inherits class from ìY classî Integer Metaclass Class class class class Meta Class Hierarchy oop Õ Ö.
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