Case Study: Meta Classes

Transcription

1 1 Case Study: Meta Classes Class representation in memory Class variables and methods Meta Classes 3 Level System 4 Level System 5 Level System 1 Level System Infinite Levels System

2 Class Representation in Memory? 2 A is defined in the source code, so why should it be represented in the runtime environment? Answer: Dynamic Binding! The actual method bound to a message is determined at run time based on the type () of an object. There must be a link between the object and its methods. Since these links are the same for all objects of the same, it makes sense to share the representation. Schematic diagram, common to all OO languages: Method 1 Msg Object Class Rep Methods Table Method 2 Method n

3 A Class as an Object? Dynamic binding implies that es must have some memory representation. Why not as an object? Answer: Let's go for it! Each object o has a pointer to its object c which is the object representing the to which o belongs. 3 Class Novel Play 1984 War & Peace Macbeth Othello Oliver Twist As you lik e

4 Metaes 4 Meta-: Beyond; transcending; more comprehensive; e.g., as in metalinguistics. Class: generates instances. Meta: a whose instances are es. Terminal Instance: cannot be instantiated further. O b j e c t s C l a s s e s I n s t a n t i a b l e o b j e c t s T e r m N i n a l I n s t a n c e s o n i n s t a n t i a b l e o b j e c t s M e t a c l a s s e s I n s t a n c e s a r e c l a s s e s Main Text Book Reference: G.Masini et al. Object-Oriented Languages, APIC series, No.34

5 Taxonomy of Meta Systems 1-Level System: Objects only Each object describes itself No need for es: objects are instantiated or inherited from other objects Example: Self, Javascript 2-Level System: Objects, Classes Objects: described by es Classes: need no description--do not exist in run-time. Not first objects: Not instances of es Not created by constructors Cannot receive messages Examples: Eiffel 3-Level System: Objects, Classes, Meta Objects: described by es Classes: described by the meta The Meta: describes itself Examples: Smalltalk-76, Little-Smalltalk 5

6 oop Object The 3 Level System 6 INSTANCE_OF SUBCLASS_OF VARIABLES METHODS print instance _of Class INSTANCE_OF SUBCLASS_OF VARIABLES METHODS sub _of variables methods Oliver Twist INSTANCE_OF AUTHOR :DICKENS YEAR: 1838 Novel INSTANCE_OF SUBCLASS_OF VARIABLES METHODS set_author get_author set_year get_year author year new add sub -

7 One Meta Structure Simple and elegant model: courtesy of Smalltalk-76, the first language to introduce the meta concept. Later adopted by Little-Smalltalk The Class meta: The only meta in the system Instance of itself Avoids the infinite regress of the instantiation relationship Inherits from Object Holds behaviour common to all objects: How to add methods How to instantiate a Holds structure common to all objects: Super Instance variables List of methods Uniform behaviour and structure of all es: Different es cannot have different new methods The new defined in Class calls new defined in the object s 7

8 oop The 1 Level System 8 Novel Traits Object Traits P RINT method for printing obj ects PA RENET RENT T Oliver Twist IT LE A UT HOR A UT HOR : D Oliver Twist T IT LE : <- ICKENS <- PA RENET RENT ST ORE Novel Prototype PA RENET RENT T IT LE??? T IT LE : <- A UT HOR A UT HOR :??? < PA RENET RENT A UT HOR method for storing in library T IT LE 1984 T IT LE : <- A UT HOR : Orwell <- Clone Clone

9 Principles of the 1 Level Sysem In a 1-level system, each object describes itself. An object is a essentially a map from names (strings) to either values or methods Very flexible, but static type checking is hard No distinction between objects and es Every object is the of itself Instantiation of a new object: Take an existing object and clone it. 9

10 Class Variables and Class Methods 10 If a is an object, then it makes sense to ask what are the variables and methods of that object. Simple answer (LST approach): Class Methods: Constructor! Send a message to the, requesting it to create a new instance. Class Variables: Data structure defining the structure of instances. Data structure determining the protocol/implementation of instances. No per- variables The more sophisticated answer: Smalltalk-80 (a 5 level system): Class specific variables Class specific methods

11 oop A 4 Level System 11 Obj ect INST A NCE SUBCLA SS VA RIA BLES MET HODS instance_of Meta Novel INST A NCE SUBCLA SS Meta Class INST A NCE SUBCLA SS VA RIA BLES MET HODS sub_of variables methods print Novel INST A NCE VA RIA BLES MET HODS new SUBCLA SS Oliver Twist INST A NCE A UT HOR : D ICKENS Other instance variables VA RIA BLES MET HODS N _ NOVELS : 1 Other variables set_author get_author author year add_novel del_novel n_novels

12 Properties of the 4 Level System 12 Each object: is an instance of a Each : is an instance of a meta May have its own singleton meta May share a meta with other es May use the default meta Meta Object Each meta: is an instance of a meta-meta. One meta-meta: Instance of itself Inherits from Object (or Class) Examples: (with minor variations) LOOPS: Lisp Object and data Oriented Programming System ObjVlisp: OBJect Virtual extension of LISP A model more than a real language

13 oop Infinite Object-Class Regression 13 3-KRS: an infinite number of levels system Objects: instances of metaobjects (aka es). Classes: instances of metaes. Metaes: instances of meta-metaes. Meta-metaes: instances of meta-metametaes. x x For each object X, there is a meta-object X Lazy creation of meta-objects enables the system to work in a finite world. x x x

14 A 5 Level System Three Kinds of Entities: Objects Classes: each X has an object X describing it. Integer Object Class: the of all objects Meta: the of all meta objects Metaes: the object X describing X is an instance of a singleton meta: X describing it. Integer : the of Integer Class : the of Class Meta Class: the of Meta Integer Meta Meta Object 14 oop The meta Novel is also the result of evaluating the Smalltalk expression: Novel Integer Object

15 oop Inheritance in the 5 Level System A meta object X : is an instance of Meta inherits from Class Class: an abstract parent of all X objects Metaes hierarchy: X inherits from Y X inherits from Y Class Hierarchy Meta Instance Of Object Class Object Integer 15 Meta Class Integer Meta Class Hierarchy