Basic Mechanism of OOPL

Basic Mechanism of Object-Oriented Programming Language 2009-09-11 : Released 2009-09-14 : Fixed makoto kuwata <kwa@kuwata-lab.com> http://www.kuwata-lab.com/ copyright© 2009 kuwata-lab.com all right reserved.

Purpose ✤ Describes basic mechanism of Object-Oriented Programming Language (OOPL) ✤ Not only Perl but also Python, Ruby, Java, ... ✤ You must be familiar with terms of Object-Oriented ✤ Class, Instance, Method, Override, Overload, ... copyright© 2009 kuwata-lab.com all right reserved.

Overview Function Class obj Method Tbl func (self) { Variable ..... m1 } m2 Z: 123 m3 func (self) { ..... Instance obj Class obj Method Tbl } m2 x: 10 m3 func (self) { y: 20 Z: 456 m4 ..... } copyright© 2009 kuwata-lab.com all right reserved.

Instance Object Pointer to Class object ✤ Instance variables + Is-a pointer ✤ Instance object knows "what I am" Variable Instance obj Class obj x: 10 Hash data (in script y: 20 lang) or Struct data (in Java or C++) copyright© 2009 kuwata-lab.com all right reserved.

Class Object Class obj ✤ Class variables Method Table ( + Is-a pointer) + Parent pointer Z: 30 + Method Table Class obj Instance methods Method Table belong to Class Z: 35

Method Lookup Table ✤ Method signatures(≒names) + function pointers ✤ May have pointer to parent method table Class obj Method Table func (self) { print "hello"; m1 } m2 m3 func (self, x) { return x + 1; } copyright© 2009 kuwata-lab.com all right reserved.

Method Function ✤ Instance object is passed as hidden argument OOPL non-OOPL class Point { def move(self, x, y) { var x=0, y=0; self['x'] = x; def move(x, y) { self['y'] = y; this.x = x; } this.y = y; almost } equiv. p = {isa: Point, x: 0, y: 0}; } func = lookup(p, 'move'); p = new Point(); func(p, 10, 20); p.move(10, 20); copyright© 2009 kuwata-lab.com all right reserved.

Difference bet. method and function ✤ Method requires instance object ✤ Typically, instance obj is passed as 1st argument ✤ Method call is dynamic, function call is static ✤ Method signature(≒name) is determined statically ✤ Method lookup is performed dynamically (This is why method call is slower than function call) copyright© 2009 kuwata-lab.com all right reserved.

Overview (again) Function Class obj Method Tbl func (self) { Variable ..... m1 } m2 Z: 123 m3 func (self) { ..... Instance obj Class obj Method Tbl } m2 x: 10 m3 func (self) { y: 20 Z: 456 m4 ..... } copyright© 2009 kuwata-lab.com all right reserved.

Conslusion ✤ Instance object knows "what I am" (by is-a pointer) ✤ Instance variables belong to instance object ✤ Instance methods belong to class object ✤ function-call is static, method-call is dynamic copyright© 2009 kuwata-lab.com all right reserved.

Method Signature ✤ Method name + Argument types (+ Return type) (in static language) ✤ Or same as Method name (in dynamic language) Example (Java): Method Declaration Method Signature void hello(int v, char ch) hello(IC)V void hello(String s) hello(Ljava.lang.String;)V copyright© 2009 kuwata-lab.com all right reserved.

Method Override ✤ Child class can deﬁne methods which has same method signature as method in parent class Parent Class Method Table hello(IC) hello(Ljava.lang.String;) Child Class Method Table hello(Ljava.lang.String;) : copyright© 2009 kuwata-lab.com all right reserved.

Polymorphism (1) not used ✤ Polymorphism depends on ... used ✤ Receiver object's data type Animal a; a = new Dog(); a.bark(); ✤ Variable data type a = new Cat(); a.bark(); Determined dynamically Instance obj Class obj Method Tbl Method Func bark(...) func (self) { ..... : : } : : copyright© 2009 kuwata-lab.com all right reserved.

Polymorphism (2) ✤ Polymorphism depends on ... void bark(Object o) { ... } ✤ Data type of formal args a.bark("Wee!"); ✤ Data type of actual args Determined statically Instance obj Class obj Method Tbl Method Func bark(...) func (self) { ..... : : } : : copyright© 2009 kuwata-lab.com all right reserved.

"Object" class vs. "Class" class (1) class Class "Class" extends "Object" extends Object {...} class Foo "Object" class extends Object {...} NULL "Class" class Instance obj "Foo" class x: 10 y: 20 "Foo" extends "Object" copyright© 2009 kuwata-lab.com all right reserved.

"Object" class vs. "Class" class (2) Object = new Class() "Object" is-a "Class" Foo = new Class() Instobj = new Foo() "Object" class "Class" is-a "Class" Instance is-a "Foo" NULL "Class" class Instance obj "Foo" class x: 10 y: 20 "Foo" is-a "Class" copyright© 2009 kuwata-lab.com all right reserved.

Conclusion (1) ✤ Method Signature = Method Name + Arg Types ✤ Method Overload : Same method name can have different method signature ✤ Method Override : Child class can have same method signature as parent class ✤ Super : Skips current class when method lookup copyright© 2009 kuwata-lab.com all right reserved.

Conclusion (2) ✤ Polymorphism ✤ Depends on Receiver's data type, Method Name, and Temporary Args' data type ✤ Not depends on Variable data type and Actual Args' data type ✤ Relation between "Object" class and "Class" class is complicated copyright© 2009 kuwata-lab.com all right reserved.

Case Study: Ruby from ruby.h: from ruby.h: struct RBasic { struct RClass { unsigned long ﬂags; struct RBasic basic; VALUE klass; struct st_table *iv_tbl; }; struct st_table *m_tbl; Is-a pointer VALUE super; struct RObject { }; struct RBasic basic; Parent pointer struct st_table *iv_tbl; }; Method table Instance variables copyright© 2009 kuwata-lab.com all right reserved.

Case Study: Perl package Point; sub new { my $classname = shift @_; bless() binds hash data my ($x, $y) = @_; with class name my $this = {x=>$x, y=>$y}; (instead of is-a pointer) return bless $this, $classname; } sub move_to { my $this = shift @_; Instance object is the my ($x, $y) = @_; ﬁrst argument of $this->{x} = $x; instance method $this->{y} = $y; } copyright© 2009 kuwata-lab.com all right reserved.

Case Study: Python class Point(object): def __init__(self, x=0, y=0): self.x = x Instance object is the self.y = y ﬁrst argument of instance method def move_to(self, x, y): self.x = x self.y = y Possible to call instance p = Point(0, 0) method as function p.move_to(10, 20) Point.move_to(p, 10, 20) Python uses function as method, and Ruby uses method as function. copyright© 2009 kuwata-lab.com all right reserved.

Case Study: Java Class obj Method Table m1 func (this) { ... } m2 func (this) { ... } Class obj Copy parent entries Method Table not to follow parent table (for performance) m1 m2 m3 func (this) { ... } copyright© 2009 kuwata-lab.com all right reserved.

Case Study: JavaScript (1) /// (1) ✤ instance.__proto__ points prototype function Animal(name) { object (= Constructor.prototype) this.name = name; } ✤ Trace __proto__ recursively /// (2) (called as 'prototype chain') Animal.prototype.bark = function() { ('__proto__' property is anim alert(this.name); } available on Firefox) name /// (3) var anim = __proto__ new Animal("Doara"); (3) Animal prototype (1) (2) prototype bark prototype this.name = name; __proto__ alert(this.name); copyright© 2009 kuwata-lab.com all right reserved.

Case Study: JavaScript (2) /// (4) /// (5) function Dog(name) { Dog.prototype.bark2 = Animal.call(this, name); dog function() { } name alert("BowWow"); }; Dog.prototype = anim; /// (6) __proto__ delete Dog.prototype.name; var dog = new Dog("so16"); (6) Dog anim (4) (5) prototype bark2 prototype Animal.call(this); __proto__ alert("BowWow"); Animal prototype prototype bark prototype this.name = name; __proto__ alert(this.name); copyright© 2009 kuwata-lab.com all right reserved.

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Basic Mechanism of OOPL

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