metawhat?

metawhat? a look into the mysterious metaclass and Ruby’s object model Brandon Dimcheff @bdimcheff Ruby Midwest 2010

class << self

class Foo class << self def bar "hello" end end end Foo.bar # => "hello"

what’s an object?

struct RObject { struct RBasic basic; struct st_table *iv_tbl; }; ruby.h

struct RObject { struct RBasic basic; struct st_table *iv_tbl; }; struct RBasic { unsigned long ﬂags; VALUE klass; }; ruby.h

what is an object? foo class ivar hash

objects don’t have methods

struct RClass { struct RBasic basic; struct st_table *iv_tbl; struct st_table *m_tbl; VALUE super; }; ruby.h

RObject struct RClass { struct RBasic basic; struct st_table *iv_tbl; struct st_table *m_tbl; VALUE super; }; ruby.h

struct RClass { struct RBasic basic; struct st_table *iv_tbl; struct st_table *m_tbl; VALUE super; methods }; ruby.h

struct RClass { struct RBasic basic; struct st_table *iv_tbl; struct st_table *m_tbl; VALUE super; methods }; superclass pointer ruby.h

what is a class? super Bar class ivar hash methods

super Bar class ivar hash foo class ivar hash methods

super Object Bar class ivar hash foo class ivar hash methods

method dispatch

method dispatch 1. Follow my class pointer

method dispatch 1. Follow my class pointer 2. Check the methods table for my method

method dispatch 1. Follow my class pointer 2. Check the methods table for my method 3. Call the method, or follow the super pointer if the method doesn’t exist

method dispatch 1. Follow my class pointer 2. Check the methods table for my method 3. Call the method, or follow the super pointer if the method doesn’t exist 4. Repeat 2-3 if the method was not found

method dispatch super Bar class ivar hash foo class ivar hash methods

method dispatch super 1. Follow the class pointer Bar class ivar hash foo class ivar hash methods

method dispatch super 1. Follow the class pointer Bar class ivar hash foo class ivar hash methods 2. Check the method table

method dispatch super 3. Follow the super pointer 1. Follow the class pointer Bar class ivar hash foo class ivar hash methods 2. Check the method table

class Primate def opposable_thumbs true end end bob = Primate.new bob.opposable_thumbs # => true

Object super class bob Primate

Object bob.opposable_thumbs super class bob Primate

Object bob.opposable_thumbs super class bob Primate me tho ds

Object bob.opposable_thumbs super class bob Primate me tho ds #opposable_thumbs

Object bob.opposable_thumbs super class bob Primate me tho ds #opposable_thumbs # => true

joe = Primate.new def joe.opposable_thumbs false end joe.opposable_thumbs # => false bob.opposable_thumbs # => true

Pretend this is missing Joe

Object super class joe Primate

Object joe.opposable_thumbs super class joe Primate

Object joe.opposable_thumbs super class joe Primate me tho ds

Object joe.opposable_thumbs super class joe Primate me tho ds #opposable_thumbs

Object joe.opposable_thumbs super class joe Primate me tho ds #opposable_thumbs # => true :( :( :(

Object super Primate super class joe joe'

Object super Primate joe.opposable_thumbs super class joe joe'

Object super Primate joe.opposable_thumbs super class meth joe joe' ods

Object super Primate joe.opposable_thumbs super class meth joe joe' ods #opposable_thumbs

Object super Primate joe.opposable_thumbs super class meth joe joe' ods #opposable_thumbs # => false :)

Object super meth Primate ods joe.opposable_thumbs super class meth joe joe' ods #opposable_thumbs # => false :)

Object super meth Primate ods #opposable_thumbs joe.opposable_thumbs super class meth joe joe' ods #opposable_thumbs # => false :)

Object super meth Primate ods #opposable_thumbs joe.opposable_thumbs # => true super class meth joe joe' ods #opposable_thumbs # => false :)

Object super class meth bob Primate ods #opposable_thumbs joe.opposable_thumbs # => true super class meth joe joe' ods #opposable_thumbs # => false :)

Object super Primate super Joe’s Singleton Class class joe joe'

Metaclass Singleton Class Eigenclass Virtual Class

bob.class # => Primate

bob.class # => Primate joe.class # => Primate

class Primate class << self def species ["lemur", "monkey", "human"] end end end Primate.species # => ["lemur", ...]

joe = Primate.new def joe.opposable_thumbs false end joe.opposable_thumbs # => false

Primate = Class.new def Primate.species ["lemur", "monkey", "human"] end Primate.species # => ["lemur", "monkey", "human"]

Object super class Primate Class

Object Primate.species super class Primate Class

Object Primate.species super class Primate Class me tho ds

Object Primate.species super class Primate Class me tho ds #species NoMethodError

Object super Class super class Primate Primate'

Object super Class Primate.species super class Primate Primate'

Object super Class Primate.species super class meth Primate Primate' ods

Object super Class Primate.species super class meth Primate Primate' ods #species

Object super Class Primate.species super class meth Primate Primate' ods #species # => [“lemur”, ...]

Object super meth ods Class Primate.species super class meth Primate Primate' ods #species # => [“lemur”, ...]

Object super meth ods Class #species Primate.species super class meth Primate Primate' ods #species # => [“lemur”, ...]

Object super Class super Primate’s Singleton Class (Metaclass) class Primate Primate'

a metaclass is a singleton class of a class (to some people, anyways)

super Object Class super super class Primate Primate'

all classes have singletons/metaclasses automatically

super Class super class Object Object' super super class Primate Primate'

super Class super def Object.foo class Object Object' super super class Primate Primate'

super Class super def Object.foo class Object Object' super super Primate.foo class Primate Primate'

super Class super def Object.foo class Object Object' super super Primate.foo meth class ods Primate Primate' #species

super Class super def Object.foo meth ods class Object Object' #foo super super Primate.foo meth class ods Primate Primate' #species

super Module super Class super class Object Object' super class super Primate Primate'

super Module super Class super class Object Object' super super class Primate Primate' super class class joe joe'

super Module super Kernel Class super super class Object Object' super super class Primate Primate' super class class joe joe'

super BasicObject Module 1 .9 super super Kernel Class super super class Object Object' super super class Primate Primate' super class class joe joe'

give me your singleton class!

class Object # The hidden singleton lurks behind everyone def metaclass class << self self end end def meta_eval(&blk) metaclass.instance_eval(&blk) end # Adds methods to a metaclass def meta_def(name, &blk) meta_eval { define_method(name, &blk) } end # Defines an instance method within a class def class_def(name, &blk) class_eval { define_method(name, &blk) } end end http://code.whytheluckystiff.net/metaid/

ruby 1.9 # Object.singleton_class joe.singleton_class # Object#deﬁne_singleton_method joe.deﬁne_singleton_method(:opposable_thumbs) do false end

def method deﬁnes an instance method on Primate class Primate # def deﬁnes a method on self def opposable_thumbs true end end

def object.method deﬁnes a method on the singleton of Primate class Primate # deﬁnes a method on the singleton of self # self is the class Primate def self.species ["Lemur", "Monkey", "Human"] end end

class << o; def deﬁnes a method on the singleton of Primate class << Primate # in the singleton class of Primate def species ["Lemur", "Monkey", "Human"] end end

class << class << my_object # in the context of the # singleton class of my_object end

class << class << my_object # in the context of the # singleton class of my_object end class Primate # in the context # of class Primate end

class Foo class << self def bar "hello" end end end Foo.bar # => "hello"

include vs. extend module ThumbsUp def opposable_thumbs; true; end end module Species def species; ["Lemur", "Monkey", "Human"]; end end class Primate # adds methods to the class Primate include ThumbsUp # adds methods to the singleton of Primate extend Species end

instance.extend module ThumbsDown def opposable_thumbs; false; end end joe = Primate.new joe.extend(ThumbsDown)

object.extend(Foo) object.singleton_class.include(Foo)

Brandon Dimcheff http://spkr8.com/t/3756 brandon@dimcheff.com twitter: @bdimcheff http://brandon.dimcheff.com #midwest.rb on irc.freenode.net

Bob - http://en.wikipedia.org/wiki/File:Tamarin_portrait_2_edit3.jpg Joe - http://upload.wikimedia.org/wikipedia/commons/e/ec/Tarsier_Hugs_Mossy_Branch.jpg

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