The document provides an overview of Ruby's object model and metaclasses. It discusses what objects and classes are at a low level, how method dispatch works, and what metaclasses (or singleton classes) are. Key points include: - Objects have an ivar hash and classes have an ivar hash and methods table - Method dispatch follows the class pointer, checks the methods table, and follows the super pointer if needed - Classes are objects that have a methods table and superclass pointer - All classes have singleton classes/metaclasses that are used to define class methods - Method definitions on objects add methods to the singleton class

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

2. class << self

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

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

5. what’s an object?

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

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

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

9. what is an object?
foo class
ivar hash

10. objects don’t have
methods

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

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

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

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

15. what is a class?
super
Bar
class
ivar hash
methods

16. super
Bar
class
ivar hash
foo class
ivar hash methods

17. super
Object
Bar
class
ivar hash
foo class
ivar hash methods

18. method dispatch

19. method dispatch
1. Follow my class pointer

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

21. 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

22. 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

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

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

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

26. 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

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

28. Bob

29. Object
super
class
bob Primate

30. Object
bob.opposable_thumbs
super
class
bob Primate

31. Object
bob.opposable_thumbs
super
class
bob Primate

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

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

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

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

36. Joe

37. Pretend this is
missing
Joe

38. Object
super
class
joe Primate

39. Object
joe.opposable_thumbs
super
class
joe Primate

40. Object
joe.opposable_thumbs
super
class
joe Primate

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

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

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

44. Object
super
Primate
super
class
joe joe'

45. Object
super
Primate
joe.opposable_thumbs
super
class
joe joe'

46. Object
super
Primate
joe.opposable_thumbs
super
class
joe joe'

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

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

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

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

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

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

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

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

55. Metaclass
Singleton Class
Eigenclass
Virtual Class

56. Metaclass
Singleton Class
Eigenclass
Virtual Class

57. bob.class # => Primate

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

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

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

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

62. Object
super
class
Primate Class

63. Object
Primate.species
super
class
Primate Class

64. Object
Primate.species
super
class
Primate Class

65. Object
Primate.species
super
class
Primate Class
me
tho
ds

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

67. Object
super
Class
super
class
Primate Primate'

68. Object
super
Class
Primate.species
super
class
Primate Primate'

69. Object
super
Class
Primate.species
super
class
Primate Primate'

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

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

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

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

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

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

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

77. super
Object Class
super
super
class
Primate Primate'

78. all classes have
singletons/metaclasses
automatically

79. super
Class
super
class
Object Object'
super
super
class
Primate Primate'

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

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

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

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

84. super
Module
super
Class
super
class
Object Object'
super
class super
Primate Primate'

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

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

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

88. give me your
singleton class!

89. 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/

90. 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/

91. 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/

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

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

94. 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

95. 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

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

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

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

99. 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

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

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

102. ?

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

104. 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