pure-code explanation of the Eigenclass

1. singleton_method_on_object.rb

my_object = Object.new
another_object = Object.new

def my_object.a_method
  self
end

my_object.a_method # => #<Object:0x33c0c>
another_object.a_method
# ~> -:9: undefined method `a_method' for #<Object:0x33bf8> (NoMethodError)

2. singleton_method_on_class.rb

my_class = Class.new
another_class = Class.new

def my_class.a_method
  self
end

my_class.a_method # => #<Class:0x33c20>
another_class.a_method
# ~> -:9: undefined method `a_method' for #<Class:0x33bf8> (NoMethodError)

3. method_on_singleton_class.rb

class MyClass; end
class AnotherClass; end

class MyClass
  def self.a_method
    self
  end
end

MyClass.a_method # => MyClass
AnotherClass.a_method
# ~> -:11: undefined method `a_method' for AnotherClass:Class (NoMethodError)

4. use_of_metaclass_methods.rb

class Floobie; end
class Snapple; end

class Floobie
  @@floobies = []
  def self.all
    @@floobies
  end
  
  def initialize name
    @name = name
    
    @@floobies << self
  end
end

Floobie.new :one
Floobie.new :two
Floobie.new :three
Floobie.new :four

Floobie.all # => [#<Floobie:0x33644 @name=:one>, #<Floobie:0x335a4 @name=:two>, #<Floobie:0x33590 @name=:three>, #<Floobie:0x3357c @name=:four>]

Snapple.all
# ~> -:24: undefined method `all' for Snapple:Class (NoMethodError)