Rails class << self

Question

I would like to understand what class << self stands for in the next example.

module Utility   class Options #:nodoc:     class << self       def parse(args)                 end     end   end end 

Answer 1

this

module Utility   class Options #:nodoc:     class << self       # we are inside Options's singleton class       def parse(args)        end     end   end end 

is equivalent to:

module Utility   class Options #:nodoc:     def Options.parse(args)      end   end end 

A couple examples to help you understand :

class A   HELLO = 'world'   def self.foo     puts "class method A::foo, HELLO #{HELLO}"   end    def A.bar     puts "class method A::bar, HELLO #{HELLO}"   end    class << self     HELLO = 'universe'     def zim       puts "class method A::zim, HELLO #{HELLO}"     end   end  end A.foo A.bar A.zim puts "A::HELLO #{A::HELLO}"  # Output # class method A::foo, HELLO world # class method A::bar, HELLO world # class method A::zim, HELLO universe # A::HELLO world 

Answer 2

This is an eigenclass. This question's been asked before.