class << self idiom in Ruby

Question

What does class << self do in Ruby?

Answer 1

What class << thing does:

class Hi
  self #=> Hi
  class << self #same as 'class << Hi'
    self #=> #
    self == Hi.singleton_class #=> true
  end
end

[it makes self == thing.singleton_class in the context of its block].

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What is thing.singleton_class?

hi = String.new
def hi.a
end

hi.class.instance_methods.include? :a #=> false
hi.singleton_class.instance_methods.include? :a #=> true

hi object inherits its #methods from its #singleton_class.instance_methods and then from its #class.instance_methods.
Here we gave hi's singleton class instance method :a. It could have been done with class << hi instead.
hi's #singleton_class has all instance methods hi's #class has, and possibly some more (:a here).

[instance methods of thing's #class and #singleton_class can be applied directly to thing. when ruby sees thing.a, it first looks for :a method definition in thing.singleton_class.instance_methods and then in thing.class.instance_methods]

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By the way - they call object's singleton class == metaclass == eigenclass.