CODEWITHSUNDEEP
rubysingletonmetaprogramming
Tesseract
Tesseract

Reputation: 123

Value of self in a method defined on singleton class

Why is self, in the method (singleton2) defined on a singleton class, not equal to that singleton class like in the other three cases?

    class A
      def self.singleton1
        class << self
          self
        end
      end

      class << self
        def singleton2
          self
        end
      end
    end

    def singleton3
      class << A
        self
      end
    end

    p [A.singleton1, A.singleton1.object_id]           # [#<Class:A>, 70003841656620]
    p [A.singleton2, A.singleton2.object_id]           # [A, 70003841656640]
    p [singleton3, singleton3.object_id]               # [#<Class:A>, 70003841656620]
    p [A.singleton_class, A.singleton_class.object_id] # [#<Class:A>, 70003841656620]

Upvotes: 1

Views: 78

Answers (1)

Arup Rakshit
Arup Rakshit

Reputation: 118261

I'll explain it part by part.

 def self.singleton1
   class << self
     self
   end
 end

Here, self inside the method is A. Now within the method you are creating another scope, which is the scope of the singleton class of A, using class << self. And the self of the singleton class is being returned when you called A.singeton1, as self inside the singleton class of A is the last statement of the method singleton1.

class << self
  def singleton2
    self
  end
end

Here, self inside the body of the singleton class of A is, singleton class itself. Now you defined a singleton method singleton2 on class A. Inside the method, last expression is self, which is always will be set to receiver, here it is A. One more thing, def singleton2 creates a new scope, which is different from the scope of the singleton class of A.

As per the above explanation, I hope the below code output must be clear to you :-

class A
  def self.singleton1
    class << self
      self
    end
  end

  class << self
    def singleton2
      self
    end
  end
end

A.singleton2 # => A 
A.singleton1 # => #<Class:A>

output A and #<Class:A> clearly telling, why the object_ids were different.

From the above 2 explanations, it is clear what is going on for singleton3.

Upvotes: 3

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