Question

I learning ruby singletons and have misunderstanding with such code:

class MyClass
  def self.class_singleton_mymethod
  end
end

class_singleton = class << MyClass
  self
end

puts class_singleton.methods.grep(/mymethod/)  # => []


obj = MyClass.new

def obj.object_singleton_mymethod
end

object_singleton = class << obj
  self
end

puts object_singleton.methods.grep(/mymethod/) # => class_singleton_mymethod

Why class_singleton not contains class’s Class method and object_singleton instead of Object’s singleton method contains class’s Class method?

Answer 1

I think you have the notion of methods and instance_methods mixed up. If you were to replace all instances of methods with instance_methods, you will see the results you expect.

instance_methods is used to enumerate which methods a class’s instances have. methods is used to enumerate what methods the object has. (Class objects are objects too, and have their own methods like new that are not instance methods.

For example, String#slice is an instance method; you can call slice on string instances. On the other hand, String.new is a method on String itself; you don’t call new on string instances, but you can call String.new (i.e., on the String class object itself) to create a new string.)