Question

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class << self idiom in Ruby

I have a quick Ruby question. I come from a Java/c background, so I understand in Ruby “self” when referenced inside a instance method acts like “this”. And “self.” prefix for method defines it as a class method.

But what does this mean here??

class << self
  def some_method
  end
end

Answer 1

What’s happening here is that we’re reopening an object’s class from within itself and defining a new instance method on it. This is one of the ways you can do so-called “monkey-patching” in Ruby. This method adds the method only to the current object rather than all objects of the class.

It’s equivalent to doing this:

my_obj = MyClass.new

class << my_obj
  def some_method
  end
end

# or...

def my_obj.some_method
end

Here’s a good article that covers it: Learning Ruby: class << self.