Class << self idiom

ruby-talk
1

Hello,

I have trouble understanding the need for the class << self idiom. For
example, let's say I have this class:

class My_Class
def method_1
end
end

Now, I want to extend that class. I can do:

class My_Class
def method_2
end
end

or:

class My_Class
class << self
   def method_2
   end
end
end

I understand that in the second case we're extending the singleton
class of My_Class, but why is the second form so often used in Ruby
vs. the other form? Isn't the end result identical?

2

surge wrote:

Now, I want to extend that class. I can do:

class My_Class
def method_2
end
end

or:

class My_Class
class << self
   def method_2
   end
end
end

I understand that in the second case we're extending the singleton
class of My_Class, but why is the second form so often used in Ruby
vs. the other form? Isn't the end result identical?

They are *not* identical. The first defines an *instance* method (My_Class.new.method_2); the second defines a *class* method (My_Class.method_2

Now, there are other ways of defining class methods. Which one you use depends a lot on personal preference but depends also on constants (which have a static scope) ->

  class Foo
    K = "foo"
  end

  #can access K
  class Foo
    class << self
      def foo1; K; end
    end
    def self.foo2; K; end
    def Foo.foo3; K; end
  end

  #must specify Foo::K
  class << Foo
    def foo4; Foo::K; end
  end
  def Foo.foo5; Foo::K; end

Daniel

3

You're completely right and I understand my mistake now. Thanks for
the comprehensive answer.

Surge