Ruby Class Methods vs. Methods in Eigenclasses

Question

Are class methods and methods in the eigenclass (or metaclass) of that class just two ways to define one thing?

Otherwise, what are the differences?

cla         


        

Answer 1

In Ruby there really are no such things as class methods. Since everything is an object in Ruby (including classes), when you say def self.class_method, you are just really defining a singleton method on the instance of the class Class. So to answer your question, saying

class X
  def self.a
    puts "Hi"
  end

  class << self
    def b
      puts "there"
    end
  end
end

X.a # => Hi
X.b # => there

is two ways of saying the same thing. Both these methods are just singeton (eigen, meta, ghost, or whatever you want to call them) methods defined in the instance of your Class object, which in your example was X. This topic is part of metaprogramming, which is a fun topic, that if you have been using Ruby for a while, you should check out. The Pragmatic Programmers have a great book on metaprogramming that you should definitely take a look at if you interested in the topic.