class << self vs self.method with Ruby: what's better?

Question

This Ruby Style Guide tells that is better using self.method_name instead of class method_name. But Why?

class TestClass   # bad   class << self     def first_method       # body omitted     end      def second_method_etc       # body omitted     end   end    # good   def self.first_method     # body omitted   end    def self.second_method_etc     # body omitted   end end 

Are there performance issues?

Answer 1

class << self is good at keeping all of your class methods in the same block. If methods are being added in def self.method form then there's no guarantee (other than convention and wishful thinking) that there won't be an extra class method tucked away later in the file.

def self.method is good at explicitly stating that a method is a class method, whereas with class << self you have to go and find the container yourself.

Which of these is more important to you is a subjective decision, and also depends on things like how many other people are working on the code and what their preferences are.

Answer 2

Generally, class << self is used in metaprogramming to set the class as self for a prolonged period of time. If I'm trying to write 10 methods, I would use it like so:

METHOD_APPENDICES = [1...10] class << self   METHOD_APPENDICES.each do |n|     define_method("method#{n}") { n }   end end 

This would create 10 methods (method1, method2, method3, etc.) that would just return the number. I would use class << self for clarity in this case because in metaprogramming self is crucial. Littering self. inside there would actually make things less readable.

If you're just defining class methods normally, stick to self.class_method_name because more people are likely to understand it. No need to bring in meta-syntax unless you expect your audience to understand it.