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

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.

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.

As noted above, both styles seem to be equivalent, however using class << self allows one to mark class methods as private or protected. For example:

class UsingDefSelf
  def self.a; 'public class method'; end
  private
  def self.b; 'public class method!'; end
end

class UsingSingletonClass
  class << self
    def a; 'public class method'; end
    private
    def b; 'private class method'; end
  end
end

private only affects instance methods. Using the singleton class, we are defining instance methods of that class, which turn into class methods of the containing class!

We can also mark class methods as private with def self:

class UsingDefSelf
  def self.a; 'private class method'; end
  def self.b; 'private class method!'; end
  private_class_method :a, :b
  # In Ruby 2.1 there is an alternative syntax
  private_class_method def self.c; 'private class method!'; end
end

But we cannot mark them as protected, there is no protected_class_method. (However, since class is the only instance of its singletonclass, private class method and protected class methods are almost the same except their calling syntax is different.)

Also it is less easier than using class << self to mark private class methods, since you have to list all method names in private_class_method or prefix private_class_method to every private class method definition.