実は回字には7つの書き方があります...

class T
  def T.class_method1
    #   
  end
 class << self
   def class_method2
     #   
   end
  end
  def self.class_method3
    #   
  end
end


def T.class_method4
  #   
end


T.class_eval do
  def T.class_method5
    #   
  end
  def self.class_method6
    #   
  end
end
class Object
  # The hidden singleton lurks behind everyone
  def metaclass; class << self; self; end; end
  def meta_eval &blk; metaclass.instance_eval &blk; end
  # Adds methods to a metaclass
  def meta_def name, &blk
    meta_eval { define_method name, &blk }
  end
end
T.meta_def :class_method7 do 
  #   
end
p T.singleton_methods.sort#["class_method1", "class_method2", "class_method3", "class_method4", "class_method5", "class_method6", "class_method7", "yaml_tag_subclasses?"]

class TT # YD   ...
  class << self
    private
    def private_class_method
      #     ？      ！
    end
  end
end
TT.private_class_method# private method `private_class_method' called for TT:Class (NoMethodError)

class TT# YD  ..
  class <<self
    def class_m
      puts "class method"
    end
    alias alias_class_m class_m
  end
end
TT.alias_class_m#class method