ruby - Variables declared within `class << self` -

how should consider @variable declared within class << self block outside method definition? @ final part of script:

class vardemo   @@class_var_1 = "this @@class_var_1"   @class_i_var = "this @class_i_var"    attr_accessor :ivar_1   attr_accessor :ivar_2    def initialize(ivar_1: "base_ivar_1", ivar_2: "base_ivar_2")     @@class_var_2 = "this @@class_var_2, defined within instance method"     @ivar_1 = ivar_1     @ivar_2 = ivar_2     vardemo.class_i_var_2 = "class_i_var_2 defined through accessor on class object"   end    def print_vars     puts "from within instance method. here 'self' is: #{self}"      # test instance variables     puts "@ivar_1: #{@ivar_1}"     puts "@ivar_2: #{@ivar_2}"     puts "self.ivar_1: #{self.ivar_1}"     puts "self.ivar_2: #{self.ivar_2}"     puts "ivar_1: #{ivar_1}"     puts "ivar_2: #{ivar_2}"      # test class variables     puts "@@class_var_1: #{@@class_var_1}"     puts "@@class_var_2: #{@@class_var_2}"      # test class instance variables (ivars on class object)     puts "vardemo.class_i_var: #{vardemo.class_i_var}"     puts "vardemo.class_i_var_2: #{vardemo.class_i_var_2}"     puts "@class_i_var: #{@class_i_var} (should empty, doesn't exist in instance"      puts "self.class_i_var: #{self.class_i_var}"   rescue exception => ex     puts "self.class_i_var not defined in scope (self instance)"   end    # open class object   class << self     attr_accessor :class_i_var     attr_accessor :class_i_var_2      @what_about_this = "what be?"      attr_accessor :what_about_this      def print_what_about_this       puts "@what_about_this: #{@what_about_this}" # empty       puts "vardemo.what_about_this: #{vardemo.what_about_this}" # empty     end   end  end 

what @what_about_this? defined? interpreter lets me do, without practical use? thought defined on class, doesn't appear case.

the class <<self doesn't open self's class; opens self's singleton class. (described here.)

class foo   class << self     @bar = "baz"   # not inside class here.   end end  foo.singleton_class.instance_eval { @bar }  #baz 

yehuda katz has article describes what's going on here.