[Ruby] @v ? @@v ?

class C
  attr_reader :v, :w
  @v = 5
  @@v = 10
  def initialize
    @w = 15
    @@w = 20
  end
end

eval('@v', C.send(:binding)) # -> 5
eval('@@v', C.send(:binding)) # -> undefined

eval('@w', C.new.send(:binding)) # -> 15
eval('@@w', C.new.send(:binding)) # -> undefined

eval('@v', Class.send(:binding)) # -> nil
eval('@@v', Class.send(:binding)) # -> undefined

Class.module_eval{attr_reader, :v, :w}
C.v # -> 5
C.w # -> nil
C.instance_eval{puts @v} # -> 5
C.instance_eval{puts @@v} # -> undefined
C.instance_eval{puts @w} # -> nil
C.instance_eval{puts @@w} # -> undefined
C.module_eval{puts @v} # -> 5
C.module_eval{puts @@v} # -> undefined
C.module_eval{puts @w} # -> nil
C.module_eval{puts @@w} # -> undefined

where are the class variables? @@

class C
  def orz
    puts [@@v, @@w].join(', ')
  end
  def self.xd
    puts [@@v, @@w].join(', ')
  end
end

C.new.orz # -> 10, 20
C.orz # -> 10, 20

eval('@@v', C.new.method(:orz).send(:binding)) # -> undefined
eval('@@v', C.method(:xd).send(:binding)) # -> undefined

class variable can only be bound in a real method?