檢視類變數

類和模組具有與任何其他物件相同的內省例項變數的方法。類和模組也有類似的方法來查詢類變數（@@these_things）：

p Module.methods.grep(/class_variable/)
#=> [:class_variables, :class_variable_get, :remove_class_variable,
#=>  :class_variable_defined?, :class_variable_set]

class Foo
  @@instances = 0
  def initialize
    @@instances += 1
  end
end

class Bar < Foo; end

5.times{ Foo.new }
3.times{ Bar.new }
p Foo.class_variables                   #=> [:@@instances]
p Bar.class_variables                   #=> [:@@instances]
p Foo.class_variable_get(:@@instances)  #=> 8
p Bar.class_variable_get(:@@instances)  #=> 8

與例項變數類似，類變數的名稱必須以 @@ 開頭，否則你將收到錯誤：

p Bar.class_variable_defined?( :instances )
#=> NameError: `instances' is not allowed as a class variable name