在RoR中，如何让服务类的子类识别其超类中的类变量？

class SuperClass
  @@class_variable = "Hello, World!"

  def self.class_variable
    @@class_variable
  end
end

class SubClass < SuperClass
  def self.sub_class_variable
    class_variable
  end
end

puts SubClass.sub_class_variable
# 输出：Hello, World!

class SuperClass
  @@class_variable = "Hello, World!"
end

class SubClass < SuperClass
end

puts SubClass.class_variable
# 输出：Hello, World!