定义在哪里?方法定义?我到处都找不到关于它的任何文件

Here's the sample codes:

这里的示例代码:

class Square
  def initialize

    if defined?(@@number_of_squares)
      @@number_of_squares += 1
    else
      @@number_of_squares = 1
    end
  end

  def Square.count
    @@number_of_squares
  end
end
class Squ