当 Class attribute 遇见 inheritance

Ruby的类变量遇到继承的时候：

  class F
    @@a = 'f'
    def foo
      puts @@a
    end
  end

  class A < F
    @@a = 'a'
  end

  class B < F
    @@a = 'b'
  end


  a = A.new
  a.foo ＃ => 'b'

Rails是这么解决的：[url]http://dev.rubyonrails.org/browser/trunk/activesupport/lib/active_support/core_ext/class/inheritable_attributes.rb?rev=7220[/url]