C++ already allows such a thing using the "curiously recurring template pattern" (http://en.wikipedia.org/wiki/Curiously_recurring_template_pa...), often now called "mixins":

  class person
  {
  protected:
      int age;
      int wieght;
  };

  template <class Base>
  class people: public Base
  {
  private:
      Base *prev;
      Base *next;
  };

  people<person> plist;

Your language extension sounds a lot like the friend keyword.

That's what I thought at first, but friend is for when you want class A to be able to view private things of class B. That is, B needs to know ahead of time what A is interested in. He wants B to remain ignorant of what A wants - he wants A to be able to insert new fields into B. But we can accomplish the same thing with mixins.