I think its about recursive types/members, e.g.

    class Tree { Tree _left, _right; };

Won't compile because it has an infinite size. However

    class Tree { Tree *_left, *_right; };

Compiles, but the author contends that a more explicit description of the ownership semantics is better:

    class Tree { value_ptr<Tree> _left, _right; }

This is also why `value_ptr` is different than `optional`.