Daniel Frey:
I attached a complete implementation of enable_shared_from_this_light which has an overhead of a single plain pointer. [...]
I agree that providing the ability to build this lightweight helper may be valuable to some. But I'm not sure that we should provide several esft classes in Boost (except possibly as examples). This of course raises the question of which esft base (of the three candidates we have) should be provided.
The thing to note is that I added two more things to shared_ptr: _internal_shared_count() and a ctor which takes a detail::weak_count and the raw pointer.
I'd change the first argument of the new constructor to detail::shared_count. I don't like your preferred spelling of the function. It doesn't reflect its semantics in my opinion. In shared_ptr<T> p( new T ), the T object is not an observer, but it does accept p as its owner.