Understanding of Scott Meyers' third example of std::weak_ptr

The final example at page 137 of Effective Modern C++ draws the scenario of a data structure with objects A, B, and C in it, connected to each other via std::shared_ptr in the following way:

   std::shared_ptr       std::shared_ptr
A -----------------> B <----------------- C

To me, this implies that the classes which objects A and C are instance of (two unrelated classes, in general) must contain a std::shared_ptr<classOfB> member.

Then the supposition is made that we need a pointer from B back to A, and the available options are listed: the pointer can be raw, shared, or weak, and the last one is picked up as the best candidate.

   std::shared_ptr       std::shared_ptr
A -----------------> B <----------------- C
^                    |
|                    |
\    std::weak_ptr   /
 \__________________/

I do understand the weaknesses (ahahah) of the first two alternatives, but I also see that the third alternative requires that member A be already managed by some std::shared_ptr, otherwise how can a std::weak_ptr point to it at all?

However the book does not refer to this "limitation"/assumption/whatever, so the truth is either

• I'm wrong
• I'm right but that assumption is obvious for some reason I don't understand
• The assumption is obvious for the exact reason that a std::weak_ptr needs an already existing std::shared_ptr to the same object, but it's a bit strange it's not even mentioned at the beginning of the example, I think.

and I'm asking this question to understand this.