shared_ptr<> is to weak_ptr<> as unique_ptr<> is to… what?

Question

In C++11, you can use a shared_ptr<> to establish an ownership relation with an object or variable and weak_ptr<> to safely reference t

Answer 1

A function taking a raw pointer or reference implicitly promises not to hold on to a copy of that pointer after the function has returned. In return the caller promises that the pointer is valid (or nullptr) until the callee has returned.

If you want to hold on to the pointer, you are sharing it (and should use shared_ptr). A unique_ptr manages a single copy of the pointer. You use raw pointers (or references) to refer to call functions involving that object.

This is the same for shared_ptr objects. weak_ptr only comes into play when you want to have an additional reference to the pointed too object that outlives the involved function. The main purpose of weak_ptr is to break reference cycles where two objects hold references to each other (and are therefore never released).

Remember however that taking shared_ptr or weak_ptr implies that the function taking that parameter will (optionally) modify some other object to retain a reference to the pointed to object that outlives the invocation of the function. In the vast majority of cases you use raw pointer (if nullptr is a valid value) or ref (when a value is guaranteed) even for shared_ptr or weak_ptr.