Why shared_ptr's reference counting object needs to keep track of the number of weak_ptrs pointing to the object too?

Question

Hi I am reading through this document and some other documents about C++'s shared_ptr and they all seem to suggest that apart from the number of shared_ptr pointing to the allocated object, the reference count object has to keep track of how many weak_ptr pointer pointing to the object as well. My question is why? From my understanding, weak_ptr is non-owning so if the count of shared_ptr pointing to the object reaches zero the object can be deleted. That is why sometimes we need to use expired to check the availability of an object pointed by a weak_ptr. Could you explain the reason for needing to keep track of the number of weak_ptrs?

Why do we need weak count here?

Answer 1

Adding to the answer by François Andrieux:

There is a side effect of this that is very important to understand.

If you use an implementation of std::make_shared that uses the WKWYL optimization (We Know Where You Live) of allocating the control block and the actual object together as one contiguous allocation, the memory WILL NOT BE FREED until after all weak_ptr objects have also gone out of scope.

(my account is seldom used, so I cannot add comments, due to not having enough reputation points, thus adding as an answer rather than a comment)

Answer 2

std::weak_ptr refers to the control block to know if the object still exists and if so, to provide a std::shared_ptr to it when needed. For that reason, the control block must exist as long as either a std::weak_ptr or a std::shared_ptr exists. You need to track the number of instances of std::weak_ptr to know when the last one is destroyed, just like for std::shared_ptr.

Answer 3

The shared_ptr reference count is the count of owners of the object. The weak_ptr reference count is the count of owners of the reference count control block.