`std::weak_ptr` 何时为空？

Question

According to [util.smartptr.weak.obs]/4 , the weak_ptr::owner_before strict weak ordering is such that "two shared_ptr or weak_ptr instances are equivalent if and only if they share ownership or are both empty."

However, I see nothing in the standard that defines what it means for a weak_ptr to be empty. Obviously a default-constructed weak_ptr is empty, and a weak_ptr constructed from an empty shared_ptr is empty, but it doesn't seem to be explicitly stated whether an expired weak_ptr is empty.

Answer 1

While I don't see it guaranteed by the Standard, usability ¹ of the owner_before ordering requires that:

• A weak_ptr which has become expired ² is NOT EMPTY.

This is because only mutation of an object should change its placement in an ordering, and a weak_ptr may expire without ever being mutated.

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¹ For example, here someone has used weak_ptr as a key in std::set : How to compute hash of std::weak_ptr? . Doing that, and std::map likewise, requires the ordering to survive expiration. Were there to be an owner_hash to allow use with std::unordered_set and std::unordered_map , that hash would also have to survive expiration.

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² Note the subtle difference between "has become expired" and "is expired" -- a weak_ptr constructed empty, or assigned with an empty pointer value, has expired() == true . But it didn't expire (action verb), it was created in an expired state. My answer only applies to weak_ptr values which are at some point not expired, then (passively) become expired due to detachment of shared_ptr values from the target.