C++: why doesn't weak_ptr have a shared_ptr&& constructor?

Question

The C++ draft standard mandates the following constructors for weak_ptr :

weak_ptr(const weak_ptr& r) noexcept;
template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;

weak_ptr(weak_ptr&& r) noexcept;
template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

I'm surprised to not see the following constructor:

template<class Y> weak_ptr(shared_ptr<Y>&& r) noexcept;

Of course the existing constructor from const reference to shared_ptr works functionally in any context where the rvalue reference version would work. But I believe the latter would allow saving at least two atomic operations in the case where the caller wants to destructively convert a shared reference to a weak one by using a shared_ptr rvalue to initialize a weak_ptr .

Answer 1

A weak_ptr<> holds a non-owning pointer to an object managed by a shared_ptr<> . The object is deleted when the last shared_ptr<> reference to it is deleted.

If the constructor you miss would exist, it could happen that you pass the last existing shared_ptr<> as parameter, which would destroy the object in the process of the move. So you would be left with a weak_ptr<> that points to an already deleted object.

Does not make much sense, right?