std::weak_ptr<T>::use_count (3) - Linux Manuals
long use_count() const noexcept; (since C++11)
Returns the number of shared_ptr instances that share ownership of the managed object, or 0 if the managed object has already been deleted, i.e. *this is empty.
The number of shared_ptr instances sharing the ownership of the managed object at the instant of the call.
expired() may be faster than use_count(). This function is inherently racy, if the managed object is shared among threads that might be creating and destroying copies of the shared_ptr: then, the result is reliable only if it matches the number of copies uniquely owned by the calling thread, or zero; any other value may become stale before it can be used.
This section is incomplete
Reason: no example
expired (public member function)