std::weak_ptr<T>::expired (3) - Linux Man Pages
bool expired() const noexcept; (since C++11)
Equivalent to use_count() == 0. The destructor for the managed object may not yet have been called, but this object's destruction is imminent (or may have already happened).
true if the managed object has already been deleted, false otherwise.
This function is inherently racy if the managed object is shared among threads. In particular, a false result may become stale before it can be used. A true result is reliable.
Demonstrates how expired is used to check validity of the pointer.
// Run this code
lock (public member function)
use_count (public member function)