std::weak_ptr::expired(3)

NAME
std::weak_ptr::expired - std::weak_ptr::expired

Synopsis
   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).

Parameters
   (none)

Return value
   true if the managed object has already been deleted, false otherwise.

Notes
   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.

Example
   Demonstrates how expired is used to check validity of the pointer. // Run this code
 #include <iostream>
 #include <memory>
 std::weak_ptr<int> gw;
 void f()
 {
     if (!gw.expired())
         std::cout << "gw is valid\n";
     else
         std::cout << "gw is expired\n";
 }
 int main()
 {
     {
         auto sp = std::make_shared<int>(42);
         gw = sp;
         f();
     }
     f();
 }

Output:
 gw is valid
 gw is expired

See also
   lock creates a shared_ptr that manages the referenced object
             (public member function)
   use_count returns the number of shared_ptr objects that manage the object
             (public member function)