std::weak_ptr

WeakPtr.cpp

/* A ​std::shared_ptr​ uses reference counting, so circular references are potentially a problem. 
   To break up cycles, ​std::weak_ptr​ can be used to access the stored object. 
   The stored object will be deleted if the only references to the object are ​weak_ptr​ references.
   ​weak_ptr​ therefore does not ensure that the object will continue to exist, but it can ask for the resource. */
std::shared_ptr<int> p1(new int(5));
std::weak_ptr<int> wp1 = p1; //p1 owns the memory.
 
{
  std::shared_ptr<int> p2 = wp1.lock(); //Now p1 and p2 own the memory.
  if(p2) //Always check to see if the memory still exists
  { 
    //Do something with p2
  }
} //p2 is destroyed. Memory is owned by p1.
 
p1.reset(); //Memory is deleted.
 
std::shared_ptr<int> p3 = wp1.lock(); //Memory is gone, so we get an empty shared_ptr.
if(p3)
{
  //Will not execute this.
}