1.1.10.6 weak_ptr

• A truly smart pointer would deal with this problem by tracking when it dangles, i.e., when the object it is supposed to point to no longer exists. That’s precisely the kind of smart pointer std::weak_ptr is. You can’t test if a raw pointer is dangle or not.
• The relationship begins at birth. std::weak_ptrs are typically created from std::shared_ptrs. You can only create a weak_ptr out of a shared_ptr or another weak_ptr. So the idea would be that the owner of the pointer hold a shared_ptr instead of a raw pointer.
• std::weak_ptrs can’t be dereferenced, nor can they be tested for nullness. That’s because std::weak_ptr isn’t a standalone smart pointer. It’s an augmentation of std::shared_ptr. Almost the only things you can do are to interrogate it to see if the managed object is still there, or construct a shared_ptr from it.
  auto spw = 
  std::make_shared<Widget>(); 
  // the pointed-to Widget’s ref count (RC) is 1. 
   
  std::weak_ptr<Widget> wpw(spw); 
  // wpw points to same Widget as spw. RC remains 1 
  ... 
  spw = nullptr; 
  // RC goes to 0, and the Widget is destroyed. 
  // wpw now dangles 
  ____________________________________if (wpw.expired()) 
  // if wpw doesn’t point to an objectâĂę
  std::shared_ptr<Widget> spw1 = wpw.lock(); 
  // if wpw’s expired, spw1 is null 
  auto spw2 = wpw.lock(); 
  // same as above, but uses auto 
   
  std::shared_ptr<Widget> spw3(wpw); 
  // if wpw’s expired, throw std::bad_weak_ptr
• Potential use cases for std::weak_ptr includes: caching, observer lists, and the prevention of std::shared_ptr cycles. All the detail can be seen in "Effective Modern ++" Item 20.
  std::shared_ptr<const Widget> fastLoadWidget(WidgetID id) 
  { 
  static std::unordered_map<WidgetID, 
  std::weak_ptr<const Widget>> cache; 
  auto objPtr = cache[id].lock(); 
  // objPtr is std::shared_ptr to cached object (or null 
  // if object’s not in cache) 
  if (!objPtr) { // if not in cache, 
  objPtr = loadWidget(id); // load it 
  cache[id] = objPtr; // cache it 
  } 
  return objPtr; 
  }