shared_ptr<> is to weak_ptr<> as unique_ptr<> is to… what?

Question

In C++11, you can use a shared_ptr<> to establish an ownership relation with an object or variable and weak_ptr<> to safely reference t

Answer 1

In the new C++ world with shared_ptr, weak_ptr, and unique_ptr you should not be storing long lived references to objects, like your trebuchet, using raw pointers or references. Instead World should have a shared_ptr to the trebuchet and Victim should store either a shared_ptr or a weak_ptr, depending on whether the trebuchet should stick around with the victim if the world goes away. Using a weak_ptr allows you to tell if the pointer is still valid (i.e. the world still exists), there is no way to do this with a raw pointer or reference.

When you use a unique_ptr you are declaring that only the World instance will own the trebuchet. Clients of the World class can use the World object's trebuchet by calling the "get" method but should not hold on to the reference or pointer returned by the method when they are done using it. Instead they should "borrow" the trebuchet every time they want to use it by calling the "get" method.

The above being said there could be instances where you want to store a reference or raw pointer for future use to avoid the overhead of the shared_ptr. But those instances are few and far between and you need to be completely sure that you won't use the pointer or reference after the World object that owns the trebuchet has gone away.