In the last recipe, we learned how to use unique_ptr. This is an enormously useful and important class because it helps us manage dynamically allocated objects. However, it can only handle single ownership. It is not possible to let multiple objects own the same dynamically allocated object because, then, it would be unclear who has to delete it later.
The pointer type, shared_ptr, was designed for specifically this case. Shared pointers can be copied arbitrarily often. An internal reference counting mechanism tracks how many objects are still maintaining a pointer to the payload object. Only the last shared pointer that goes out of scope will call delete on the payload object. This way, we can be sure that we do not get memory leaks because objects are deleted automatically after use. At the same time, we can be sure that they are not deleted too early, or too often (every created object must only be deleted once).
In this recipe...