How does a reference-counting smart pointer's reference counting work?

Question

In other words, how does the implementation keeps track of the count?

Is there a map-like object maintained which is accessible by all the shared_ptr instances whose key is the pointer's address and value is the number of references? If I've to implement a shared_ptr, this is the first idea that's coming to my mind.

Is there a possibility for a memory leak in case of these reference-counting smart pointers? If so, how can I avoid them?

Answer 1

I've seen two different non-intrusive approaches to this:

1. The smart pointer allocates a small block of memory to contain the reference counter. Each copy of the smart pointer then receives a pointer to the actual object and a pointer to the reference count.
2. In addition to an object pointer, each smart pointer contains a previous and next pointer, thereby forming a doubly-linked list of smart pointers to a particular object. The reference count is implicit in the list. When a smart pointer is copied, it adds itself to the list. Upon destruction, each smart pointer removes itself from the list. If it's the last one in the list it then frees the referenced object as well.

If you go here and scroll to the bottom, there is an excellent diagram which explains these methods much more clearly.

Answer 2

Creating a memory leak with reference-counting smart pointers is very easy. Just create any graph-like structure of objects that has a cycle in the graph. The objects in the cycle will prevent each other from being released. This can't be resolved automatically - for example, when you create a double-link list you have to take care of never removing more than one object at a time.