In one edge case, reference counts can cause a memory leak, that is, accidentally preventing resources from ever being deleted. This happens when two objects exist that both contain an Rc pointing to each other. Because of this circular dependence, none of them will stop using the other and so the two objects will continue existing and pointing at each other long after your code has stopped using them. The solution here is to pick the weaker link in the hierarchy and replace its Rc for a Weak, which contains a non-owning reference instead. Because this situation is fairly rare, however, we are not going to look at it in detail. Instead, simply remember the possibility of a memory leak and come back to read this again when it arises.

Rc is inherently singlethreaded. If you need its functionality in a multithreaded environment (such as the one we are going to explore in Chapter 7, Parallelism and Rayon; Sharing resources in multithreaded closures), you can use Arc instead. It stands for Atomically Reference Counted and behaves the same way as Rc.