Smart Pointers

Why Box, Rc, Arc, and RefCell exist as controlled escape hatches from strict single-owner rules, and what interior mutability actually trades away.

Why and when to reach past plain references.

Heap allocation, recursive types, and trait objects.

Shared ownership, single- vs. multi-threaded.

Interior mutability and runtime borrow checking.

Breaking reference cycles.

Borrow-or-own without needless allocation.

How smart pointers behave like references.

Choosing the least-powerful tool that works.