Box, Rc, Arc, RefCell — Smart Pointer Guide
Tools for flexibly working around ownership rules
Ownership rules are powerful but don't cover every pattern.
Box<T> — Allocates on the heap. Required for recursive types (linked lists, etc.). Also for types with unknown compile-time size.
Rc<T> — Reference Counted. For multiple owners. Value freed when last Rc drops. Single-thread only.
Arc<T> — Atomic Rc. Thread-safe shared ownership for multi-threaded contexts.
RefCell<T> — Runtime borrow checking. For patterns where compile-time safety can't be proven. Panics at runtime on misuse.
In practice, Arc<Mutex<T>> is very common — the standard pattern for shared mutable data across threads.
Key Points
Box<T> — heap allocation + ownership transfer (recursive types, trait objects)
Rc<T> / Arc<T> — reference-counted shared ownership (single/multi thread)
RefCell<T> — runtime borrow checking (Interior Mutability pattern)
Arc<Mutex<T>> — standard pattern for multi-thread shared mutable data
Pros
- ✓ Safely implement patterns that ownership rules forbid
- ✓ Rc/Arc provide automatic memory release without GC
Cons
- ✗ Rc/Arc leak memory on circular references (use Weak)
- ✗ RefCell can panic at runtime — compile-time safety sacrificed