Rust: What is the difference between Box and Rc?

Box = exclusive ownership, heap allocation, no runtime overhead. Rc = shared ownership, heap allocation, reference counting overhead. Box is simpler and faster; Rc enables multiple owners.

Box — single owner:

• One variable owns the data at a time.
• Ownership moves on assignment (no Copy).
• Data freed when the Box goes out of scope.
• Zero runtime overhead.

1let boxed = Box::new(String::from("important"));
2let taken = boxed;  // Move ownership
3println!("{}", taken);  // important
4// boxed is invalid here

Rc — shared ownership:

• Multiple variables own the same data.
• Reference count tracks active owners.
• Data freed when count reaches zero.
• Small runtime cost.

1use std::rc::Rc;
2
3let shared = Rc::new(vec![10, 20, 30]);
4let r1 = Rc::clone(&shared);
5let r2 = Rc::clone(&shared);
6
7// All three point to the same Vec
8assert_eq!(shared[0], r1[0]);
9assert_eq!(r1[1], r2[1]);
10println!("Count: {}", Rc::strong_count(&shared));  // 3

When to use Box:

• Recursive types, trait objects.
• Single-owner scenarios.
• Performance-critical code.

When to use Rc:

• Shared state across components.
• Multiple readers of same data.
• Single-threaded contexts.

Common mistakes:

• Using Rc when Box works.
• Mutating Rc without RefCell.
• Creating reference cycles.