Rust: What is the difference between Box and Rc?

Box provides exclusive ownership — one owner, immediate cleanup. Rc provides shared ownership — many owners, cleanup when last owner drops. Both allocate on the heap.

Box — exclusive:

• Exactly one owner.
• Move semantics (no Copy).
• Immediate, deterministic cleanup.
• Zero runtime overhead.

1fn take_ownership(b: Box<String>) {
2    println!("Got: {}", *b);
3    // b dropped here — memory freed
4}
5
6let s = Box::new(String::from("hello"));
7take_ownership(s);
8// s is invalid here

Rc — shared:

• Multiple owners.
• Clone increments count (doesn't copy data).
• Cleanup when count reaches zero.
• Small runtime cost.

1use std::rc::Rc;
2
3let data = Rc::new(vec![10, 20, 30]);
4let r1 = Rc::clone(&data);
5let r2 = Rc::clone(&data);
6
7println!("{} {} {}", data[0], r1[1], r2[2]);  // 10 20 30
8drop(r1);
9drop(r2);
10// data still valid — count is 1
11drop(data);  // Now freed

When to use Box:

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

When to use Rc:

• Shared state across components.
• Graphs with shared nodes.
• Read-only shared data.

Common mistakes:

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