What is the difference between Box and Rc?

Question

Accepted Answer

**Box** = single owner, heap allocation, no runtime cost. **Rc** = multiple owners, heap allocation, reference counting. Box is simpler; Rc is more flexible for shared data.

**Box — exclusive ownership:**
- One owner at a time.
- Move semantics on assignment.
- Immediate cleanup when dropped.
- Zero runtime overhead.

```rust
let data = Box::new(vec![1, 2, 3]);
let moved = data;  // Ownership moves
// data is no longer valid
println!("{:?}", moved);  // [1, 2, 3]
```

**Rc — shared ownership:**
- Multiple owners.
- Reference count tracks owners.
- Cleanup when count reaches zero.
- Small runtime cost.

```rust
use std::rc::Rc;

let shared = Rc::new(String::from("shared data"));
let owner1 = Rc::clone(&shared);
let owner2 = Rc::clone(&shared);

println!("{}", *shared);    // shared data
println!("{}", *owner1);   // shared data
println!("Refs: {}", 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.

Rust: What is the difference between Box and Rc?