Rust: What is the difference between Box and Rc?

Box = one owner, heap data, immediate cleanup, no overhead. Rc = many owners, heap data, reference-counted cleanup, small overhead. Box for exclusive ownership; Rc for shared.

Box — single owner:

• Exactly one variable owns the data.
• Ownership transfers on move.
• Data freed deterministically when dropped.
• Zero runtime overhead.

1fn process(data: Box<dyn std::fmt::Display>) {
2    println!("Data: {}", data);
3    // data freed when function returns
4}
5
6let value = Box::new(42);
7process(value);  // value moved, now invalid

Rc — shared ownership:

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

1use std::rc::Rc;
2
3struct SharedBuffer {
4    data: Vec<u8>,
5}
6
7let buf = Rc::new(SharedBuffer { data: vec![0; 1024] });
8let reader1 = Rc::clone(&buf);
9let reader2 = Rc::clone(&buf);
10
11println!("Buffer size: {}", reader1.data.len());  // 1024
12println!("Refs: {}", Rc::strong_count(&buf));     // 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.