Arc

Arc<T> allows shared read-only access via Arc::clone:

use std::sync::Arc;
use std::thread;

fn main() {
    let v = Arc::new(vec![10, 20, 30]);

    let handles = (0..5).map(|_| {
        let v = Arc::clone(&v);

        thread::spawn(move || {
            let thread_id = thread::current().id();
            println!("{thread_id:?}: {v:?}");
        })
    }).collect::<Vec<_>>();

    for handle in handles {
        handle.join().unwrap();
    }

    println!("v: {v:?}");
}
  • Arc stands for “Atomic Reference Counted”, a thread safe version of Rc that uses atomic operations.
  • Arc<T> implements Clone whether or not T does. It implements Send and Sync if and only if T implements them both.
  • Arc::clone() has the cost of atomic operations that get executed, but after that the use of the T is free.
  • Beware of reference cycles, Arc does not use a garbage collector to detect them.
    • std::sync::Weak can help.