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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 mut handles = Vec::new();
    for _ in 1..5 {
        let v = Arc::clone(&v);
        handles.push(thread::spawn(move || {
            let thread_id = thread::current().id();
            println!("{thread_id:?}: {v:?}");
        }));
    }

    handles.into_iter().for_each(|h| h.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.