Arc

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

use std::thread;
use std::sync::Arc;
fn main() {
    let v = Arc::new(vec![10, 20, 30]);
    let mut handles = Vec::new();
    for _ in 1..5 {
        let v = v.clone();
        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:?}");
}
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Speaker Notes

  • Arc stands for โ€œAtomic Reference Countedโ€, a thread safe version of Rc that uses atomic operations.

    โ€œThe type Arc provides shared ownership of a value of type T, allocated in the heap. Invoking clone on Arc produces a new Arc instance, which points to the same allocation on the heap as the source Arc, while increasing a reference count. When the last Arc pointer to a given allocation is destroyed, the value stored in that allocation (often referred to as โ€œinner valueโ€) is also dropped.โ€ from 1.

    Also see How arc works in rust

  • Arc<T> implements Clone whether or not T does. It implements Send and Sync iff 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.