# Module alloc::arc [−] [src]

Threadsafe reference-counted boxes (the `Arc<T>` type).

The `Arc<T>` type provides shared ownership of an immutable value. Destruction is deterministic, and will occur as soon as the last owner is gone. It is marked as `Send` because it uses atomic reference counting.

If you do not need thread-safety, and just need shared ownership, consider the `Rc<T>` type. It is the same as `Arc<T>`, but does not use atomics, making it both thread-unsafe as well as significantly faster when updating the reference count.

The `downgrade` method can be used to create a non-owning `Weak<T>` pointer to the box. A `Weak<T>` pointer can be upgraded to an `Arc<T>` pointer, but will return `None` if the value has already been dropped.

For example, a tree with parent pointers can be represented by putting the nodes behind strong `Arc<T>` pointers, and then storing the parent pointers as `Weak<T>` pointers.

# Examples

Sharing some immutable data between threads:

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

    let five = Arc::new(5);

    for _ in 0..10 {
        let five = five.clone();

        thread::spawn(move || {
            println!("{:?}", five);
        });
    }

Sharing mutable data safely between threads with a `Mutex`:

    use std::sync::{Arc, Mutex};
    use std::thread;

    let five = Arc::new(Mutex::new(5));

    for _ in 0..10 {
        let five = five.clone();

        thread::spawn(move || {
            let mut number = five.lock().unwrap();

            *number += 1;

            println!("{}", *number); // prints 6
        });
    }

## Structs

Arc | An atomically reference counted wrapper for shared state.
Weak | A weak pointer to an `Arc`.

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