Arc

当需要在线程之间共享所有权时，可以使用Arc（原子引用计数）。这个结构体，通过 Clone 实现，可以为内存堆中某个值的位置创建一个引用指针，同时增加引用计数。由于它在线程之间共享所有权，当最后一个指向某个值的引用指针超出作用域时，该变量将被释放。

use std::time::Duration;
use std::sync::Arc;
use std::thread;

fn main() {
    // This variable declaration is where its value is specified.
    let apple = Arc::new("the same apple");

    for _ in 0..10 {
        // Here there is no value specification as it is a pointer to a
        // reference in the memory heap.
        let apple = Arc::clone(&apple);

        thread::spawn(move || {
            // As Arc was used, threads can be spawned using the value allocated
            // in the Arc variable pointer's location.
            println!("{:?}", apple);
        });
    }

    // Make sure all Arc instances are printed from spawned threads.
    thread::sleep(Duration::from_secs(1));
}