Arc<T> 与跨线程共享所有权

  源码:
      1 | use std::sync::Arc;
      2 | use std::thread;
      3 | fn main() {
      4 |     let data = Arc::new(vec![1, 2, 3, 4, 5, 6, 7, 8]);
      5 |     let mut hs = vec![];
      6 |     for i in 0..4 {
      7 |         let d = Arc::clone(&data);
      8 |         hs.push(thread::spawn(move || { let s: i32 = d[i * 2..i * 2 + 2].iter().sum(); (i, s) }));
      9 |     }
     10 |     for h in hs { let (i, s) = h.join().unwrap(); println!("线程 {i} 求和 = {s}"); }
     11 |     println!("全部线程结束后 strong_count = {}", Arc::strong_count(&data));
     12 | }
  编译:成功
  运行(退出码 0):
    线程 0 求和 = 3
    线程 1 求和 = 7
    线程 2 求和 = 11
    线程 3 求和 = 15
    全部线程结束后 strong_count = 1

  源码:
      1 | use std::sync::Arc;
      2 | fn main() { let a = Arc::new(5); *a += 1; }
  编译:失败
    error[E0594]: cannot assign to data in an `Arc`
     --> <实验目录>/src/u30b.rs:2:34
      |
    2 | fn main() { let a = Arc::new(5); *a += 1; }
      |                                  ^^^^^^^ cannot assign
      |
      = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `Arc<i32>`
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0594`.
  源码:
      1 | use std::sync::{Arc, Mutex};
      2 | use std::thread;
      3 | fn main() {
      4 |     let counter = Arc::new(Mutex::new(0u64));
      5 |     let hs: Vec<_> = (0..4).map(|_| { let c = Arc::clone(&counter); thread::spawn(move || { for _ in 0..10_000 { *c.lock().unwrap() += 1; } }) }).collect();
      6 |     for h in hs { h.join().unwrap(); }
      7 |     println!("4 个线程各加 10000 次,counter = {}(期望 40000)", *counter.lock().unwrap());
      8 | }
  编译:成功
  运行(退出码 0):
    4 个线程各加 10000 次,counter = 40000(期望 40000)

  源码:
      1 | use std::cell::Cell;
      2 | use std::sync::Arc;
      3 | use std::thread;
      4 | fn main() {
      5 |     let a = Arc::new(Cell::new(1));
      6 |     let b = Arc::clone(&a);
      7 |     thread::spawn(move || b.set(2)).join().unwrap();
      8 | }
  编译:失败
    error[E0277]: `Cell<i32>` cannot be shared between threads safely
       --> <实验目录>/src/u30d.rs:7:19
        |
      7 |     thread::spawn(move || b.set(2)).join().unwrap();
        |     ------------- ^^^^^^^^^^^^^^^^ `Cell<i32>` cannot be shared between threads safely
        |     |
        |     required by a bound introduced by this call
        |
        = help: the trait `Sync` is not implemented for `Cell<i32>`
        = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
        = note: required for `Arc<Cell<i32>>` to implement `Send`
    note: required because it's used within this closure
       --> <实验目录>/src/u30d.rs:7:19
        |
      7 |     thread::spawn(move || b.set(2)).join().unwrap();
        |                   ^^^^^^^
    note: required by a bound in `spawn`
       --> <rust 标准库源码>/library/std/src/thread/functions.rs:128:8
        |
    125 | pub fn spawn<F, T>(f: F) -> JoinHandle<T>
        |        ----- required by a bound in this function
    ...
    128 |     F: Send + 'static,
        |        ^^^^ required by this bound in `spawn`
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0277`.

  编译:成功
  运行(退出码 0):
    Rc::clone + drop                          每次约 3.98 ns
    Arc::clone + drop(单线程)                每次约 3.98 ns
    4 个线程各克隆 2000000 次,共用同一个 Arc:    墙钟 272 ms
    4 个线程各克隆 2000000 次,各用自己的 Arc:    墙钟 8 ms
    共用同一个计数器比各用各的慢 34.1 倍(我的推断:同一个计数器所在的缓存行在 4 个核之间来回传递,没有用性能计数器验证)