为什么局部变量的生存时间不足以支持thread :: scoped? [英] Why doesn't a local variable live long enough for thread::scoped?

问题描述

鉴于示例2可以很好地编译，为什么示例1不能编译?这些示例之间的唯一区别在于，在示例1中，value是函数局部变量，在示例2中，value是函数的参数.

Why doesn't Example 1 compile given that Example 2 compiles just fine? The only difference between the examples is that in Example 1 value is a function local variable, and in Example 2 value is an argument to the function.

示例1

#![feature(scoped)]

use std::thread;
use std::sync::atomic::{AtomicUsize, Ordering};

pub fn foo<F>() {
    let mut join_guards = Vec::new();
    let value = AtomicUsize::new(0);

    for _ in 0..10 {
        join_guards.push(thread::scoped(|| {
            value.fetch_add(1, Ordering::SeqCst);
        }));
    }
}

示例2

#![feature(scoped)]

use std::thread;
use std::sync::atomic::{AtomicUsize, Ordering};

pub fn foo<F>(value: AtomicUsize) {
    let mut join_guards = Vec::new();

    for _ in 0..10 {
        join_guards.push(thread::scoped(|| {
            value.fetch_add(1, Ordering::SeqCst);
        }));
    }
}

这些是我尝试编译的错误消息示例1:

These are the error messages I get trying to compile Example 1:

error: `value` does not live long enough
         join_guards.push(thread::scoped(|| {
             value.fetch_add(1, Ordering::SeqCst);
         }));
note: reference must be valid for the block at 6:20...
pub fn foo<F>() {
    let mut join_guards = Vec::new();
    let value = AtomicUsize::new(0);

     for _ in 0..10 {
         join_guards.push(thread::scoped(|| {

note: ...but borrowed value is only valid for the block suffix following statement 1 at 8:40
    let value = AtomicUsize::new(0);

     for _ in 0..10 {
         join_guards.push(thread::scoped(|| {
             value.fetch_add(1, Ordering::SeqCst);
         }));

推荐答案

join_guards是具有一定共享生存期的作用域线程的向量.该生存期由其结束的变量确定:value是唯一的生存期.因此，作用域线程的生存期为value的范围. value在哪里合法? 在定义之后，直到超出范围为止，即在8:40的语句1之后的块后缀". join_guards的生存期必须不超过此生存期，但是如果之前 value定义了join_guards，则情况并非如此.

join_guards is a vector of scoped threads with a certain shared lifetime. That lifetime is determined by which variables it closes over: value is the only one. Therefore, the lifetime of the scoped threads is the scope of value. Where is value legal? After it is defined and until it goes out of scope—that is, "the block suffix following statement 1 at 8:40". join_guards must have as its lifetime a lifetime no greater than this, but if join_guards is defined before value, this is not the case.

解决方案是颠倒声明的顺序:

The solution is to reverse the order of declaration:

let value = AtomicUsize::new(0);
let mut join_guards = Vec::new();

这说明了第二个示例也可以工作的原因.

This explains why the second example works, also.

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