rust - 向量借用和所有权

Question

这个问题在这里已经有了答案:





What does it mean to pass in a vector into a `for` loop versus a reference to a vector?

(1 个回答)



How can I solve "use of moved value" and "which does not implement the `Copy` trait"?

(1 个回答)



What do I have to do to solve a "use of moved value" error?

(3 个回答)



What does "cannot move out of index of" mean?

(1 个回答)


去年关闭。




这不起作用:

let vectors = vec![1, 2, 3, 4, 5, 6, 7];

for i in vectors {
    println!("Element is {}", i);
}

let largest = vectors[0];

错误信息:

error[E0382]: borrow of moved value: `vectors`
 --> src/main.rs:8:19
  |
2 |     let vectors = vec![1, 2, 3, 4, 5, 6, 7];
  |         ------- move occurs because `vectors` has type `std::vec::Vec<i32>`, which does not implement the `Copy` trait
3 | 
4 |     for i in vectors {
  |              -------
  |              |
  |              value moved here
  |              help: consider borrowing to avoid moving into the for loop: `&vectors`
...
8 |     let largest = vectors[0];
  |                   ^^^^^^^ value borrowed here after move

向量已移入循环中。它的所有权 - 及其各个元素的所有权 - 已永久转移到那里。

但这有效:

let largest = vectors[0];
let largest2 = vectors[0];

我不知道为什么； vectors[0]值应该已移至 largest和 largest2然后应该失败，但它没有。

Answer 1

当您使用 vectors里面for..in循环，Rust 将调用 IntoIterator::into_iter Vec的trait方法，它拥有 self 的所有权.因此您不能使用 vectors然后。

use std::iter::IntoIterator;

// these are equivalent
for i in vectors { /* ... */ }
for i in IntoIterator::into_iter(vectors) { /* ... */ }

index operator ，另一方面，调用 Index::index Vec的trait方法，这需要 self引用。此外，它会自动取消引用该值，以便如果向量中的项实现 Copy ，它们将被复制出向量而不是借用(如果需要引用，则需要显式借用):

use std::ops::Index;

// these are equivalent
let x = vectors[0];
let x = *Index::index(&vectors, 0);

// these are equivalent
let x = &vectors[0];
let x = Index::index(&vectors, 0);