问题描述
我正在尝试存储对可变向量元素的引用以备后用。但是,一旦我改变了向量,我就不能再使用存储的引用。我理解这是因为借用对元素的引用还需要借用对向量本身的引用。因此,不能修改向量,因为这需要借用可变引用,当已经借用了向量的另一个引用时,这是不允许的。
这是一个简单的例子
struct Person {
name: String,}
fn main() {
// Create a mutable vector
let mut people: Vec<Person> = ["Joe","Shavawn","Katie"]
.iter()
.map(|&s| Person {
name: s.to_string(),})
.collect();
// Borrow a reference to an element
let person_ref = &people[0];
// Mutate the vector
let new_person = Person {
name: "Tim".to_string(),};
people.push(new_person);
// Attempt to use the borrowed reference
assert!(person_ref.name == "Joe");
}
产生以下错误
error[E0502]: cannot borrow `people` as mutable because it is also borrowed as immutable
--> src/main.rs:21:5
|
15 | let person_ref = &people[0];
| ------ immutable borrow occurs here
...
21 | people.push(new_person);
| ^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
...
24 | assert!(person_ref.name == "Joe");
| --------------- immutable borrow later used here
我还尝试按照建议的 here 对矢量元素进行装箱,但这无济于事。我认为这可能允许我在保留对元素的引用的同时删除对向量的引用,但显然不是。
struct Person {
name: String,}
fn main() {
// Create a mutable vector
let mut people: Vec<Box<Person>> = ["Joe","Katie"]
.iter()
.map(|&s| {
Box::new(Person {
name: s.to_string(),})
})
.collect();
// Borrow a reference to an element
let person_ref = people[0].as_ref();
// Mutate the vector
let new_person = Box::new(Person {
name: "Tim".to_string(),});
people.push(new_person);
// Attempt to use the borrowed reference
assert!(person_ref.name == "Joe");
}
这仍然产生相同的错误
error[E0502]: cannot borrow `people` as mutable because it is also borrowed as immutable
--> src/main.rs:23:5
|
17 | let person_ref = people[0].as_ref();
| ------ immutable borrow occurs here
...
23 | people.push(new_person);
| ^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
...
26 | assert!(person_ref.name == "Joe");
| --------------- immutable borrow later used here
有没有办法做到这一点,或者我是在尝试做一些不可能的事情?
解决方法
我发现使用 reference counted smart pointer 可以让我完成我正在尝试的事情。共享所有权是必要的,否则如果原始向量超出范围(这将释放元素,无论是否使用 Box
),元素引用都会无效。
以下代码编译成功。
use std::rc::Rc;
struct Person {
name: String,}
fn main() {
// Create a mutable vector
let mut people: Vec<Rc<Person>> = ["Joe","Shavawn","Katie"]
.iter()
.map(|&s| {
Rc::new(Person {
name: s.to_string(),})
})
.collect();
// Borrow a reference to an element
let person_ref = Rc::clone(&people[0]);
// Mutate the vector
let new_person = Rc::new(Person {
name: "Tim".to_string(),});
people.push(new_person);
// Attempt to use the borrowed reference
assert!(person_ref.name == "Joe");
}
如果其他人有任何更正、改进或进一步的见解,我会很高兴听到。但如果没有,我现在对这个答案感到满意。