简介:我对Rust还很陌生,所以决定通过实现一个双向链表来练习。为了调试方便,我实现了get()
方法,但是我无法从Rc<RefCell<_>>
中复制值出来。(抱歉问了个愚蠢的问题)
问题:我试图在.get()
中返回一个Result<T, &'static str>
,其中T
是节点中存储的数据类型,&str
是错误消息字符串。借用检查器告诉我不能返回一个方法内部变量的引用,所以我尝试将内部值复制出来并返回,但是失败了。
源代码:
use std::{rc::Rc, cell::RefCell};
struct Node<T> {
data: Option<T>,
prev: Option<Rc<RefCell<Node<T>>>>,
next: Option<Rc<RefCell<Node<T>>>>,
}
impl<T> Node<T> {
/// Instantiate a new dummy node.
/// This node is used to mark the start and end of the list.
/// It is not counted in the size of the list.
fn new() -> Self {
Node {
data: None,
prev: None,
next: None,
}
}
/// Instantiate a new content node.
/// This node is used to store data.
/// It is counted in the size of the list.
fn from(data: T) -> Self {
Node {
data: Some(data),
prev: None,
next: None,
}
}
}
struct List<T> {
head: Rc<RefCell<Node<T>>>,
tail: Rc<RefCell<Node<T>>>,
size: usize,
}
impl<T> List<T> {
pub fn new() -> Self {
let head = Rc::new(RefCell::new(Node::new()));
let tail = Rc::new(RefCell::new(Node::new()));
head.borrow_mut().next = Some(Rc::clone(&tail));
tail.borrow_mut().prev = Some(Rc::clone(&head));
List { head, tail, size: 0 }
}
pub fn prepend(&self, data: T) {
let node = Rc::new(RefCell::new(Node::from(data)));
let mut head = self.head.borrow_mut();
node.borrow_mut().next = Some(head.next.take().unwrap());
node.borrow_mut().prev = Some(Rc::clone(&self.head));
head.next = Some(Rc::clone(&node));
if let Some(next) = node.borrow().next.as_ref() {
next.borrow_mut().prev = Some(Rc::clone(&node));
};
}
pub fn append(&self, data: T) {
let node = Rc::new(RefCell::new(Node::from(data)));
let mut tail = self.tail.borrow_mut();
node.borrow_mut().prev = Some(Rc::clone(&tail.prev.take().unwrap()));
node.borrow_mut().next = Some(Rc::clone(&self.tail));
tail.prev = Some(Rc::clone(&node));
if let Some(prev) = node.borrow().prev.as_ref() {
prev.borrow_mut().next = Some(Rc::clone(&node));
};
}
pub fn get(&self, index: isize) -> Result<T, &'static str> {
let mut current: Rc<RefCell<Node<T>>> = Rc::clone(self.head.borrow().next.as_ref().unwrap());
for _ in 0..index {
let tmp = Rc::clone(current.borrow().next.as_ref().ok_or("Index out of range")?);
current = tmp;
}
let result = current.borrow().data.as_ref().ok_or("Index out of range")?; // error[E0716]
Ok(*result) // error[E0507]
}
}
/*
error[E0716]: temporary value dropped while borrowed
--> src\linked.rs:74:22
|
74 | let result = current.borrow().data.as_ref().ok_or("Index out of range")?;
| ^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
| |
| creates a temporary value which is freed while still in use
75 | Ok(*result)
| ------- borrow later used here
|
help: consider using a `let` binding to create a longer lived value
|
74 ~ let binding = current.borrow();
75 ~ let result = binding.data.as_ref().ok_or("Index out of range")?;
|
error[E0507]: cannot move out of `*result` which is behind a shared reference
--> src\linked.rs:75:12
|
75 | Ok(*result)
| ^^^^^^^ move occurs because `*result` has type `T`, which does not implement the `Copy` trait
*/
我已经尝试过:
- 关于如何将参考转换为值的帖子, 但这正是我尝试做的却失败了。
- 关于修改
RefCell
的帖子, 但这并没有帮助。我试图返回它,而不是改变它。 - 关于如何借用
RefCell
的帖子, 但我不能返回借用的值,因为借用的Rc
寿命很短(但内部值不是)。 - 关于如何在
RefCell
中返回某个内容的帖子 和 关于如何使用.map()
返回帖子, 但当我尝试使用.into()
时编译器显示"trait bound not satisfied",如果我移除.into()
则借用检查器会报错"cannot move out"。 - 关于使用
Rc::try_unwarp()
的帖子, 但它不起作用,因为内部数据有多个所有者。
还有:如果其中一篇帖子解决了我的问题,但我没有正确实施,请原谅我,并请教我如何正确操作。非常感谢。