rust - 循环引用的生命周期不够长

Question

我有一个类似循环图的结构，其中节点引用其他节点。为了简洁起见，我删除了一些内容。我刚开始学习 Rust，我用过 Rust borrowed pointers and lifetimes和 https://github.com/nrc/r4cppp/blob/master/graphs/src/rc_graph.rs作为引用

use std::cell::RefCell;
use std::rc::*;

fn main() {
    let node1 = Node::new(1);
    let node0 = Node::new(0);

    node0.borrow_mut().parent = Some(node1.clone());
    node1.borrow_mut().parent = Some(node0.clone());

    //works
    println!("Value of node0: {}", node0.borrow().value);

    //neither of the following work
    println!("Value of node0.parent: {}", node0.borrow().parent.as_ref().unwrap().borrow().value);
    println!("Value of node0: {}", node0.borrow().get_parent().borrow().value);
}

struct Node {
    value: i32,
    parent:  Option<Rc<RefCell<Node>>>
}

impl Node{

    fn new(val: i32) -> Rc<RefCell<Node>> {
        Rc::new(RefCell::new(Node {
            value: val,
            parent: None
        }))
    }


    fn get_parent(&self) -> Rc<RefCell<Node>> {
        self.parent.as_ref().unwrap().clone()
    }
}

我正在尝试输出节点父节点的值，但出现以下编译错误:

16 |     println!("Value of node0.parent: {}", node0.borrow().parent.as_ref().unwrap().borrow().value);
   |                                           ^^^^^^^^^^^^^^ does not live long enough

和

17 |     println!("Value of node0: {}", node0.borrow().get_parent().borrow().value);
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^ does not live long enough

我做错了什么？

Answer 1

您需要从 println! 调用中分离出借用:

// Borrow them separately, so their scopes are larger.
let n0_borrowed = node0.borrow();
let n1_borrowed = node1.borrow();
let n0_parent = n0_borrowed.parent.as_ref().unwrap();
let n1_parent = n1_borrowed.parent.as_ref().unwrap();

println!("Value of node0: {}", node0.borrow().value);
println!("Value of node0.parent: {}", n0_parent.borrow().value);

println!("Value of node1: {}",node1.borrow().value);
println!("Value of node1.parent: {}", n1_parent.borrow().value);

Here it is running in the Playground .

从本质上讲，当您将所有调用链接在一起时，借用的引用不会存在足够长的时间。将它们分离出来会增加它们的范围并延长它们的生命周期。