从同一个 HashMap 中借用两个可变值

问题描述

我有以下代码:

use std::collections::{HashMap, HashSet};

fn populate_connections(
    start: i32,
    num: i32,
    conns: &mut HashMap<i32, HashSet<i32>>,
    ancs: &mut HashSet<i32>,
) {
    let mut orig_conns = conns.get_mut(&start).unwrap();
    let pipes = conns.get(&num).unwrap();

    for pipe in pipes.iter() {
        if !ancs.contains(pipe) && !orig_conns.contains(pipe) {
            ancs.insert(*pipe);
            orig_conns.insert(*pipe);
            populate_connections(start, num, conns, ancs);
        }
    }
}

fn main() {}

逻辑不是很重要，我正在尝试创建一个函数，它会自行遍历管道.

The logic is not very important, I'm trying to create a function which will itself and walk over pipes.

我的问题是这不能编译:

My issue is that this doesn't compile:

error[E0502]: cannot borrow `*conns` as immutable because it is also borrowed as mutable
  --> src/main.rs:10:17
   |
9  |     let mut orig_conns = conns.get_mut(&start).unwrap();
   |                          ----- mutable borrow occurs here
10 |     let pipes = conns.get(&num).unwrap();
   |                 ^^^^^ immutable borrow occurs here
...
19 | }
   | - mutable borrow ends here

error[E0499]: cannot borrow `*conns` as mutable more than once at a time
  --> src/main.rs:16:46
   |
9  |     let mut orig_conns = conns.get_mut(&start).unwrap();
   |                          ----- first mutable borrow occurs here
...
16 |             populate_connections(start, num, conns, ancs);
   |                                              ^^^^^ second mutable borrow occurs here
...
19 | }
   | - first borrow ends here

我不知道如何让它工作.一开始，我试图将两个 HashSet 存储在 HashMap(orig_conns 和 pipes)中.

I don't know how to make it work. At the beginning, I'm trying to get two HashSets stored in a HashMap (orig_conns and pipes).

Rust 不会让我同时拥有可变和不可变变量.我有点困惑，因为这将是完全不同的对象，但我想如果 &start == &num，那么我会有两个不同的引用对象(一个可变，一个不可变).

Rust won't let me have both mutable and immutable variables at the same time. I'm confused a bit because this will be completely different objects but I guess if &start == &num, then I would have two different references to the same object (one mutable, one immutable).

没关系，但是我怎样才能做到这一点?我想遍历一个 HashSet 并读取和修改另一个.让我们假设它们不是相同的 HashSet.

Thats ok, but then how can I achieve this? I want to iterate over one HashSet and read and modify other one. Let's assume that they won't be the same HashSet.

推荐答案

如果您可以更改数据类型和函数签名，则可以使用 RefCell 来创建 内部可变性:

If you can change your datatypes and your function signature, you can use a RefCell to create interior mutability:

use std::cell::RefCell;
use std::collections::{HashMap, HashSet};

fn populate_connections(
    start: i32,
    num: i32,
    conns: &HashMap<i32, RefCell<HashSet<i32>>>,
    ancs: &mut HashSet<i32>,
) {
    let mut orig_conns = conns.get(&start).unwrap().borrow_mut();
    let pipes = conns.get(&num).unwrap().borrow();

    for pipe in pipes.iter() {
        if !ancs.contains(pipe) && !orig_conns.contains(pipe) {
            ancs.insert(*pipe);
            orig_conns.insert(*pipe);
            populate_connections(start, num, conns, ancs);
        }
    }
}

fn main() {}

请注意，如果 start == num，线程将恐慌，因为这是尝试对同一 HashSet 进行可变和不可变访问.

Note that if start == num, the thread will panic because this is an attempt to have both mutable and immutable access to the same HashSet.

根据您的确切数据和代码需求，您还可以使用诸如 Cell 或 原子.它们比 RefCell 具有更低的内存开销，并且对代码生成的影响很小.

Depending on your exact data and code needs, you can also use types like Cell or one of the atomics. These have lower memory overhead than a RefCell and only a small effect on codegen.

在多线程情况下，您可能希望使用 Mutex 或 RwLock.

In multithreaded cases, you may wish to use a Mutex or RwLock.

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