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rust - 内部 Rust 结构可以传递对其所有者的引用吗？

问题描述

从副本中，@Shepmaster 在此答案中记录了一组真正出色的解决此问题的模式。谢谢你

AccountBank调用方法时需要访问的字段。Bank有deposit()调用set()帐户，但set()需要了解有关银行的信息，因此必须将银行传递到Account::set. 我确信有其他方法可以解决这个问题，这在 Rust 中更有意义。我很难找到更好的替代模式来替代我会用其他语言做的事情。

这是一个来自Twitch 流的最小示例，我尝试在 Rust 中创建一个简单的流行病学模型——如果我理解了，我将在下一个中给出答案。

fn main() {
    // create a bank and fill with accounts:
    let mut bank = Bank { accounts: vec![] };
    for i in 0..100 {
        bank.accounts.push(Account {
            id: i,
            balance: 0,
            mean_deviation: 0,
        });
    }

    // set the balance of an account
    bank.deposit(42, 10000);
}

// BANK:
struct Bank {
    accounts: Vec<Account>,
}

impl Bank {
    pub fn deposit(&mut self, id: usize, balance: i32) {
        let account = self.accounts.get_mut(id).unwrap();

        // this fails, because account needs access to information from the bank struct,
        // and we cannot borrow bank as mutable twice, or immutable when already mutable:
        account.set(balance, self);
    }
}

// ACCOUNT:
struct Account {
    id: i32,
    balance: i32,
    mean_deviation: i32,
}

impl Account {
    pub fn set(&mut self, balance: i32, bank: &Bank) {
        self.balance = balance;

        /* use bank to calculate how far from the mean account value this account is */
    }
}

error[E0502]: cannot borrow `*self` as immutable because it is also borrowed as mutable
  --> src/main.rs:27:30
   |
23 |         let account = self.accounts.get_mut(id).unwrap();
   |                       ------------- mutable borrow occurs here
...
27 |         account.set(balance, self);
   |                 ---          ^^^^ immutable borrow occurs here
   |                 |
   |                 mutable borrow later used by call

标签： rust

解决方案

老实说，最好的方法是尽可能避免它。循环引用一点都不好。

这是另一种思考方式：通过使用Cell包装帐户余额的 a ，您可以根据需要读取/写入新值。

use std::cell::Cell;
fn main() {
    // create a bank and fill with accounts:
    let mut bank = Bank { accounts: vec![] };
    for i in 0..100 {
        bank.accounts.push(Account {
            id: i,
            balance: Cell::new(0),
            mean_deviation: Cell::new(0),
        });
    }

    // set the balance of an account
    bank.deposit(42, 10000);
}

// BANK:
struct Bank {
    accounts: Vec<Account>,
}

impl Bank {
    pub fn deposit(&self, id: usize, balance: i32) {
        let account = self.accounts.get(id).unwrap();
        account.set(balance, &self);
    }
}

// ACCOUNT:
struct Account {
    id: i32,
    balance: Cell<i32>,
    mean_deviation: Cell<i32>,
}

impl Account {
    pub fn set(&self, balance: i32, bank: &Bank) {
        let old_balance = self.balance.get();
        self.balance.set(old_balance + balance);
        /* use bank to calculate how far from the mean account value this account is */
        let _ = bank.accounts.len();
    }
}

如果出于某种原因您愿意在多线程中使用它，则应该将atomic类型用于简单的原语。

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