函数返回时，为什么从HashMap :: get借用没有结束? [英] Why does the borrow from `HashMap::get` not end when the function returns?

问题描述

这里是我的问题的模仿，当借贷的结账时间太晚了

Here is emulation of my problem, when a borrow ends too late

use std::collections::HashMap;

struct Item {
    capacity: u64
}

struct Petrol {
    name: String,
    fuel: HashMap<&'static str, Item>
}

fn buy_gaz(p: &mut Petrol) {
   match p.fuel.get("gaz") {
      Some(gaz) => {
        fire_petrol(p); 
      }
      None => ()
   }
}

fn fire_petrol(p: &mut Petrol) {
    println!("Boom!");
    p.fuel.remove("gaz");
    p.fuel.remove("benzin");
}

fn main() {
    let mut bt = Petrol {
        name: "Britii Petrovich".to_string(),
        fuel: HashMap::new()
    };

    bt.fuel.insert("gaz", Item { capacity: 1000 });
    bt.fuel.insert("benzin", Item { capacity: 5000 });

    buy_gaz(&mut bt);
}

编译时我得到:

note: previous borrow of `p.fuel` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `p.fuel` until the borrow ends
match p.fuel.get("gaz") {
      ^~~~~~

为什么借款这么晚才结束，而不是从HashMap::get退出?我该如何解决我的情况?

Why does the borrow end so late and not on exit from HashMap::get? How do I fix my case?

PS:我编辑了第一篇关于将结构添加到HashMap的文章，因为下面的决定适用于简单的类型(我认为具有默认的Clone特性)，但不适用于自定义结构

PS: I edited my first post for adding struct to HashMap, because decision below worked for simply types (with default Clone trait, I think), but doesn't work for custom structures

推荐答案

如果您查看

If you look at the documentation of HashMap::get you can see, that it returns an Option<&V>. The reference into the map allows you to do zero-copy accesses into a hash-map. The downside is, as long as you have a reference, you cannot modify the hash map as that might invalidate your reference.

分支Some(gaz)使绑定gaz具有类型&u64，其中引用指向您的哈希映射.如果将其更改为Some(&gaz)，则将获得值的副本而不是引用，并且甚至可以在该分支内部修改哈希映射.

The branch Some(gaz) causes the binding gaz to have type &u64, where the reference points into your hashmap. If you change that to Some(&gaz) you get a copy of the value instead of a reference, and may modify the hash map even inside that branch.

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