获取 HashMap 条目，如果没有则添加它

Question

我想做这样的事情：

fn some_fn() {
    let mut my_map = HashMap::from([
        (1, "1.0".to_string()),
        (2, "2.0".to_string()),
    ]);

    let key = 3;

    let res = match my_map.get(&key) {
        Some(child) => child,
        None => {
            let value = "3.0".to_string();
            my_map.insert(key, value);
            &value // HERE IT FAILS
        }
    };

    println!("{}", res);
}

但编译时出现错误：

error[E0597]: `value` does not live long enough
  --> src/lib.rs:16:13
   |
16 |             &value // HERE IT FAILS
   |             ^^^^^^
   |             |
   |             borrowed value does not live long enough
   |             borrow later used here
17 |         }
   |         - `value` dropped here while still borrowed

error[E0382]: borrow of moved value: `value`
  --> src/lib.rs:16:13
   |
14 |             let value = "3.0".to_string();
   |                 ----- move occurs because `value` has type `String`, which does not implement the `Copy` trait
15 |             my_map.insert(key, value);
   |                                ----- value moved here
16 |             &value // HERE IT FAILS
   |             ^^^^^^ value borrowed here after move

^操场

我怎样才能优雅地修复它？在我看来，复制字符串似乎不是最佳选择。

Answer 1

这里的问题是，您插入value字符串然后尝试返回对它的引用，尽管它已经被移动到 HashMap 中。您真正想要的是插入值，然后获取对插入值的引用，这可能如下所示：

let res = match my_map.get(&key) {
    Some(child) => child,
    None => {
        let value = "3.0".to_string();
        my_map.insert(key, value);
        my_map.get(&key).unwrap() // unwrap is guaranteed to work
    }
};

但不要这样做。它丑陋且缓慢，因为它必须在地图中查找密钥两次。Rust 有一个方便的Entry类型，可以让你获取 a 的条目HashMap，然后对其执行操作：

// or_insert returns the value if it exists, otherwise it inserts a default and returns it
let res = my_map.entry(key).or_insert("3.0".to_string());

或者，如果生成默认值的操作成本很高，您可以使用or_insert_with传递闭包：

// this won't call "3.0".to_string() unless it needs to
let res = my_map.entry(key).or_insert_with(|| "3.0".to_string());