关于Rust HashMap和String借用的困惑 [英] Confusion about Rust HashMap and String borrowing

问题描述

此程序接受整数N，后接N行，其中包含由空格分隔的两个字符串。我想使用第一个字符串作为键，第二个字符串作为值，将这些行放入 HashMap 中：

This program accepts an integer N, followed by N lines containing two strings separated by a space. I want to put those lines into a HashMap using the first string as the key and the second string as the value:

use std::collections::HashMap;
use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input)
        .expect("unable to read line");
    let desc_num: u32 = match input.trim().parse() {
        Ok(num) => num,
        Err(_) => panic!("unable to parse")
    };

    let mut map = HashMap::<&str, &str>::new();
    for _ in 0..desc_num {
        input.clear();
        io::stdin().read_line(&mut input)
            .expect("unable to read line");
        let data = input.split_whitespace().collect::<Vec<&str>>();
        println!("{:?}", data);
        // map.insert(data[0], data[1]);
    }
}

程序按预期工作：

3
a 1
["a", "1"]
b 2
["b", "2"]
c 3
["c", "3"]

当我尝试将解析后的字符串放入 HashMap 并取消注释 map.insert（data [0]，data [1]）; ，编译失败并显示以下错误：

When I try to put those parsed strings into a HashMap and uncomment map.insert(data[0], data[1]);, the compilation fails with this error:

error: cannot borrow `input` as mutable because it is also borrowed as immutable [E0502]
        input.clear();
        ^~~~~
note: previous borrow of `input` occurs here; the immutable borrow prevents subsequent moves or mutable borrows of `input` until the borrow ends
        let data = input.split_whitespace().collect::<Vec<&str>>();
                   ^~~~~
note: previous borrow ends here
fn main() {
...
}
^

我不明白为什么会出现此错误，因为我认为 map.insert（ ）表达式根本不借用字符串 input 。

I don't understand why this error would come up, since I think the map.insert() expression doesn't borrow the string input at all.

推荐答案

split_whitespace（）不会给您两个新的 String 包含（副本）输入的非空白部分。而是在类型为& str 的 input 管理的内存中获得两个引用。因此，当您随后尝试清除 input 并读取输入的下一行时，尝试覆盖哈希映射仍在使用的内存。

split_whitespace() doesn't give you two new Strings containing (copies of) the non-whitespace parts of the input. Instead you get two references into the memory managed by input, of type &str. So when you then try to clear input and read the next line of input into it, you try overwriting memory that's still being used by the hash map.

为什么 split_whitespace （以及许多其他字符串方法，我应该添加）通过返回& str <使事情复杂化/ code>？因为通常就足够了，在这种情况下，避免不必要的复制。但是，在这种特定情况下，最好最好显式复制字符串的相关部分：

Why does split_whitespace (and many other string methods, I should add) complicate matters by returning &str? Because it's often enough, and in those cases it avoid unnecessary copies. In this specific case however, it's probably best to explicitly copy the relevant parts of the string:

map.insert(data[0].clone(), data[1].clone());

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