string - 连接字符串并将它们转换为 &str

Question

fn lyrics_more_bottles(song_template: &mut String, number: i32) {
    let mut template_partials = HashMap::new();
    let mut start_num = number.to_string();
    let mut remaining_num = (number - 1).to_string();

    template_partials.insert("start", start_num + " bottles");
    template_partials.insert("repeat", start_num + " bottles");
    template_partials.insert("remaining", remaining_num + " bottles");
    template_partials.insert("message", "Take one down and pass it around");

    resolve_template(song_template, template_partials);
}

我无法将这些连接的字符串转换为 &str .我试图将它们放在括号中并调用 .as_str() .没有运气。

我需要一个 HashMap<&str, &str>所以有一个String在这里不是一个选项。

Answer 1

如果您需要从函数返回&str，那您就不走运了，参见Return local String as a slice (&str) .

如果您只是需要将切片放入HashMap 并将其进一步向下传递，您需要将String 存储在某处并且用对 String 的引用填充 map :

use std::collections::HashMap;

fn resolve_template(_: &mut String, _: HashMap<&str, &str>) {}

fn lyrics_more_bottles(song_template: &mut String, number: i32) {
    let mut start_num = number.to_string();
    let mut remaining_num = (number - 1).to_string();

    let start = start_num.clone() + " bottles";
    let repeat = start_num + " bottles";
    let remaining = remaining_num + " bottles";

    let mut template_partials = HashMap::new();

    template_partials.insert("start", &*start);
    template_partials.insert("repeat", &*repeat);
    template_partials.insert("remaining", &*remaining);
    template_partials.insert("message", "Take one down and pass it around");

    resolve_template(song_template, template_partials);
}

fn main() {}

我们使用 &* 将 String Deref 为 str ，然后进行另一个引用以结束在 &str 处。

请注意，let mut template_partials 行必须在创建字符串后 移动，否则当删除字符串时，映射可能包含对无效内存的引用。此外，start_num 需要克隆，因为它被使用了两次。

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就个人而言，我只是使用 format! 并使用相同的字符串两次，而不是克隆和修改它两次:

use std::collections::HashMap;

fn resolve_template(_: &mut String, _: HashMap<&str, &str>) {}

fn lyrics_more_bottles(song_template: &mut String, number: i32) {
    let start_and_repeat = format!("{} bottles", number);
    let remaining = format!("{} bottles", number - 1);

    let mut template_partials = HashMap::new();

    template_partials.insert("start", &*start_and_repeat);
    template_partials.insert("repeat", &*start_and_repeat);
    template_partials.insert("remaining", &*remaining);
    template_partials.insert("message", "Take one down and pass it around");

    resolve_template(song_template, template_partials);
}

fn main() {}