为什么我需要为返回引用的函数添加生命周期? [英] Why do I need to add a lifetime to a function that returns a reference?

问题描述

我编写了以下代码:

const DIGIT_SPELLING: [&str; 10] = [
    "", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"
];

fn to_spelling_1(d: u8) -> &str {
    DIGIT_SPELLING[d as usize]
}

fn main() {
    let d = 1;
    let s = to_spelling_1(d);
    println!("{}", s);
}

这会导致以下编译器错误:

This gives the following compiler error:

error[E0106]: missing lifetime specifier
 --> src/main.rs:5:28
  |
5 | fn to_spelling_1(d: u8) -> &str {
  |                            ^ expected lifetime parameter
  |
  = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
  = help: consider giving it an explicit bounded or 'static lifetime

为解决此问题，我将代码更改为此:

To fix the problem, I changed my code to this:

const DIGIT_SPELLING: [&str; 10] = [
    "", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"
];

fn to_spelling_1<'a>(d: u8) -> &'a str { // !!!!! Added the lifetime. !!!!!
    DIGIT_SPELLING[d as usize]
}

fn main() {
    let d = 1;
    let s = to_spelling_1(d);
    println!("{}", s);
}

此代码编译并运行没有错误.为什么我需要增加'a 寿命?为什么添加'a 有效期可以解决该错误?

This code compiles and runs without error. Why did I need to add the 'a lifetime? Why does adding the 'a lifetime fix the error?

推荐答案

任何返回引用的函数都必须包含此引用的生存期.如果该函数还使用至少一个by-reference参数，则终身淘汰表示您可以省略返回寿命，但是如果没有by-reference参数，就不会出现省略，就像您的示例一样.

Any function that returns a reference must include a lifetime for this reference. If the function also takes at least one by-reference parameter then lifetime elision means that you can omit the return lifetime, but elision does not occur if there is no by-reference parameter, like in your example.

请注意，在您的情况下，使用显式的'static 生命周期而不是泛型会更有意义，因为您返回的值始终为'static :

Note that in your case, it would make more sense to use an explicit 'static lifetime rather than a generic since the value you return is always 'static:

fn to_spelling_1(d: u8) -> &'static str {
    DIGIT_SPELLING[d as usize]
}

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