什么时候需要在Rust中指定显式生存期?

问题描述

如果我有两个功能

// implicit
fn foo(x: &i32) {
}

// explicit
fn bar<'a>(x: &'a i32) {
}

什么时候foo返回错误并且bar是正确的函数头?我对为什么会明确声明生命周期感到困惑:

When would foo return an error and bar be the correct function header? I'm confused as to why I would explicitly declare a lifetime:

我了解生命周期是什么，但是为我明确指定生命周期'a 做什么是什么?作为参考，我使用锈皮书作为阅读材料

I understand what a lifetime is, but what does explicitly specifying a lifetime 'a do for me? For reference I'm using the Rust book as reading material

推荐答案

实际上，必须编写生命周期注释的#1原因是，因为编译器会要求您.它将拒绝终身淘汰规则未涵盖的功能签名. >.

Practically speaking, the #1 reason you'll have to write lifetime annotations is because the compiler asks you so. It will reject function signatures which are not covered by lifetime elision rules.

我假设您想要一个简单的示例，其中必须使用生命周期.想象以下情况:

I assume you would like an simple example where lifetimes are mandatory. Imagine the following scenario:

struct Blah<'a> {
    hoy: &'a u8
}

fn want_a_hoy(blah: &Blah) -> &u8 {
    blah.hoy
}

意图很明显，但是编译器无法处理它:

The intention is obvious, but the compiler doesn't handle it:

<anon>:7:35: 7:38 error: missing lifetime specifier [E0106]
<anon>:7     fn want_a_hoy(blah: &Blah) -> &u8 {
                                           ^~~
<anon>:7:35: 7:38 help: see the detailed explanation for E0106
<anon>:7:35: 7:38 help: this function's return type contains a borrowed value, but
                        the signature does not say which one of `blah`'s 2 elided
                        lifetimes it is borrowed from

在这种情况下，注释可以解决问题:

In this case, annotations solve the problem:

fn want_a_hoy<'a, 'b>(blah: &'b Blah<'a>) -> &'a u8 {
    blah.hoy
}

在这里您两次指定'a(在Blah<'a>和&'a上).这是一辈子！因此，您在这里对编译器说的是:此函数对包含内部引用的blah进行引用.我将返回与该blah的内部引用一样长的生命."在这种情况下，签名强烈地暗示了您很可能会返回来自blah内幕的东西.

Here you're specifying 'a twice (on Blah<'a> and &'a). This is the same lifetime! So what you're saying to the compiler here is: "This function takes a reference to a blah containing an inner reference. I will return something which lives exactly as long as the inner reference of the blah." In this case, the signature gives a strong hint that you're likely to return something coming from the innards of the blah.

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