为什么“需要明确的生命周期限制"?对于 Box<T>指导? [英] Why &quot;explicit lifetime bound required&quot; for Box&lt;T&gt; in struct?

问题描述

编者注:此代码在 RFC 599 已实施，但答案中讨论的概念仍然有效.

Editor's note: This code no longer produces the same error after RFC 599 was implemented, but the concepts discussed in the answers are still valid.

我正在尝试编译此代码:

I'm trying to compile this code:

trait A {
    fn f(&self);
}

struct S {
    a: Box<A>,
}

我收到此错误:

a.rs:6:13: 6:14 error: explicit lifetime bound required
a.rs:6     a: Box<A>,

我希望 S.a 拥有 A 的一个实例，但不知道这个生命周期在这里是多么合适.我需要做什么才能让编译器满意?

I want S.a to own an instance of A, and don't see how that lifetime is appropriate here. What do I need to do to make the compiler happy?

我的 Rust 版本:

My Rust version:

rustc --version
rustc 0.12.0-pre-nightly (79a5448f4 2014-09-13 20:36:02 +0000)

推荐答案

这里的问题是也可以为引用实现 trait，所以如果你没有为 Box 指定所需的生命周期，任何东西都可以存储在那里.

The problem here is that a trait can be implemented for references too, so if you don't specify the required lifetime for Box anything could be stored in there.

您可以在此 rfc.

所以一种可能的解决方案是绑定生命周期所以 Send(我们把 I 放在 S 中):

So one possible solution is to bind the lifetime so Send (we put I in S):

trait A {
    fn f(&self);
}

struct I;

impl A for I {
    fn f(&self) {
        println!("A for I")
    }
}

struct S {
    a: Box<A + Send>
}

fn main() {
    let s = S {
        a: box I
    };
    s.a.f();
}

另一个是将生命周期设置为 'a(我们可以将引用 &I 或 I 设置为 S):

The other is setting the lifetime to 'a (we can put a reference &I or I to S):

trait A {
    fn f(&self);
}

struct I;

impl A for I {
    fn f(&self) {
        println!("A for I")
    }
}

impl <'a> A for &'a I {
    fn f(&self) {
        println!("A for &I")
    }
}

struct S<'a> {
    a: Box<A + 'a>
}

fn main() {
    let s = S {
        a: box &I
    };
    s.a.f();
}

请注意，这是更通用的，我们可以存储引用和拥有的数据(Send 类型，其生命周期为 'static)，但您需要在任何地方使用生命周期参数类型被使用.

Note that this is more general and we can store both references and owned data (Send kind that has a lifetime of 'static) but you require a lifetime parameter everywhere the type is used.

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