Question

我看过多个帖子，例如this或this，但我认为这不重复。我想我还不太明白如何使用寿命比彼此更长寿。这是一个MWE：

struct Point;

pub struct Line<'a> {
    pub start: &'a Point,
    pub end: &'a Point,
}

impl<'a> Line<'a> {
    pub fn new(start: &Point, end: &Point) -> Self {
        Line {
            start: start,
            end: end,
        }
    }
}

fn main() {}

我收到错误消息

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> src/main.rs:10:9
   |
10 |         Line {
   |         ^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 9:51...
  --> src/main.rs:9:52
   |
9  |       pub fn new(start: &Point, end: &Point) -> Self {
   |  ____________________________________________________^
10 | |         Line {
11 | |             start: start,
12 | |             end: end,
13 | |         }
14 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:12:18
   |
12 |             end: end,
   |                  ^^^
note: but, the lifetime must be valid for the lifetime 'a as defined on the body at 9:51...
  --> src/main.rs:9:52
   |
9  |       pub fn new(start: &Point, end: &Point) -> Self {
   |  ____________________________________________________^
10 | |         Line {
11 | |             start: start,
12 | |             end: end,
13 | |         }
14 | |     }
   | |_____^
note: ...so that expression is assignable (expected Line<'a>, found Line<'_>)
  --> src/main.rs:10:9
   |
10 | /         Line {
11 | |             start: start,
12 | |             end: end,
13 | |         }
   | |_________^

我完全迷失了如何解释它。

Answer 1

您需要明确指定两个参数的生命周期，使它们相同：

impl<'a> Line<'a> {
    pub fn new(start: &'a Point, end: &'a Point) -> Self {
        Line {
            start: start,
            end: end,
        }
    }
}

否则编译器无法决定为输出选择哪个输入生命周期。我推荐相关的Rust Book section on lifetime elision，尤其是以下3条规则：

函数参数中的每个省略生命周期变为不同的生命周期   参数。


如果只有一个输入生命周期，无论是否已经过去，那就是生命周期   被赋予所有退出生命周期的返回值   功能



如果有多个输入生命周期，但其中一个是＆amp; self或   ＆amp; mut self，self的生命周期分配给所有省略的输出   寿命。



否则，忽略输出生命周期是错误的。

在构建具有相同生命周期的多个引用的结构时，无法推断出适当的生命周期

1 个答案: