生命周期注解

A reference has a lifetime, which must not “outlive” the value it refers to. This is verified by the borrow checker.

The lifetime can be implicit - this is what we have seen so far. Lifetimes can also be explicit: &'a Point, &'document str. Lifetimes start with ' and 'a is a typical default name. Read &'a Point as “a borrowed Point which is valid for at least the lifetime a“.

Lifetimes are always inferred by the compiler: you cannot assign a lifetime yourself. Explicit lifetime annotations create constraints where there is ambiguity; the compiler verifies that there is a valid solution.

当考虑向函数传递值和从函数返回值时，生命周期会变得更加复杂。

  1. #[derive(Debug)]
  2. struct Point(i32, i32);
  4. fn left_most(p1: &Point, p2: &Point) -> &Point {
  5.     if p1.0 < p2.0 {
  6.         p1
  7.     } else {
  8.         p2
  9.     }
  10. }
  12. fn main() {
  13.     let p1: Point = Point(10, 10);
  14.     let p2: Point = Point(20, 20);
  15.     let p3 = left_most(&p1, &p2); // What is the lifetime of p3?
  16.     println!("p3: {p3:?}");
  17. }

This slide should take about 10 minutes.

In this example, the compiler does not know what lifetime to infer for p3. Looking inside the function body shows that it can only safely assume that p3‘s lifetime is the shorter of p1 and p2. But just like types, Rust requires explicit annotations of lifetimes on function arguments and return values.

将 'a 适当添加到 left_most 中：

  1. fn left_most<'a>(p1: &'a Point, p2: &'a Point) -> &'a Point {

这表示.，“假设 p1 和 p2 的存在时间都比 'a 更长，则返回值至少在 'a 内有效”。

在一般情况下，可以省略生命周期，如下一张幻灯片中所述。