Question

我正在尝试为一个结构实现Iterator特性，该结构充当i32值数组的借用者，但我一直在编译器中抱怨无法推断出一生在下一个方法中。

我知道Need help understanding Iterator lifetimes，但由于我的结构只是借用了数组的一部分，我将实际元素的记忆与IntegerArrayBag分开。

#[derive(Debug)]
struct IntegerArrayBag<'a> {
    arr: &'a [i32],
    idx: usize,
}

impl<'a> IntegerArrayBag<'a> {
    fn len(&self) -> usize {
        self.arr.len()
    }

    fn get(&self, idx: usize) -> Option<&i32> {
        if self.arr.len() > idx {
            Some(&self.arr[idx])
        } else {
            None
        }
    }
}

impl<'a> Iterator for IntegerArrayBag<'a> {
    type Item = &'a i32;

    fn next(&mut self) -> Option<&'a i32> {
        let idx = self.idx;
        self.idx += 1;
        self.get(idx)
    }
}

如果我尝试编译this code，编译器会抱怨：

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
  --> src/main.rs:27:14
   |
27 |         self.get(idx)
   |              ^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 24:5...
  --> src/main.rs:24:5
   |
24 | /     fn next(&mut self) -> Option<&'a i32> {
25 | |         let idx = self.idx;
26 | |         self.idx += 1;
27 | |         self.get(idx)
28 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:27:9
   |
27 |         self.get(idx)
   |         ^^^^
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 21:1...
  --> src/main.rs:21:1
   |
21 | / impl<'a> Iterator for IntegerArrayBag<'a> {
22 | |     type Item = &'a i32;
23 | |
24 | |     fn next(&mut self) -> Option<&'a i32> {
...  |
28 | |     }
29 | | }
   | |_^
note: ...so that expression is assignable (expected std::option::Option<&'a i32>, found std::option::Option<&i32>)
  --> src/main.rs:27:9
   |
27 |         self.get(idx)
   |         ^^^^^^^^^^^^^

Answer 1

您需要更新get方法以返回具有更长使用寿命的参考：

// Use 'a from impl<'a> IntegerArrayBag<'a>
fn get(&self, idx: usize) -> Option<&'a i32> {

然后它将编译。

Answer 2

将get(…)更改为返回Option<&'a i32>会使其编译。

游乐场网址：https://play.rust-lang.org/?gist=10783e90287b7111c126&version=stable

Gist网址：https://gist.github.com/10783e90287b7111c126

从Iterator实现调用方法时，无法推断autoref的适当生命周期

2 个答案: