我想写一个泛型函数,它接受任何不可变的可移植容器,比如数组、Vec
、BTreeSet
等等.因为这个函数是我正在实现的特性的一部分,所以我无法更改它的签名,所以不可能直接将迭代器作为参数,我也不能在函数签名中引入任何生命周期参数.
上下文
我试图在Rust中实现观察者模式.可观察者和观察者如下所示:
struct Observable<T> {
value: T,
}
impl<T> Observable<T> {
pub fn get(&self) -> &T {
&self.value
}
}
trait Observer<T> {
fn update(&self, &Observable<T>);
}
(省略了一些与我的问题无关的函数)
现在,我的目标是编写一个观察器,它可以与任意iterable容器一起使用,这些容器包含可以赋值的项.它应该跟踪容器中项目值的总和,因此保存当前总和和计算任何项目值的函数.它应该实现Observer
特征,这样每次容器更改时都可以更新总和.
use std::cell::RefCell;
struct SumObserver<T> {
current_sum: RefCell<i64>,
get_value: Fn(&T) -> i64,
}
到目前为止
很长一段时间以来,我一直试图编译update
函数,但没有成功.以下是我try 过的函数的一个版本:
impl<'a, T, L> Observer<L> for SumObserver<T>
where
&'a L: IntoIterator<Item = &'a T>,
{
fn update(&self, observable: &Observable<L>) {
let mut sum: i64 = 0;
for item in observable.get() {
sum += (self.get_value)(item);
}
*self.current_sum.borrow_mut() = sum;
}
}
然而,编译器抱怨参数类型T
和L
的生命周期 可能不够长:
error[E0309]: the parameter type `T` may not live long enough
--> src/lib.rs:22:1
|
22 | impl<'a, T, L> Observer<L> for SumObserver<T>
| ^ - help: consider adding an explicit lifetime bound `T: 'a`...
| _|
| |
23 | | where
24 | | &'a L: IntoIterator<Item = &'a T>,
25 | | {
... |
32 | | }
33 | | }
| |_^
|
note: ...so that the reference type `&'a T` does not outlive the data it points at
--> src/lib.rs:22:1
|
22 | / impl<'a, T, L> Observer<L> for SumObserver<T>
23 | | where
24 | | &'a L: IntoIterator<Item = &'a T>,
25 | | {
... |
32 | | }
33 | | }
| |_^
如果整个函数体被注释掉,错误消息甚至保持不变.如果我也删除where
条款,编译就可以了.
如果我按照编译器的建议为参数类型添加显式的生存期界限:
impl<'a, T: 'a, L: 'a> Observer<L> for SumObserver<T>
编译器给出以下错误:
error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
--> src/lib.rs:28:32
|
28 | for item in observable.get() {
| ^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the method body at 26:5...
--> src/lib.rs:26:5
|
26 | / fn update(&self, observable: &Observable<L>) {
27 | | let mut sum: i64 = 0;
28 | | for item in observable.get() {
29 | | sum += (self.get_value)(item);
30 | | }
31 | | *self.current_sum.borrow_mut() = sum;
32 | | }
| |_____^
note: ...so that reference does not outlive borrowed content
--> src/lib.rs:28:21
|
28 | for item in observable.get() {
| ^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 22:6...
--> src/lib.rs:22:6
|
22 | impl<'a, T: 'a, L: 'a> Observer<L> for SumObserver<T>
| ^^
= note: ...so that the types are compatible:
expected std::iter::IntoIterator
found std::iter::IntoIterator
我不明白这个函数的生命周期 有什么问题.在调用此函数的任何时候,编译器都应该确保borrow observable
至少持续到函数返回为止.当时,任何observable
英镑的借款都超出了范围.