【发布时间】：2021-01-23 21:06:34
【问题描述】：
我正在学习 Rust，但我被一个玩具示例困住了。我已经阅读了关于生命周期的文档、post 和一堆关于 Stack Overflow 的问题。我已经花了一个多星期了，但还是卡住了，所以我决定向社区寻求帮助。
我有一个通用特征BookSide，它返回BookIterator（它扩展了通常的Iterator）。我对BookSide 和BookIterator 都有两个实现：ArrayBookSide 和CommissionBookSide。
- 第一个是有状态的。它在引擎盖下有一个
Vec。 - 第二个是无状态的：它包装了其他一些
BookSide。
我的目标只是编译整个内容。我正在解决问题并遵循编译器的建议。这个过程产生了以下代码。
use std::marker::PhantomData;
fn main() {
println!("Hello, world!");
}
// traits
pub trait BookIterator<'a>: Iterator<Item=f64> {}
pub trait BookSide<'a> {
type BookIteratorType: BookIterator<'a>;
fn book_iterator(&self) -> Self::BookIteratorType;
}
// implementation 1: stateful
pub struct ArrayBookSide {
quotes: Vec<f64>,
}
pub struct ArrayBookSideIterator<'a> {
quotes_iter: std::slice::Iter<'a, f64>,
}
impl<'a> BookSide<'a> for ArrayBookSide {
type BookIteratorType = ArrayBookSideIterator<'a>;
fn book_iterator(&self) -> Self::BookIteratorType {
ArrayBookSideIterator { quotes_iter: self.quotes.iter() }
}
}
impl<'a> Iterator for ArrayBookSideIterator<'a> {
type Item = f64;
fn next(&mut self) -> Option<Self::Item> {
self.quotes_iter.next().map(|"e| quote)
}
}
impl<'a> BookIterator<'a> for ArrayBookSideIterator<'a> {}
// implementation 2: delegating
pub struct CommissionBookSide<'a, B>
where B: BookSide<'a> {
base_book_side: B,
multiplier: f64,
_marker: PhantomData<&'a B>,
}
impl<'a, B> CommissionBookSide<'a, B>
where B: BookSide<'a> {
pub fn new(base_book_side: B) -> CommissionBookSide<'a, B> {
CommissionBookSide { base_book_side, multiplier: 1.1, _marker: PhantomData {} }
}
}
impl<'a, B> BookSide<'a> for CommissionBookSide<'a, B>
where B: BookSide<'a> {
type BookIteratorType = CommissionIterator<'a, B::BookIteratorType>;
fn book_iterator(&self) -> Self::BookIteratorType {
CommissionIterator {
base_iterator: self.base_book_side.book_iterator(),
multiplier: self.multiplier,
_marker: PhantomData {},
}
}
}
pub struct CommissionIterator<'a, BI>
where BI: BookIterator<'a> {
base_iterator: BI,
multiplier: f64,
_marker: PhantomData<&'a BI>,
}
impl<'a, BI> Iterator for CommissionIterator<'a, BI>
where BI: BookIterator<'a> {
type Item = BI::Item;
fn next(&mut self) -> Option<Self::Item> {
self.base_iterator.next().map(|quote| quote * self.multiplier)
}
}
impl<'a, BI> BookIterator<'a> for CommissionIterator<'a, BI>
where BI: BookIterator<'a> {}
现在我有以下编译错误。
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements
--> src/main.rs:31:58
|
31 | ArrayBookSideIterator { quotes_iter: self.quotes.iter() }
| ^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 30:5...
--> src/main.rs:30:5
|
30 | / fn book_iterator(&self) -> Self::BookIteratorType {
31 | | ArrayBookSideIterator { quotes_iter: self.quotes.iter() }
32 | | }
| |_____^
note: ...so that reference does not outlive borrowed content
--> src/main.rs:31:46
|
31 | ArrayBookSideIterator { quotes_iter: self.quotes.iter() }
| ^^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime `'a` as defined on the impl at 27:6...
--> src/main.rs:27:6
|
27 | impl<'a> BookSide<'a> for ArrayBookSide {
| ^^
note: ...so that the types are compatible
--> src/main.rs:30:55
|
30 | fn book_iterator(&self) -> Self::BookIteratorType {
| _______________________________________________________^
31 | | ArrayBookSideIterator { quotes_iter: self.quotes.iter() }
32 | | }
| |_____^
= note: expected `BookSide<'a>`
found `BookSide<'_>`
我可能不应该使用PhantomData？对我来说，这看起来过于复杂和解决方法。我已经发布了full code here。
【问题讨论】：
标签： generics rust iterator lifetime borrow-checker