我已经掌握了Rust生命周期的基础知识以及如何使用迭代器,但是仍然有一些麻烦来定义一个函数,它接受一个可迭代的元组并返回一个也在堆上分配的可迭代元组 .
(我知道"iterable"在Rust中没有任何意义但是我仍然会使用它而不是 IntoInterator
)
use std::iter::{once, repeat};
fn foo<'a, I>(costs: I) -> Box<Iterator<Item = &'a (i32, f32)>>
where
I: IntoIterator<Item = &'a (usize, f32)>,
{
let preliminary_examination_costs = once(10.0).chain(repeat(20.0));
let id_assignment_costs = once(10.0).chain(repeat(20.0));
let batch_fixed = once(10.0).chain(repeat(0.0));
let temp: Vec<(usize, &(i32, f32))> = costs.into_iter().enumerate().collect();
temp.sort_by_key(|&(_, &(n, cost))| n);
Box::new(temp.into_iter().map(|(i, &(n, cost))| {
(
i,
cost + preliminary_examination_costs.next().unwrap()
+ id_assignment_costs.next().unwrap() + batch_fixed.next().unwrap(),
)
}))
}
这是错误:
error[E0277]: the trait bound `std::vec::Vec<(usize, &(i32, f32))>: std::iter::FromIterator<(usize, &'a (usize, f32))>` is not satisfied
--> src/main.rs:10:73
|
10 | let temp: Vec<(usize, &(i32, f32))> = costs.into_iter().enumerate().collect();
| ^^^^^^^ a collection of type `std::vec::Vec<(usize, &(i32, f32))>` cannot be built from an iterator over elements of type `(usize, &'a (usize, f32))`
|
= help: the trait `std::iter::FromIterator<(usize, &'a (usize, f32))>` is not implemented for `std::vec::Vec<(usize, &(i32, f32))>`
error[E0271]: type mismatch resolving `<[closure@src/main.rs:12:35: 18:6 preliminary_examination_costs:_, id_assignment_costs:_, batch_fixed:_] as std::ops::FnOnce<((usize, &(i32, f32)),)>>::Output == &(i32, f32)`
--> src/main.rs:12:5
|
12 | / Box::new(temp.into_iter().map(|(i, &(n, cost))| {
13 | | (
14 | | i,
15 | | cost + preliminary_examination_costs.next().unwrap()
16 | | + id_assignment_costs.next().unwrap() + batch_fixed.next().unwrap(),
17 | | )
18 | | }))
| |_______^ expected tuple, found &(i32, f32)
|
= note: expected type `(usize, f32)`
found type `&(i32, f32)`
= note: required because of the requirements on the impl of `std::iter::Iterator` for `std::iter::Map<std::vec::IntoIter<(usize, &(i32, f32))>, [closure@src/main.rs:12:35: 18:6 preliminary_examination_costs:_, id_assignment_costs:_, batch_fixed:_]>`
= note: required for the cast to the object type `std::iter::Iterator<Item=&(i32, f32)>`
1 回答
在遇到问题时创建MCVE非常有用,并且's something I encourage all learners to undertake. Here'是一个镜像代码的MCVE:
您正在接受引用,然后根据引用创建一个全新的值 . 这意味着返回类型不再是引用,因此不支持您的函数签名 . 你需要改变它:
这将解锁另一个错误,因为编译器对
I::IntoIter
的具体类型了解不够:我们按照建议做并添加
'static
界限:也可以看看:
此错误是因为您有一个
(usize, &'a (usize, f32))
的迭代器,并且您正尝试将它们转换为(usize, &(i32, f32))
. 你不能转换这样的类型 .