我正在编写一个在线算法,通过一系列函数实现,这些函数使用迭代器并生成迭代器。
当我这样编写函数时(内容更复杂但在类型方面没有不同):
fn decode<'a, T>(input: T) -> impl Iterator<Item = i16> + 'a
where
T: Iterator<Item = &'a u8> + 'a,
{
input.map(|b| i16::from(*b)).filter(|i| *i != 0)
}
但是,这使得调用函数不符合人体工程学:
let input: Vec<u8> = vec![1, 2, 3, 0];
let v: Vec<i16> = decode(input.iter()).collect();
我更喜欢使用T: Into<Iterator<...
,但我不能。当我写签名时:
fn decode<'a, T>(input: T) -> impl Iterator<Item = i16> + 'a
where
T: Into<Iterator<Item = &'a u8>> + 'a,
我得到一个错误,说在返回类型的大小在编译时是不知道的:
error[E0277]: the size for values of type `(dyn std::iter::Iterator<Item=&'a u8> + 'static)` cannot be known at compilation time
--> src/main.rs:1:1
|
1 | / fn decode<'a, T>(input: T) -> impl Iterator<Item = i16> + 'a
2 | | where
3 | | T: Into<Iterator<Item = &'a u8>> + 'a,
4 | | {
5 | | input.into().map(|b| i16::from(*b)).filter(|i| *i != 0)
6 | | }
| |_^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `(dyn std::iter::Iterator<Item=&'a u8> + 'static)`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: required by `std::convert::Into`
为什么会这样,有更好的方法吗?
使用IntoIterator
代替:
fn decode<'a>(input: impl IntoIterator<Item = &'a u8> + 'a) -> impl Iterator<Item = i16> + 'a {
input.into_iter().map(|b| i16::from(*b)).filter(|i| *i != 0)
}
Iterator
是一个特征,性状没有大小。这就是为什么你不能(还)写:
fn example(x: Iterator<Item = ()>) {}
error[E0277]: the size for values of type `(dyn std::iter::Iterator<Item=()> + 'static)` cannot be known at compilation time
--> src/lib.rs:1:12
|
1 | fn example(x: Iterator<Item = ()>) {}
| ^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `(dyn std::iter::Iterator<Item=()> + 'static)`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: all local variables must have a statically known size
= help: unsized locals are gated as an unstable feature
Into
定义为:
pub trait Into<T> {
fn into(self) -> T;
}
实现Into<dyn Iterator>
的东西必须具有fn into(self) -> dyn Iterator
功能,返回一个特征。由于特征没有大小,因此不能(还)返回,存储在变量中或作为参数接受。
也可以看看: