我如何在' IntoIterator ' impl中指定' map '的类型?

Question

我有以下代码:

struct Container<T>(T);
struct MyStruct<T> {
entries: Vec<T>,
}
impl<'a, T> IntoIterator for &'a MyStruct<Container<T>> {
type Item = &'a Container<T>;
type IntoIter = std::slice::Iter<'a, Container<T>>;
fn into_iter(self) -> Self::IntoIter {
let entries = &self.entries;
entries.into_iter()
}
}
fn main() {
let my_struct: MyStruct<Container<usize>> = MyStruct {
entries: vec![
Container(0),
Container(1),
Container(2),
Container(3),
Container(4),
],
};

for c in my_struct.into_iter() {
println!("{}", c.0);
// I want c to be usize not Container<usize>.
println!("{}", c); 
}
}

我希望从into_iter()得到的迭代器产生Container的内部值。

我试过了:

impl<'a, T> IntoIterator for &'a MyStruct<Container<T>> {
type Item = &'a T;
type IntoIter = std::slice::Iter<'a, T>;
fn into_iter(self) -> Self::IntoIter {
let entries = &self.entries;

// Map to the inner value.
entries.into_iter().map(|c| c.0)
}
}

但是它不会编译下面的错误:

error[E0308]: mismatched types
--> src/main.rs:25:9
|
21 |     fn into_iter(self) -> Self::IntoIter {
|                           -------------- expected `std::slice::Iter<'_, T>` because of return type
...
25 |         entries.into_iter().map(|c| c.0)
|         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::slice::Iter`, found struct `Map`
|
= note: expected struct `std::slice::Iter<'_, T>`
found struct `Map<std::slice::Iter<'_, Container<T>>, [closure@src/main.rs:25:33: 25:40]>`

如果我听从建议，我就会得到越来越复杂的错误。

Rust playground code

Answer 1

这行得通:

struct Container<T>(T);
struct MyStruct<T> {
entries: Vec<T>,
}
impl<T> IntoIterator for MyStruct<Container<T>> {
type Item = T;
type IntoIter = std::iter::Map<std::vec::IntoIter<Container<T>>, fn(Container<T>) -> T>;
fn into_iter(self) -> Self::IntoIter {
self.entries.into_iter().map(|c| c.0)
}
}
fn main() {
let my_struct: MyStruct<Container<usize>> = MyStruct {
entries: vec![Container(0), Container(1), Container(2), Container(3)],
};
for c in my_struct.into_iter() {
println!("{}", c);
}
}

Answer 2

您可以实现Deref，然后您只需要添加一些星号(*)来Deref:

use std::ops::Deref;
struct Container<T>(T);
impl<T> Deref for Container<T> {
type Target = T;
fn deref(&self) -> &Self::Target {
&self.0
}
}
struct MyStruct<T> {
entries: Vec<T>,
}
impl<T> Deref for MyStruct<T> {
type Target = Vec<T>;
fn deref(&self) -> &Self::Target {
&self.entries
}
}
fn main() {
let my_struct: MyStruct<Container<usize>> = MyStruct {
entries: vec![Container(0), Container(1), Container(2), Container(3)],
};
for c in my_struct.iter() {
println!("{}", c.0);
println!("{}", **c);
}
}