rust - 实现返回HashMap::IntoIter的IntoIterator时的“wrong number of type arguments”

Question

我有以下代码:

use std::{collections::HashMap, hash::Hash, rc::Rc};

#[derive(Debug)]
pub struct LFUCache<K: Hash + Eq, V> {
    values: HashMap<Rc<K>, ValueCounter<V>>,
    capacity: usize,
    min_frequency: usize,
}

#[derive(Debug)]
struct ValueCounter<V> {
    value: V,
    count: usize,
}

impl<K: Hash + Eq, V> IntoIterator for LFUCache<K, V> {
    type Item = (Rc<K>, V);
    type IntoIter = std::collections::HashMap::IntoIter<Rc<K>, V>;

    fn into_iter(self) -> Self::IntoIter {
        return self
            .values
            .into_iter()
            .map(|(key, valueCounter)| (key, valueCounter.value));
    }
}

它抛出一个错误，说:

error[E0107]: wrong number of type arguments: expected at least 2, found 0
  --> src/lib.rs:18:21
   |
18 |     type IntoIter = std::collections::HashMap::IntoIter<Rc<K>, V>;
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected at least 2 type arguments

我查看了文档，发现我的用例非常类似于 to the example。

我该如何解决？

Answer 1

HashMap::IntoIter 是 IntoIterator 特性的关联类型，没有任何类型参数。 HashMap可以:HashMap::<Rc<K>, V>::IntoIter。那是模棱两可的，因此您必须完全限定它:

type IntoIter = <std::collections::HashMap::<Rc<K>, V> as IntoIterator>::IntoIter;

通常表示为

type IntoIter = std::collections::hash_map::IntoIter<Rc<K>, V>;

然后，您遇到的问题是您试图对编译器撒谎，因为您没有返回该类型:

error[E0308]: mismatched types
  --> src/lib.rs:21:9
   |
21 | /         self.values
22 | |             .into_iter()
23 | |             .map(|(key, valueCounter)| (key, valueCounter.value))
   | |_________________________________________________________________^ expected struct `std::collections::hash_map::IntoIter`, found struct `std::iter::Map`
   |
   = note: expected struct `std::collections::hash_map::IntoIter<_, V>`
              found struct `std::iter::Map<std::collections::hash_map::IntoIter<_, ValueCounter<V>>, [closure@src/lib.rs:23:18: 23:65]>`

按照链接的问题提示，您最终得到

use std::{collections::hash_map::IntoIter, iter::Map};

impl<K: Hash + Eq, V> IntoIterator for LFUCache<K, V> {
    type Item = (Rc<K>, V);
    type IntoIter =
        Map<IntoIter<Rc<K>, ValueCounter<V>>, fn((Rc<K>, ValueCounter<V>)) -> (Rc<K>, V)>;

    fn into_iter(self) -> Self::IntoIter {
        fn xform<K, V>((key, vc): (Rc<K>, ValueCounter<V>)) -> (Rc<K>, V) {
            (key, vc.value)
        }

        self.values.into_iter().map(xform)
    }
}

也可以看看:

How do I return a Filter iterator from a function?

What is the correct way to return an Iterator (or any other trait)?