迭代器收集的类型问题 [英] Type issue with Iterator collect

问题描述

我正在尝试使用以下代码片段将 &str 对的向量转换为 HashMap:

I am trying to convert a vector of &str pairs into a HashMap with the following code snippet:

use std::collections::HashMap;

fn main() {
  let pairs = vec!(("foo", "bar"), ("toto", "tata"));
  let map: HashMap<&str, &str> = pairs.iter().collect();
  println!("{:?}", map);
}

但是编译失败并出现此错误:

However the compilation fails with this error:

<anon>:5:47: 5:56 error: the trait `core::iter::FromIterator<&(&str, &str)>` is not implemented for the type `std::collections::hash::map::HashMap<&str, &str>` [E0277]
<anon>:5   let map: HashMap<&str, &str> = pairs.iter().collect();

但是，如果我在调用 collect() 之前添加 .cloned() 一切正常:

However if I add .cloned() before calling collect() everything works fine:

...
let map: HashMap<&str, &str> = pairs.iter().cloned().collect();
...

即使我理解错误消息(没有实现 FromIterator<&(&str, &str)> 类型的特征str, &str>) 我不明白类型 &(&str, &str) 来自哪里(根据 Rust 文档中的方法签名)和为什么调用 cloned() 可以解决这个问题.

Even if I understand the error message (there is no implementation of the trait FromIterator<&(&str, &str)> for the type HashMap<&str, &str>) I do not understand where the type &(&str, &str) comes from (according to the method signature in the Rust documentation) and why calling cloned() fixes that problem.

推荐答案

类型&(&str, &str)来自什么iter() 返回:

The type &(&str, &str) comes from what iter() on a Vec returns:

fn iter(&self) -> Iter<T>

在哪里 Iter<T> 实现 Iterator<Item=&T>:

impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T
    ...
}

换句话说，向量上的 iter() 返回一个迭代器，产生对该向量的引用.

In other words, iter() on a vector returns an iterator yielding references into the vector.

cloned() 解决了这个问题，因为它是一个迭代器适配器，它将 Iterator 转换为 Iterator 如果 T 是可克隆的.您可以将其视为 map(|v| v.clone()) 的简写:

cloned() solves the problem because it is an iterator adapter which converts Iterator<Item=&T> to Iterator<Item=T> if T is cloneable. You can think of it as a shorthand for map(|v| v.clone()):

let v1: Vec<i32> = vec![1, 2, 3, 4];
let v2: Vec<_> = v1.iter().cloned().collect();
let v3: Vec<_> = v1.iter().map(|v| v.clone()).collect();
assert_eq!(v2, v3);

碰巧 (&str, &str) 是可克隆的，因为每个元组组件也是可克隆的(所有引用都是)，所以 cloned() 会返回一个实现 Iterator<Item=(&str, &str)> 的对象——正是 collect() 需要创建一个 HashMap.

It happens that (&str, &str) is cloneable because each tuple component is also cloneable (all references are), so cloned() would return an object which implements Iterator<Item=(&str, &str)> - exactly what collect() needs to create a HashMap.

或者，您可以使用 into_iter() 从 Vec 获取 Iterator，然后使用原始的向量将被消耗:

Alternatively, you can use into_iter() to get Iterator<Item=T> from Vec<T>, but then the original vector will be consumed:

use std::collections::HashMap;

fn main() {
    let pairs = vec!(("foo", "bar"), ("toto", "tata"));
    let map: HashMap<&str, &str> = pairs.into_iter().collect();
    println!("{:?}", map);
}

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