generics - 如何对泛型添加约束

Question

我很难弄清楚如何限制泛型类型。看起来 K 需要实现 core::cmp::Eq和 core::hash::Hash特质。我一直无法在文档中找到所需的语法。

use std::collections::HashMap;

struct Foo<K, V> {
    map: HashMap<K, V>,
}

impl<K, V> Foo<K, V> {
    fn insert_something(&mut self, k: K, v: V) {
        self.map.insert(k, v);
    }
}

编译错误是:

error[E0599]: no method named `insert` found for struct `std::collections::HashMap<K, V>` in the current scope
 --> src/lib.rs:9:18
  |
9 |         self.map.insert(k, v);
  |                  ^^^^^^ method not found in `std::collections::HashMap<K, V>`
  |
  = note: the method `insert` exists but the following trait bounds were not satisfied:
          `K: std::cmp::Eq`
          `K: std::hash::Hash`
// Edit note, the question is old so the error message from the compiler already hint about the answer, but ignore that.

我在哪里可以为 K 添加约束？

Answer 1

首先你可以导入哈希特征，use std::hash::Hash; .

您可以在 impl 上添加约束:

impl<K: Eq + Hash, V> Foo<K, V>

或者，使用新的“where”语法

impl<K, V> Foo<K, V>
where
    K: Eq + Hash,

可以引用book chapter on trait bound有关约束的更多上下文。