如何为我自己的结构实现 Eq 和 Hash 以将它们用作 HashMap 键? [英] How to implement Eq and Hash for my own structs to use them as a HashMap key?

问题描述

我有两个结构，A 和 B，我想使用 HashMap<A, B>.我有一段这样的代码:

I have two structs, A and B, and I want to use a HashMap<A, B>. I have a piece of code like this:

use std::collections::HashMap;

pub struct A {
    x: i32,
    y: i32,
    title: String,
}

pub struct B {
    a: u32,
    b: u32,
}

fn main() {
    let map = HashMap::new();
    map.insert(
        A {
            x: 10,
            y: 20,
            title: "test".to_string(),
        },
        B { a: 1, b: 2 },
    );
}

但是编译器给了我这些错误:

But the compiler gives me these errors:

error[E0277]: the trait bound `A: std::cmp::Eq` is not satisfied
  --> src/main.rs:16:9
   |
16 |     map.insert(
   |         ^^^^^^ the trait `std::cmp::Eq` is not implemented for `A`

error[E0277]: the trait bound `A: std::hash::Hash` is not satisfied
  --> src/main.rs:16:9
   |
16 |     map.insert(
   |         ^^^^^^ the trait `std::hash::Hash` is not implemented for `A`

我知道我必须实现这些特征，但是在网上搜索了几个小时后，我没有找到任何关于实现它们的信息.

I know that I must implement these traits, but after hours of searching the web, I have found nothing about implementing them.

我的实际代码更复杂，我的结构包含其他结构(我已经编辑了代码).

My actual code is more complicated, and my structs contain other structs (I've edited the code).

我已经实现了 Hash 特征:

I've implemented the Hash trait:

impl std::hash::Hash for A {
    fn hash<H>(&self, state: &mut H)
    where
        H: std::hash::Hasher,
    {
        state.write_i32(self.x);
        state.finish();
    }
}

我也为 PartialEq 做了一个实现:

I made an implementation for PartialEq also:

impl PartialEq for A {
    fn eq(&self, other: &A) -> bool {
        self.x == other.x
    }
}

但是编译器继续抱怨，这次是关于Eq:

But the compiler continues to complain, this time about Eq:

error[E0277]: the trait bound `A: std::cmp::Eq` is not satisfied
  --> src/main.rs:16:9
   |
16 |     map.insert(
   |         ^^^^^^ the trait `std::cmp::Eq` is not implemented for `A`

如何实现Eq?为什么文档中没有实现?

How can I implement Eq? Why is there no implementation in the docs?

推荐答案

Eq 就是我们所说的marker trait:它本身没有方法，只是程序员的一种方式表示该结构验证某个属性.你可以这样实现它:

Eq is what we call a marker trait: it has no method on its own, it is just a way for the programmer to express that the struct verifies a certain property. You can implement it like this:

impl Eq for Application {}

或者，在 Application 声明之上使用 #[derive(Eq)]

Or alternatively, use #[derive(Eq)] on top of the Application declaration

Eq 是由 PartialEq 绑定的特征.这意味着您只能在也实现 PartialEq 的结构上实现它(这里就是这种情况).通过实现 Eq，您承诺您的 PartialEq 实现是自反的(请参阅文档了解其含义).

Eq is a trait bound by PartialEq. This means that you can implement it only on structs that also implement PartialEq (which is the case here). By implementing Eq, you make the promise that your implementation of PartialEq is reflexive (see the docs for what it means).

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