How to implement Eq and Hash for my own structs to use them as a HashMap key?

Question

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 },
    );
}

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`

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

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

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();
    }
}

I made an implementation for PartialEq also:

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

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`

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

Your structs are mutually recursive, each one contain a field of the other type by value. This can not compile as is. — barjak
– barjak, Commented Aug 6, 2015 at 3:20
I think your impl of Hash should simply be self.pid.hash(state). To me it is a mistake to call finish. — barjak
– barjak, Commented Aug 6, 2015 at 4:45

Shepmaster · Accepted Answer · 2018-05-12 01:23:37Z

38

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 {}

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

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).

edited May 12, 2018 at 1:23

Shepmaster

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answered Aug 6, 2015 at 3:46

barjak

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Comments

Shepmaster · Accepted Answer · 2018-05-12 01:25:24Z

29

You can have the compiler derive these instances for you by inserting the following before your struct declaration:

#[derive(PartialEq, Eq, Hash)]
pub struct A {
    // ...
}

You could also implement them manually instead. If you want to do that, you should read the documentation on traits, Eq and Hash.

edited May 12, 2018 at 1:25

Shepmaster

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answered Aug 5, 2015 at 14:55

fjh

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2 Comments

Revertron Over a year ago

the question is how to write own implementation of hash?

oli_obk Over a year ago

there's rarely reason to do so... But as @fjh wrote, if you know which trait to implement, look up the documentation. If you run into trouble, post the code you have tried.

Christian Davén · Accepted Answer · 2021-01-12 06:00:02Z

18

This is how the Rust documentation says you write your own implementation of Hash:

use std::hash::{Hash, Hasher};

struct Person {
    id: u32,
    name: String,
    phone: u64,
}

impl Hash for Person {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.id.hash(state);
        self.phone.hash(state);
    }
}

Source: https://doc.rust-lang.org/std/hash/trait.Hash.html

answered Jan 12, 2021 at 6:00

Christian Davén

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