the trait `PartialEq<Option<_>>` is not implemented - rust

Question

I have a struct where I've derived a couple of things.
#[derive(PartialEq, Debug)]
struct Subscriber {
id: u16,
up_speed: u32,
down_speed: u32
}
However, when I try to use PartialEq, I get told it is not implemented.
for (id, subscriber) in &new_hashmap {
let original_subscriber = original_hashmap.get(id).unwrap();
if original_subscriber == None {
changed_hashmap.insert(subscriber.id, subscriber);
} else if subscriber != original_subscriber {
changed_hashmap.insert(subscriber.id, subscriber);
}
}
Here's the compiler error.
error[E0277]: can't compare `&Subscriber` with `Option<_>`
--> src/main.rs:34:32
|
34 | if original_subscriber == None {
| ^^ no implementation for `&Subscriber == Option<_>`
|
= help: the trait `PartialEq<Option<_>>` is not implemented for `&Subscriber`
= help: the trait `PartialEq` is implemented for `Subscriber`
If I rewrite it to not put original_subscriber into its own variable, then it works.
for (id, subscriber) in &new_hashmap {
if original_hashmap.get(id) == None {
changed_hashmap.insert(subscriber.id, subscriber);
} else if subscriber != original_hashmap.get(id).unwrap() {
changed_hashmap.insert(subscriber.id, subscriber);
}
}
The rest of the code is essentially doing the following.
Create HashMap of 2 Subscriber instances.
Create another HashMap of 3 Subscriber instances, 1 of which is new, 1 of which is the same, and 1 of which has the same key but an updated value.
That is original_hashmap HashMap and new_hashmap.
The goal is to get a third HashMap of items in new_hashmap that are new to original_hashmap or have changed values.

Answer 1

your code does not work for 2 reasons.
If you derive PartialEq it will only work for Subscriber == Subscriber checks. You need to implement PartialEq<Type>
You are using a reference when comparing. This means you need to implement PartialEq for &Subscriber and not subscriber
This should do the trick
#[derive(PartialEq, Debug)]
struct Subscriber {
id: u16,
up_speed: u32,
down_speed: u32,
}
let subscriber = Subscriber {
id: 1,
up_speed: 100,
down_speed: 100,
};
impl PartialEq<Option<Subscriber>> for &Subscriber {
fn eq(&self, other: &Option<Subscriber>) -> bool {
match other {
Some(other) => return other == *self,
None => return false,
}
}
}
if &subscriber == None {
println!("None");
} else {
println!("Some");
}
But I am not sure if this is really what you want. I will try to implement the same and edit my answer afterwards
I suppose that's what you want to implement
use std::collections::HashMap;
#[derive(Debug, PartialEq)]
struct Subscriber {
id: u16,
up_speed: u32,
down_speed: u32,
}
impl Subscriber {
fn new(id: u16, up_speed: u32, down_speed: u32) -> Subscriber {
Subscriber {
id,
up_speed,
down_speed,
}
}
}
fn main() {
let mut old_map = HashMap::new();
old_map.insert(1, Subscriber::new(1, 1, 1));
old_map.insert(2, Subscriber::new(2, 2, 2));
let mut new_map = HashMap::new();
new_map.insert(0, Subscriber::new(0, 0, 0)); //new
new_map.insert(1, Subscriber::new(1, 1, 1)); //Same
new_map.insert(2, Subscriber::new(3, 3, 3)); //Same key but different value
let mut changed_map = HashMap::new();
//
for (key, subscriber) in &new_map {
if old_map.contains_key(&key) {
if old_map[&key] != *subscriber {
changed_map.insert(key, subscriber);
}
} else {
changed_map.insert(key, subscriber);
}
}
println!("{:?}", changed_map);
}
It will return
{2: Subscriber { id: 3, up_speed: 3, down_speed: 3 }, 0: Subscriber { id: 0, up_speed: 0, down_speed: 0 }}
I used the deref operator to avoid impl PartialEq<Subscriber> for &Subscriber but you could have done that as well