PartialEq and Eq
Partial equality & Total equality.
Derivable: ✅
When to implement: Almost always.
#![allow(unused)]
fn main() {
// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0
// pub trait PartialEq<Rhs = Self>
//{
// // Required method
// fn eq(&self, other: &Rhs) -> bool;
//
// // Provided method
// fn ne(&self, other: &Rhs) -> bool { ... }
// }
//
// pub trait Eq: PartialEq { }
#[derive(PartialEq, Eq)]
pub struct User { name: String, favorite_number: i32 }
let alice = User { name: "alice".to_string(), favorite_number: 1_000_042 };
let bob = User { name: "bob".to_string(), favorite_number: 42 };
dbg!(alice == alice);
dbg!(alice == bob);
}
- Equality-related methods. If a type implements `PartialEq`/`Eq` then you can use the `==` operator with that type.
-
A type can’t implement
Eqwithout implementingPartialEq. -
Reminder: Partial means “there are invalid members of this set for this function.”
This doesn’t mean that equality will panic, or that it returns a result, just that there may be values that may not behave as you expect equality to behave.
For example, with floating point values
NaNis an outlier:NaN == NaNis false, despite bitwise equality.PartialEqexists to separate types like f32/f64 from types with Total Equality. -
You can implement
PartialEqbetween different types, but this is mostly useful for reference/smart pointer types.