How to compare user defined types in Rust
Let’s just say we have a user-defined type Person defined as follows:
pub struct Person {
name: String,
age: i32
}
We need to compare two Persons mat, mac defined as follows:
let mat: Person = Person{name: String::from("Mat"), age: 30};
let mac: Person = Person{name: String::from("Mac"), age: 35};
We cannot just do mat == mac or mat != mac. We have to define what equality means
for the type Person. We can do this in two ways.
- Deriving the PartialEq trait for Person
#[derive(PartialEq)]
pub struct Person {
name: String,
age: i32
}
By deriving the PartialEq trait, the compiler implements the equality operator on the Person type. When we do mat == mac, the compiler compares each field of mat with the corresponding field of mat i.e. mat.age == mac.age and mat.name == mac.name.
println!("Is mat == mac ? {}", mat == mac);
- Implementing the PartialEq trait for Person
We can implement the PartialEq trait for Person ourselves. For that, we have to implement the required method eq of the trait PartialEq.
impl PartialEq for Person {
fn eq(&self, other: &Self) -> bool {
self.name == other.name && self.age == other.age
}
}
Now, lets say if we want to know if mac is older or younger than mat i.e. mat > mac or mat < mac or mat <= mac or mat >= mac. We would have to implement the Ord, PartialOrd and Eq traits for the Person type. These can be implemented as follows:
impl PartialOrd for Person {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
Some(self.cmp(other))
}
}
impl Ord for Person {
fn cmp(&self, other: &Self) -> Ordering {
self.age.cmp(&other.age)
}
}
impl PartialEq for Person {
fn eq(&self, other: &Self) -> bool {
self.age == other.age
}
}
impl Eq for Person {}
Note: The Eq trait inherits from the PartialEq trait.
Now when we do
println!("Is mac older than mat ? {}", mac > mat);
we get
Is mac older than mat ? true