# Operators and Overloading

Rust allows for a limited form of operator overloading. There are certain operators that are able to be overloaded. To support a particular operator between types, there’s a specific trait that you can implement, which then overloads the operator.

For example, the + operator can be overloaded with the Add trait:

use std::ops::Add;

#[derive(Debug)]
struct Point {
x: i32,
y: i32,
}

impl Add for Point {
type Output = Point;

fn add(self, other: Point) -> Point {
Point { x: self.x + other.x, y: self.y + other.y }
}
}

fn main() {
let p1 = Point { x: 1, y: 0 };
let p2 = Point { x: 2, y: 3 };

let p3 = p1 + p2;

println!("{:?}", p3);
}


In main, we can use + on our two Points, since we’ve implemented Add<Output=Point> for Point.

There are a number of operators that can be overloaded this way, and all of their associated traits live in the [std::ops]stdops module. Check out its documentation for the full list.

Implementing these traits follows a pattern. Let’s look at [Add]add in more detail:

# mod foo {
pub trait Add<RHS = Self> {
type Output;

fn add(self, rhs: RHS) -> Self::Output;
}
# }


There’s three types in total involved here: the type you impl Add for, RHS, which defaults to Self, and Output. For an expression let z = x + y, x is the Self type, y is the RHS, and z is the Self::Output type.

# struct Point;
# use std::ops::Add;
impl Add<i32> for Point {
type Output = f64;

fn add(self, rhs: i32) -> f64 {
// add an i32 to a Point and get an f64
# 1.0
}
}


will let you do this:

let p: Point = // ...
let x: f64 = p + 2i32;


# Using operator traits in generic structs

Now that we know how operator traits are defined, we can define our HasArea trait and Square struct from the traits chapter more generically:

use std::ops::Mul;

trait HasArea<T> {
fn area(&self) -> T;
}

struct Square<T> {
x: T,
y: T,
side: T,
}

impl<T> HasArea<T> for Square<T>
where T: Mul<Output=T> + Copy {
fn area(&self) -> T {
self.side * self.side
}
}

fn main() {
let s = Square {
x: 0.0f64,
y: 0.0f64,
side: 12.0f64,
};

println!("Area of s: {}", s.area());
}


For HasArea and Square, we declare a type parameter T and replace f64 with it. The impl needs more involved modifications:

impl<T> HasArea<T> for Square<T>
where T: Mul<Output=T> + Copy { ... }


The area method requires that we can multiply the sides, so we declare that type T must implement std::ops::Mul. Like Add, mentioned above, Mul itself takes an Output parameter: since we know that numbers don't change type when multiplied, we also set it to T. T must also support copying, so Rust doesn't try to move self.side into the return value.