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 module. Check out its documentation for the full list.

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

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.

impl Add<i32> for Point {
    type Output = f64;

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

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.