rust/src/doc/book/operators-and-overloading.md

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2015-04-07 21:16:02 -05:00
% 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, theres a specific trait that you can implement, which then
overloads the operator.
For example, the `+` operator can be overloaded with the `Add` trait:
```rust
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 `Point`s, since weve 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.
[stdops]: ../std/ops/index.html
Implementing these traits follows a pattern. Lets look at [`Add`][add] in more
detail:
```rust
# mod foo {
pub trait Add<RHS = Self> {
type Output;
fn add(self, rhs: RHS) -> Self::Output;
}
# }
```
[add]: ../std/ops/trait.Add.html
Theres 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.
```rust
# 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:
```rust,ignore
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][traits] more generically:
[traits]: traits.html
```rust
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 just declare a type parameter `T` and replace
`f64` with it. The `impl` needs more involved modifications:
```ignore
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.