1. 1. Introduction
  2. 2. Getting Started
  3. 3. Tutorial: Guessing Game
  4. 4. Syntax and Semantics
    1. 4.1. Variable Bindings
    2. 4.2. Functions
    3. 4.3. Primitive Types
    4. 4.4. Comments
    5. 4.5. if
    6. 4.6. Loops
    7. 4.7. Vectors
    8. 4.8. Ownership
    9. 4.9. References and Borrowing
    10. 4.10. Lifetimes
    11. 4.11. Mutability
    12. 4.12. Structs
    13. 4.13. Enums
    14. 4.14. Match
    15. 4.15. Patterns
    16. 4.16. Method Syntax
    17. 4.17. Strings
    18. 4.18. Generics
    19. 4.19. Traits
    20. 4.20. Drop
    21. 4.21. if let
    22. 4.22. Trait Objects
    23. 4.23. Closures
    24. 4.24. Universal Function Call Syntax
    25. 4.25. Crates and Modules
    26. 4.26. `const` and `static`
    27. 4.27. Attributes
    28. 4.28. `type` aliases
    29. 4.29. Casting between types
    30. 4.30. Associated Types
    31. 4.31. Unsized Types
    32. 4.32. Operators and Overloading
    33. 4.33. Deref coercions
    34. 4.34. Macros
    35. 4.35. Raw Pointers
    36. 4.36. `unsafe`
  5. 5. Effective Rust
    1. 5.1. The Stack and the Heap
    2. 5.2. Testing
    3. 5.3. Conditional Compilation
    4. 5.4. Documentation
    5. 5.5. Iterators
    6. 5.6. Concurrency
    7. 5.7. Error Handling
    8. 5.8. Choosing your Guarantees
    9. 5.9. FFI
    10. 5.10. Borrow and AsRef
    11. 5.11. Release Channels
    12. 5.12. Using Rust without the standard library
  6. 6. Nightly Rust
    1. 6.1. Compiler Plugins
    2. 6.2. Inline Assembly
    3. 6.3. No stdlib
    4. 6.4. Intrinsics
    5. 6.5. Lang items
    6. 6.6. Advanced linking
    7. 6.7. Benchmark Tests
    8. 6.8. Box Syntax and Patterns
    9. 6.9. Slice Patterns
    10. 6.10. Associated Constants
    11. 6.11. Custom Allocators
  7. 7. Glossary
  8. 8. Syntax Index
  9. 9. Bibliography

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);
}Run

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;
}Run

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
    }
}Run

will let you do this:

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

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());
}Run

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 { ... }Run

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.