Operators

Operator overloading is implemented via traits in std::ops:

  1. #[derive(Debug, Copy, Clone)]
  2. struct Point {
  3.     x: i32,
  4.     y: i32,
  5. }
  7. impl std::ops::Add for Point {
  8.     type Output = Self;
  10.     fn add(self, other: Self) -> Self {
  11.         Self { x: self.x + other.x, y: self.y + other.y }
  12.     }
  13. }
  15. fn main() {
  16.     let p1 = Point { x: 10, y: 20 };
  17.     let p2 = Point { x: 100, y: 200 };
  18.     println!("{p1:?} + {p2:?} = {:?}", p1 + p2);
  19. }

This slide should take about 5 minutes.

Discussion points:

  • You could implement Add for &Point. In which situations is that useful?
    • Answer: Add:add consumes self. If type T for which you are overloading the operator is not Copy, you should consider overloading the operator for &T as well. This avoids unnecessary cloning on the call site.
  • Why is Output an associated type? Could it be made a type parameter of the method?
    • Short answer: Function type parameters are controlled by the caller, but associated types (like Output) are controlled by the implementer of a trait.
  • You could implement Add for two different types, e.g. impl Add<(i32, i32)> for Point would add a tuple to a Point.

The Not trait (! operator) is notable because it does not “boolify” like the same operator in C-family languages; instead, for integer types it negates each bit of the number, which arithmetically is equivalent to subtracting it from -1: !5 == -6.