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Operators

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

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

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

impl std::ops::Add for Point {
    type Output = Self;

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

fn main() {
    let p1 = Point { x: 10, y: 20 };
    let p2 = Point { x: 100, y: 200 };
    println!("{p1:?} + {p2:?} = {:?}", p1 + p2);
}
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 convert the argument to bool like the same operator in C-family languages; instead, for integer types it flips each bit of the number, which, arithmetically, is equivalent to subtracting the argument from -1: !5 == -6.