operator-overloading - 如何为不同的 RHS 类型和返回值重载运算符？

Question

给定以下结构:

struct Vector3D {
    x: f32,
    y: f32,
    z: f32
}

我想重载它的 * 运算符，当右侧是 Vector3D 时做点积，当 RHS 是时做逐元素乘法一个 f32。我的代码如下所示:

// Multiplication with scalar
impl Mul<f32, Vector3D> for Vector3D {
    fn mul(&self, f: &f32) -> Vector3D {
        Vector3D {x: self.x * *f, y: self.y * *f, z: self.z * *f} 
    }   
}
// Multiplication with vector, aka dot product
impl Mul<Vector3D, f32> for Vector3D {
    fn mul(&self, other: &Vector3D) -> f32 {
        self.x * other.x + self.y * other.y + self.z * other.z
    }   
}

编译器对第一个 impl block 说:

Vector3D.rs:40:1: 44:2 error: conflicting implementations for trait `std::ops::Mul`
Vector3D.rs:40 impl Mul<f32, Vector3D> for Vector3D { 
...
Vector3D.rs:53:1: 57:2 note: note conflicting implementation here
Vector3D.rs:53 impl Mul<Vector3D, f32> for Vector3D { 
...

对于其他实现，反之亦然。

Answer 1

从 Rust 1.0 开始，您现在可以实现这个:

use std::ops::Mul;

#[derive(Copy, Clone, PartialEq, Debug)]
struct Vector3D {
    x: f32,
    y: f32,
    z: f32,
}

// Multiplication with scalar
impl Mul<f32> for Vector3D {
    type Output = Vector3D;

    fn mul(self, f: f32) -> Vector3D {
        Vector3D {
            x: self.x * f,
            y: self.y * f,
            z: self.z * f,
        }
    }
}

// Multiplication with vector, aka dot product
impl Mul<Vector3D> for Vector3D {
    type Output = f32;

    fn mul(self, other: Vector3D) -> f32 {
        self.x * other.x + self.y * other.y + self.z * other.z
    }
}

fn main() {
    let a = Vector3D {
        x: 1.0,
        y: 2.0,
        z: 3.0,
    };
    let b = a * -1.0;
    let c = a * b;

    println!("{:?}", a);
    println!("{:?}", b);
    println!("{:?}", c);
}

允许这样做的最大变化是引入了关联类型，它在每个实现中显示为 type Output = 位。另一个值得注意的变化是运算符特征现在按值获取参数，使用它们，所以我继续为结构实现 Copy。