我如何要求泛型类型在泛型函数中实现诸如 Add、Sub、Mul 或 Div 之类的操作? [英] How do I require a generic type implement an operation like Add, Sub, Mul, or Div in a generic function?

问题描述

我正在尝试在 Rust 中实现一个通用函数，其中参数的唯一要求是应该定义乘法运算.我正在尝试实现一个通用的电源"，但会使用一个更简单的 cube 函数来说明问题:

I'm trying to implement a generic function in Rust where the only requirement for the argument is that the multiplication operation should be defined. I'm trying to implement a generic "power", but will go with a simpler cube function to illustrate the problem:

use std::ops::Mul;

fn cube<T: Mul>(x: T) -> T {
    x * x * x
}

fn main() {
    println!("5^3 = {}", cube(5));
}

编译时出现这个错误:

error[E0369]: binary operation `*` cannot be applied to type `<T as std::ops::Mul>::Output`
 --> src/main.rs:4:5
  |
4 |     x * x * x
  |     ^^^^^^^^^
  |
  = note: an implementation of `std::ops::Mul` might be missing for `<T as std::ops::Mul>::Output`

这是什么意思?我是否选择了错误的特质?我该如何解决?

What does this mean? Did I choose the wrong trait? How can I resolve this?

推荐答案

让我们稍微分解一下您的示例:

Let's break down your example a bit:

fn cube<T: Mul>(x: T) -> T {
    let a = x * x;
    let b = a * x;
    b
}

a 和 b 的类型是什么?在这种情况下，a 的类型是 ::Output — 从错误消息中听起来很熟悉?然后，我们再次尝试将该类型乘以 x，但不能保证 Output 能够乘以任何东西！

What are the types of a and b? In this case, the type of a is <T as std::ops::Mul>::Output — sound familiar from the error message? Then, we are trying to multiply that type by x again, but there's no guarantee that Output is able to be multiplied by anything!

让我们做最简单的事情，说 T * T 需要产生一个 T:

Let's do the simplest thing and say that T * T needs to result in a T:

fn cube<T: Mul<Output = T>>(x: T) -> T {
    x * x * x
}

不幸的是，这给出了两个类似的错误:

Unfortunately, this gives two similar errors:

error[E0382]: use of moved value: `x`
 --> src/lib.rs:6:9
  |
6 |     x * x * x
  |     -   ^ value used here after move
  |     |
  |     value moved here
  |
  = note: move occurs because `x` has type `T`, which does not implement the `Copy` trait

这是因为 Mul trait 需要参数按值，所以我们添加了Copy，这样我们就可以复制这些值.

Which is because the Mul trait takes arguments by value, so we add the Copy so we can duplicate the values.

我也切换到 where 子句，因为我更喜欢它，而且有那么多内联很笨拙:

I also switched to the where clause as I like it better and it is unwieldy to have that much inline:

fn cube<T>(x: T) -> T
where
    T: Mul<Output = T> + Copy
{
    x * x * x
}

另见:

• 如何实现 Add trait 以引用结构?
• 如何编写一个 trait bound 来添加两个泛型类型的引用?

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