如何在Rust中自动实现具有浮点数的结构的比较？ [英] How do I automatically implement comparison for structs with floats in Rust?

问题描述

我正在尝试自动为类似这样的简单结构派生比较功能：

I'm trying to automatically "derive" comparison functionality for a simple struct like this:

#[derive(PartialEq, Eq)]
struct Vec3 {
    x: f64,
    y: f64,
    z: f64,
}

但是，Rust 1.15.1抱怨：

However, Rust 1.15.1 complains:

error[E0277]: the trait bound `f64: std::cmp::Eq` is not satisfied
 --> src/main.rs:3:5
  |
3 |     x: f64,
  |     ^^^^^^ the trait `std::cmp::Eq` is not implemented for `f64`
  |
  = note: required by `std::cmp::AssertParamIsEq`

实际上是什么我应该允许在这里派生默认实现吗？

What exactly am I supposed to do to allow the derivation of a default implementation here?

推荐答案

Rust有意不实现等式用于浮点类型。此 reddit讨论可能会进一步说明原因，但tl

Rust intentionally does not implement Eq for float types. This reddit discussion may shed some more light on why, but the tl;dr is that floating point numbers aren't totally orderable so bizarre edge cases are unavoidable.

但是，如果要对结构添加比较，则可以导出 PartialOrd 。这将为您提供比较和相等运算符的实现：

However, if you want to add comparison to your struct, you can derive PartialOrd instead. This will give you implementations of the comparative and equality operators:

#[derive(PartialEq, PartialOrd)]
struct Vec3 {
    x: f64,
    y: f64,
    z: f64,
}

fn main() {
    let a = Vec3 { x: 1.0, y: 1.1, z: 1.0 };
    let b = Vec3 { x: 2.0, y: 2.0, z: 2.0 };

    println!("{}", a < b); //true
    println!("{}", a <= b); //true
    println!("{}", a == b); //false
}

Eq 和 PartialEq （因此在 Ord 和 PartialOrd ）是 Eq 要求 == 运算符形成等效关系，而 PartialOrd 仅要求 == 和！= 是逆的。因此，就像使用浮点数本身一样，在对结构实例进行比较时也应牢记这一点。

The difference between Eq and PartialEq (and hence between Ord and PartialOrd) is that Eq requires that the == operator form an equivalence relation, whereas PartialOrd only requires that == and != are inverses. So just as with floats themselves, you should keep that in mind when doing comparisons on instances of your struct.

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