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<main>
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<h1 id="eq-和-partialeq"><a class="header" href="#eq-和-partialeq">Eq 和 PartialEq</a></h1>
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<p>在 Rust 中，想要重载操作符，你就需要实现对应的特征。</p>
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<p>例如 <code><</code>、<code><=</code>、<code>></code> 和 <code>>=</code> 需要实现 <code>PartialOrd</code> 特征:</p>
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
|
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</span>use std::fmt::Display;
|
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struct Pair<T> {
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x: T,
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y: T,
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}
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impl<T> Pair<T> {
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fn new(x: T, y: T) -> Self {
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Self { x, y }
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}
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}
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impl<T: Display + PartialOrd> Pair<T> {
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fn cmp_display(&self) {
|
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if self.x >= self.y {
|
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println!("The largest member is x = {}", self.x);
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} else {
|
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println!("The largest member is y = {}", self.y);
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}
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}
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}
|
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<span class="boring">}</span></code></pre>
|
|
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<p>再比如， <code>+</code> 号需要实现 <code>std::ops::Add</code> 特征，而本文的主角 <code>Eq</code> 和 <code>PartialEq</code> 正是 <code>==</code> 和 <code>!=</code> 所需的特征，那么问题来了，这两个特征有何区别？</p>
|
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<p>我相信很多同学都说不太清楚，包括一些老司机，而且就算是翻文档，可能也找不到特别明确的解释。如果大家看过标准库示例，可能会看过这个例子：</p>
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|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>enum BookFormat { Paperback, Hardback, Ebook }
|
|
|
struct Book {
|
|
|
isbn: i32,
|
|
|
format: BookFormat,
|
|
|
}
|
|
|
impl PartialEq for Book {
|
|
|
fn eq(&self, other: &Self) -> bool {
|
|
|
self.isbn == other.isbn
|
|
|
}
|
|
|
}
|
|
|
impl Eq for Book {}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这里只实现了 <code>PartialEq</code>，并没有实现 <code>Eq</code>，而是直接使用了默认实现 <code>impl Eq for Book {}</code>，奇了怪了，别急，还有呢：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>impl PartialEq<IpAddr> for Ipv4Addr {
|
|
|
#[inline]
|
|
|
fn eq(&self, other: &IpAddr) -> bool {
|
|
|
match other {
|
|
|
IpAddr::V4(v4) => self == v4,
|
|
|
IpAddr::V6(_) => false,
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
|
|
|
impl Eq for Ipv4Addr {}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>以上代码来自 Rust 标准库，可以看到，依然是这样使用，类似的情况数不胜数。既然如此，是否说明<strong>如果要为我们的类型增加相等性比较，只要实现 <code>PartialEq</code> 即可？</strong></p>
|
|
|
<p>其实，关键点就在于 <code>partial</code> 上，<strong>如果我们的类型只在部分情况下具有相等性</strong>，那你就只能实现 <code>PartialEq</code>，否则可以实现 <code>PartialEq</code> 然后再默认实现 <code>Eq</code>。</p>
|
|
|
<p>好的，问题逐步清晰起来，现在我们只需要搞清楚何为部分相等。</p>
|
|
|
<h3 id="部分相等性"><a class="header" href="#部分相等性">部分相等性</a></h3>
|
|
|
<p>首先我们需要找到一个类型，它实现了 <code>PartialEq</code> 但是没有实现 <code>Eq</code>（你可能会想有没有反过来的情况？当然没有啦，部分相等肯定是全部相等的子集！）</p>
|
|
|
<p>在 <code>HashMap</code> 章节提到过 <code>HashMap</code> 的 key 要求实现 <code>Eq</code> 特征，也就是要能完全相等，而浮点数由于没有实现 <code>Eq</code> ，因此不能用于 <code>HashMap</code> 的 key。</p>
|
|
|
<p>当时由于一些知识点还没有介绍，因此就没有进一步展开，那么让我们考虑浮点数既然没有实现 <code>Eq</code> 为何还能进行比较呢？</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let f1 = 3.14;
|
|
|
let f2 = 3.14;
|
|
|
|
|
|
if f1 == f2 {
|
|
|
println!("hello, world!");
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>以上代码是可以看到输出内容的，既然浮点数没有实现 <code>Eq</code> 那说明它实现了 <code>PartialEq</code>，一起写个简单代码验证下：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let f1 = 3.14;
|
|
|
is_eq(f1);
|
|
|
is_partial_eq(f1)
|
|
|
}
|
|
|
|
|
|
fn is_eq<T: Eq>(f: T) {}
|
|
|
fn is_partial_eq<T: PartialEq>(f: T) {}</code></pre>
|
|
|
<p>上面的代码通过特征约束的方式验证了我们的结论:</p>
|
|
|
<pre><code class="language-shell">3 | is_eq(f1);
|
|
|
| ----- ^^ the trait `Eq` is not implemented for `{float}`
|
|
|
</code></pre>
|
|
|
<p>好的，既然我们成功找到了一个类型实现了 <code>PartialEq</code> 但没有实现 <code>Eq</code>，那就通过它来看看何为部分相等性。</p>
|
|
|
<p>其实答案很简单，浮点数有一个特殊的值 <code>NaN</code>，它是无法进行相等性比较的:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let f1 = f32::NAN;
|
|
|
let f2 = f32::NAN;
|
|
|
|
|
|
if f1 == f2 {
|
|
|
println!("NaN 竟然可以比较，这很不数学啊！")
|
|
|
} else {
|
|
|
println!("果然，虽然两个都是 NaN ，但是它们其实并不相等")
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>大家猜猜哪一行会输出 :) 至于 <code>NaN</code> 为何不能比较，这个原因就比较复杂了( 有读者会说，其实就是你不知道，我只能义正严辞的说：咦？你怎么知道 :P )。</p>
|
|
|
<p>既然浮点数有一个值不可以比较相等性，那它自然只能实现 <code>PartialEq</code> 而不能实现 <code>Eq</code> 了，以此类推，如果我们的类型也有这种特殊要求，那也应该这么做。</p>
|
|
|
<h3 id="ord-和-partialord"><a class="header" href="#ord-和-partialord">Ord 和 PartialOrd</a></h3>
|
|
|
<p>事实上，还有一对与 <code>Eq/PartialEq</code> 非常类似的特征，它们可以用于 <code><</code>、<code><=</code>、<code>></code> 和 <code>>=</code> 比较，至于哪个类型实现了 <code>PartialOrd</code> 却没有实现 <code>Ord</code> 就交给大家自己来思考了：）</p>
|
|
|
<blockquote>
|
|
|
<p>小提示：Ord 意味着一个类型的所有值都可以进行排序，而 PartialOrd 则不然</p>
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</blockquote>
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