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<main>
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<h1 id="深入-rust-类型"><a class="header" href="#深入-rust-类型">深入 Rust 类型</a></h1>
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<p>弱弱地、不负责任地说，Rust 的学习难度之恶名，可能有一半来源于 Rust 的类型系统，而其中一半的一半则来自于本章节的内容。在本章，我们将重点学习如何创建自定义类型，以及了解何为动态大小的类型。</p>
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<h2 id="newtype"><a class="header" href="#newtype">newtype</a></h2>
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<p>何为 <code>newtype</code>？简单来说，就是使用<a href="https://course.rs/basic/compound-type/struct.html#元组结构体tuple-struct">元组结构体</a>的方式将已有的类型包裹起来：<code>struct Meters(u32);</code>，那么此处 <code>Meters</code> 就是一个 <code>newtype</code>。</p>
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<p>为何需要 <code>newtype</code>？Rust 这多如繁星的 Old 类型满足不了我们吗？这是因为：</p>
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<ul>
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<li>自定义类型可以让我们给出更有意义和可读性的类型名，例如与其使用 <code>u32</code> 作为距离的单位类型，我们可以使用 <code>Meters</code>，它的可读性要好得多</li>
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<li>对于某些场景，只有 <code>newtype</code> 可以很好地解决</li>
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<li>隐藏内部类型的细节</li>
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</ul>
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<p>一箩筐的理由～～ 让我们先从第二点讲起。</p>
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<h4 id="为外部类型实现外部特征"><a class="header" href="#为外部类型实现外部特征">为外部类型实现外部特征</a></h4>
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|
<p>在之前的章节中，我们有讲过，如果在外部类型上实现外部特征必须使用 <code>newtype</code> 的方式，否则你就得遵循孤儿规则：要为类型 <code>A</code> 实现特征 <code>T</code>，那么 <code>A</code> 或者 <code>T</code> 必须至少有一个在当前的作用范围内。</p>
|
|
|
<p>例如，如果想使用 <code>println!("{}", v)</code> 的方式去格式化输出一个动态数组 <code>Vec</code>，以期给用户提供更加清晰可读的内容，那么就需要为 <code>Vec</code> 实现 <code>Display</code> 特征，但是这里有一个问题： <code>Vec</code> 类型定义在标准库中，<code>Display</code> 亦然，这时就可以祭出大杀器 <code>newtype</code> 来解决：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fmt;
|
|
|
|
|
|
struct Wrapper(Vec<String>);
|
|
|
|
|
|
impl fmt::Display for Wrapper {
|
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
|
write!(f, "[{}]", self.0.join(", "))
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let w = Wrapper(vec![String::from("hello"), String::from("world")]);
|
|
|
println!("w = {}", w);
|
|
|
}</code></pre>
|
|
|
<p>如上所示，使用元组结构体语法 <code>struct Wrapper(Vec<String>)</code> 创建了一个 <code>newtype</code> Wrapper，然后为它实现 <code>Display</code> 特征，最终实现了对 <code>Vec</code> 动态数组的格式化输出。</p>
|
|
|
<h4 id="更好的可读性及类型异化"><a class="header" href="#更好的可读性及类型异化">更好的可读性及类型异化</a></h4>
|
|
|
<p>首先，更好的可读性不等于更少的代码（如果你学过 Scala，相信会深有体会），其次下面的例子只是一个示例，未必能体现出更好的可读性：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::ops::Add;
|
|
|
use std::fmt;
|
|
|
|
|
|
struct Meters(u32);
|
|
|
impl fmt::Display for Meters {
|
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
|
write!(f, "目标地点距离你{}米", self.0)
|
|
|
}
|
|
|
}
|
|
|
|
|
|
impl Add for Meters {
|
|
|
type Output = Self;
|
|
|
|
|
|
fn add(self, other: Meters) -> Self {
|
|
|
Self(self.0 + other.0)
|
|
|
}
|
|
|
}
|
|
|
fn main() {
|
|
|
let d = calculate_distance(Meters(10), Meters(20));
|
|
|
println!("{}", d);
|
|
|
}
|
|
|
|
|
|
fn calculate_distance(d1: Meters, d2: Meters) -> Meters {
|
|
|
d1 + d2
|
|
|
}</code></pre>
|
|
|
<p>上面代码创建了一个 <code>newtype</code> Meters，为其实现 <code>Display</code> 和 <code>Add</code> 特征，接着对两个距离进行求和计算，最终打印出该距离：</p>
|
|
|
<pre><code class="language-console">目标地点距离你30米
|
|
|
</code></pre>
|
|
|
<p>事实上，除了可读性外，还有一个极大的优点：如果给 <code>calculate_distance</code> 传一个其它的类型，例如 <code>struct MilliMeters(u32);</code>，该代码将无法编译。尽管 <code>Meters</code> 和 <code>MilliMeters</code> 都是对 <code>u32</code> 类型的简单包装，但是<strong>它们是不同的类型</strong>！</p>
|
|
|
<h4 id="隐藏内部类型的细节"><a class="header" href="#隐藏内部类型的细节">隐藏内部类型的细节</a></h4>
|
|
|
<p>众所周知，Rust 的类型有很多自定义的方法，假如我们把某个类型传给了用户，但是又不想用户调用这些方法，就可以使用 <code>newtype</code>：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">struct Meters(u32);
|
|
|
|
|
|
fn main() {
|
|
|
let i: u32 = 2;
|
|
|
assert_eq!(i.pow(2), 4);
|
|
|
|
|
|
let n = Meters(i);
|
|
|
// 下面的代码将报错，因为`Meters`类型上没有`pow`方法
|
|
|
// assert_eq!(n.pow(2), 4);
|
|
|
}</code></pre>
|
|
|
<p>不过需要偷偷告诉你的是，这种方式实际上是掩耳盗铃，因为用户依然可以通过 <code>n.0.pow(2)</code> 的方式来调用内部类型的方法 :)</p>
|
|
|
<h2 id="类型别名type-alias"><a class="header" href="#类型别名type-alias">类型别名(Type Alias)</a></h2>
|
|
|
<p>除了使用 <code>newtype</code>，我们还可以使用一个更传统的方式来创建新类型：类型别名</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>type Meters = u32
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>嗯，不得不说，类型别名的方式看起来比 <code>newtype</code> 顺眼的多，而且跟其它语言的使用方式几乎一致，但是：
|
|
|
<strong>类型别名并不是一个独立的全新的类型，而是某一个类型的别名</strong>，因此编译器依然会把 <code>Meters</code> 当 <code>u32</code> 来使用：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>type Meters = u32;
|
|
|
|
|
|
let x: u32 = 5;
|
|
|
let y: Meters = 5;
|
|
|
|
|
|
println!("x + y = {}", x + y);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>上面的代码将顺利编译通过，但是如果你使用 <code>newtype</code> 模式，该代码将无情报错，简单做个总结：</p>
|
|
|
<ul>
|
|
|
<li>类型别名仅仅是别名，只是为了让可读性更好，并不是全新的类型，<code>newtype</code> 才是！</li>
|
|
|
<li>类型别名无法实现<em>为外部类型实现外部特征</em>等功能，而 <code>newtype</code> 可以</li>
|
|
|
</ul>
|
|
|
<p>类型别名除了让类型可读性更好，还能<strong>减少模版代码的使用</strong>：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let f: Box<dyn Fn() + Send + 'static> = Box::new(|| println!("hi"));
|
|
|
|
|
|
fn takes_long_type(f: Box<dyn Fn() + Send + 'static>) {
|
|
|
// --snip--
|
|
|
}
|
|
|
|
|
|
fn returns_long_type() -> Box<dyn Fn() + Send + 'static> {
|
|
|
// --snip--
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><code>f</code> 是一个令人眼花缭乱的类型 <code>Box<dyn Fn() + Send + 'static></code>，如果仔细看，会发现其实只有一个 <code>Send</code> 特征不认识，<code>Send</code> 是什么在这里不重要，你只需理解，<code>f</code> 就是一个 <code>Box<dyn T></code> 类型的特征对象，实现了 <code>Fn()</code> 和<code> Send</code> 特征，同时生命周期为 <code>'static</code>。</p>
|
|
|
<p>因为 <code>f</code> 的类型贼长，导致了后面我们在使用它时，到处都充斥这些不太优美的类型标注，好在类型别名可解君忧：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>type Thunk = Box<dyn Fn() + Send + 'static>;
|
|
|
|
|
|
let f: Thunk = Box::new(|| println!("hi"));
|
|
|
|
|
|
fn takes_long_type(f: Thunk) {
|
|
|
// --snip--
|
|
|
}
|
|
|
|
|
|
fn returns_long_type() -> Thunk {
|
|
|
// --snip--
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>Bang！是不是？！立刻大幅简化了我们的使用。喝着奶茶、哼着歌、我写起代码撩起妹，何其快哉！</p>
|
|
|
<p>在标准库中，类型别名应用最广的就是简化 <code>Result<T, E></code> 枚举。</p>
|
|
|
<p>例如在 <code>std::io</code> 库中，它定义了自己的 <code>Error</code> 类型：<code>std::io::Error</code>，那么如果要使用该 <code>Result</code> 就要用这样的语法：<code>std::result::Result<T, std::io::Error>;</code>，想象一下代码中充斥着这样的东东是一种什么感受？颤抖吧。。。</p>
|
|
|
<p>由于使用 <code>std::io</code> 库时，它的所有错误类型都是 <code>std::io::Error</code>，那么我们完全可以把该错误对用户隐藏起来，只在内部使用即可，因此就可以使用类型别名来简化实现：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>type Result<T> = std::result::Result<T, std::io::Error>;
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>Bingo，这样一来，其它库只需要使用 <code>std::io::Result<T></code> 即可替代冗长的 <code>std::result::Result<T, std::io::Error></code> 类型。</p>
|
|
|
<p>更香的是，由于它只是别名，因此我们可以用它来调用真实类型的所有方法，甚至包括 <code>?</code> 符号！</p>
|
|
|
<h2 id="永不返回类型"><a class="header" href="#永不返回类型">!永不返回类型</a></h2>
|
|
|
<p>在<a href="https://course.rs/basic/base-type/function.html#永不返回的函数">函数</a>那章，曾经介绍过 <code>!</code> 类型：<code>!</code> 用来说明一个函数永不返回任何值，当时可能体会不深，没事，在学习了更多手法后，保证你有全新的体验：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let i = 2;
|
|
|
let v = match i {
|
|
|
0..=3 => i,
|
|
|
_ => println!("不合规定的值:{}", i)
|
|
|
};
|
|
|
}</code></pre>
|
|
|
<p>上面函数，会报出一个编译错误:</p>
|
|
|
<pre><code class="language-console">error[E0308]: `match` arms have incompatible types // match的分支类型不同
|
|
|
--> src/main.rs:5:13
|
|
|
|
|
|
|
3 | let v = match i {
|
|
|
| _____________-
|
|
|
4 | | 0..3 => i,
|
|
|
| | - this is found to be of type `{integer}` // 该分支返回整数类型
|
|
|
5 | | _ => println!("不合规定的值:{}", i)
|
|
|
| | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected integer, found `()` // 该分支返回()单元类型
|
|
|
6 | | };
|
|
|
| |_____- `match` arms have incompatible types
|
|
|
</code></pre>
|
|
|
<p>原因很简单: 要赋值给 <code>v</code>，就必须保证 <code>match</code> 的各个分支返回的值是同一个类型，但是上面一个分支返回数值、另一个分支返回元类型 <code>()</code>，自然会出错。</p>
|
|
|
<p>既然 <code>println</code> 不行，那再试试 <code>panic</code></p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
let i = 2;
|
|
|
let v = match i {
|
|
|
0..=3 => i,
|
|
|
_ => panic!("不合规定的值:{}", i)
|
|
|
};
|
|
|
}</code></pre>
|
|
|
<p>神奇的事发生了，此处 <code>panic</code> 竟然通过了编译。难道这两个宏拥有不同的返回类型？</p>
|
|
|
<p>你猜的没错：<code>panic</code> 的返回值是 <code>!</code>，代表它决不会返回任何值，既然没有任何返回值，那自然不会存在分支类型不匹配的情况。</p>
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