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<h1 id="无处不在的迭代器"><a class="header" href="#无处不在的迭代器">无处不在的迭代器</a></h1>
<p>Rust 的迭代器无处不在,直至你在它上面栽了跟头,经过深入调查才发现:哦,原来是迭代器的锅。不信的话,看看这个报错你能想到是迭代器的问题吗: <code>borrow of moved value: words</code>.</p>
<h2 id="报错的代码"><a class="header" href="#报错的代码">报错的代码</a></h2>
<p>以下的代码非常简单,用来统计文本中字词的数量,并打印出来:</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let s = &quot;hello world&quot;;
let mut words = s.split(&quot; &quot;);
let n = words.count();
println!(&quot;{:?}&quot;,words);
}</code></pre></pre>
<p>四行代码,行云流水,一气呵成,且看成效:</p>
<pre><code class="language-console">error[E0382]: borrow of moved value: `words`
--&gt; src/main.rs:5:21
|
3 | let mut words = s.split(&quot; &quot;);
| --------- move occurs because `words` has type `std::str::Split&lt;'_, &amp;str&gt;`, which does not implement the `Copy` trait
4 | let n = words.count();
| ------- `words` moved due to this method call
5 | println!(&quot;{:?}&quot;,words);
| ^^^^^ value borrowed here after move
</code></pre>
<p>世事难料,我以为只有的生命周期、闭包才容易背叛革命,没想到一个你浓眉大眼的<code>count</code>方法也背叛革命。从报错来看,是因为<code>count</code>方法拿走了<code>words</code>的所有权,来看看签名:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn count(self) -&gt; usize
<span class="boring">}</span></code></pre></pre>
<p>从签名来看,编译器的报错是正确的,但是为什么?为什么一个简单的标准库<code>count</code>方法就敢拿走所有权?</p>
<h2 id="迭代器回顾"><a class="header" href="#迭代器回顾">迭代器回顾</a></h2>
<p><a href="https://course.rs/advance/functional-programing/iterator.html#%E6%B6%88%E8%B4%B9%E8%80%85%E4%B8%8E%E9%80%82%E9%85%8D%E5%99%A8">迭代器</a>章节中,我们曾经学习过两个概念:迭代器适配器和消费者适配器,前者用于对迭代器中的元素进行操作,最终生成一个新的迭代器,例如<code>map</code><code>filter</code>等方法;而后者用于消费掉迭代器,最终产生一个结果,例如<code>collect</code>方法, 一个典型的示例如下:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let v1: Vec&lt;i32&gt; = vec![1, 2, 3];
let v2: Vec&lt;_&gt; = v1.iter().map(|x| x + 1).collect();
assert_eq!(v2, vec![2, 3, 4]);
<span class="boring">}</span></code></pre></pre>
<p>在其中,我们还提到一个细节,消费者适配器会拿走迭代器的所有权,那么这个是否与我们最开始碰到的问题有关系?</p>
<h2 id="深入调查"><a class="header" href="#深入调查">深入调查</a></h2>
<p>要解释这个问题,必须要找到<code>words</code>是消费者适配器的证据,因此我们需要深入源码进行查看。</p>
<p>其实。。也不需要多深,只要进入<code>words</code>的源码,就能看出它属于<code>Iterator</code>特征,那说明<code>split</code>方法产生了一个迭代器?再来看看:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn split&lt;'a, P&gt;(&amp;'a self, pat: P) -&gt; Split&lt;'a, P&gt;
where
P: Pattern&lt;'a&gt;,
//An iterator over substrings of this string slice, separated by characters matched by a pattern.
<span class="boring">}</span></code></pre></pre>
<p>还真是,从代码注释来看,<code>Split</code>就是一个迭代器类型,用来迭代被分隔符隔开的子字符串集合。</p>
<p>真相大白了,<code>split</code>产生一个迭代器,而<code>count</code>方法是一个消费者适配器,用于消耗掉前者产生的迭代器,最终生成字词统计的结果。</p>
<p>本身问题不复杂,但是在<strong>解决方法上,可能还有点在各位客官的意料之外</strong>,且看下文。</p>
<h2 id="最-rusty-的解决方法"><a class="header" href="#最-rusty-的解决方法">最 rusty 的解决方法</a></h2>
<p>你可能会想用<code>collect</code>来解决这个问题,先收集成一个集合,然后进行统计。当然此方法完全可行,但是很不<code>rusty</code>(很符合 rust 规范、潮流的意思),以下给出最<code>rusty</code>的解决方案:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let words = s.split(&quot;,&quot;);
let n = words.clone().count();
<span class="boring">}</span></code></pre></pre>
<p>在继续之前,我得先找一个地方藏好,因为俺有一个感觉,烂西红柿正在铺天盖地的呼啸而来,伴随而来的是读者的正义呵斥:
<strong>你管<code>clone</code>叫最好、最<code>rusty</code>的解决方法??</strong></p>
<p>大家且听我慢慢道来,事实上,在 Rust 中<code>clone</code>不总是性能低下的代名词,因为<code>clone</code>的行为完全取决于它的具体实现。</p>
<h4 id="迭代器的clone代价"><a class="header" href="#迭代器的clone代价">迭代器的<code>clone</code>代价</a></h4>
<p>对于迭代器而言,它其实并不需要持有数据才能进行迭代,事实上它包含一个引用,该引用指向了保存在堆上的数据,而迭代器自身的结构是保存在栈上。</p>
<p>因此对迭代器的<code>clone</code>仅仅是复制了一份栈上的简单结构,性能非常高效,例如:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct Split&lt;'a, T: 'a, P&gt;
where
P: FnMut(&amp;T) -&gt; bool,
{
// Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
pub(crate) v: &amp;'a [T],
pred: P,
// Used for `SplitAsciiWhitespace` `as_str` method
pub(crate) finished: bool,
}
impl&lt;T, P&gt; Clone for Split&lt;'_, T, P&gt;
where
P: Clone + FnMut(&amp;T) -&gt; bool,
{
fn clone(&amp;self) -&gt; Self {
Split { v: self.v, pred: self.pred.clone(), finished: self.finished }
}
}
<span class="boring">}</span></code></pre></pre>
<p>以上代码实现了对<code>Split</code>迭代器的克隆,可以看出,底层的的数组<code>self.v</code>并没有被克隆而是简单的复制了一个引用,依然指向了底层的数组<code>&amp;[T]</code>,因此这个克隆非常高效。</p>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>看起来是无效借用导致的错误,实际上是迭代器被消费了导致的问题,这说明 Rust 编译器虽然会告诉你错误原因,但是这个原因不总是根本原因。我们需要一双慧眼和勤劳的手,来挖掘出这个宝藏,最后为己所用。</p>
<p>同时,克隆在 Rust 中也并不总是<strong>bad guy</strong>的代名词,有的时候我们可以大胆去使用,当然前提是了解你的代码场景和具体的<code>clone</code>实现,这样你也能像文中那样作出非常<code>rusty</code>的选择。</p>
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