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<h1 id="切片和切片引用"><a class="header" href="#切片和切片引用">切片和切片引用</a></h1>
<p>关于 <code>str</code> / <code>&amp;str</code><code>[u8]</code> / <code>&amp;[u8]</code> 区别,你能清晰的说出来嘛?如果答案是 No ,那就跟随我一起来看看切片和切片引用到底有何区别吧。</p>
<blockquote>
<p>在继续之前,查看<a href="https://course.rs/basic/compound-type/string-slice.html#%E5%88%87%E7%89%87slice">这里</a>了解何为切片</p>
</blockquote>
<p>切片允许我们引用集合中部分连续的元素序列,而不是引用整个集合。例如,字符串切片就是一个子字符串,数组切片就是一个子数组。</p>
<h2 id="无法被直接使用的切片类型"><a class="header" href="#无法被直接使用的切片类型">无法被直接使用的切片类型</a></h2>
<p>Rust 语言特性内置的 <code>str</code><code>[u8]</code> 类型都是切片,前者是字符串切片,后者是数组切片,下面我们来尝试下使用 <code>str</code> </p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let string: str = &quot;banana&quot;;
<span class="boring">}</span></code></pre></pre>
<p>上面代码创建一个 <code>str</code> 类型的字符串,看起来很正常,但是编译就会报错:</p>
<pre><code class="language-shell">error[E0277]: the size for values of type `str` cannot be known at compilation time
--&gt; src/main.rs:4:9
|
4 | let string: str = &quot;banana&quot;;
| ^^^^^^ doesn't have a size known at compile-time
</code></pre>
<p>编译器准确的告诉了我们原因:<code>str</code> 字符串切片它是 <a href="https://course.rs/advance/into-types/sized.html#%E5%8A%A8%E6%80%81%E5%A4%A7%E5%B0%8F%E7%B1%BB%E5%9E%8B-dst"><code>DST</code> 动态大小类型</a>,这意味着编译器无法在编译期知道 <code>str</code> 类型的大小,只有到了运行期才能动态获知,这对于强类型、强安全的 Rust 语言来说是不可接受的。</p>
<p>也就是说,我们无法直接使用 <code>str</code>,而对于 <code>[u8]</code> 也是类似的,大家可以自己动手试试。</p>
<p>总之,我们可以总结出一个结论:<strong>在 Rust 中,所有的切片都是动态大小类型,它们都无法直接被使用</strong></p>
<h4 id="为何切片是动态大小类型"><a class="header" href="#为何切片是动态大小类型">为何切片是动态大小类型</a></h4>
<p>原因在于底层的切片长度是可以动态变化的,而编译器无法在编译期得知它的具体的长度,因此该类型无法被分配在栈上,只能分配在堆上。</p>
<h4 id="为何切片只能通过引用来使用"><a class="header" href="#为何切片只能通过引用来使用">为何切片只能通过引用来使用</a></h4>
<p>既然切片只能分配到堆上,我们就无法直接使用它,大家可以想想,所有分配在堆上的数据,是不是都是通过一个在栈上的引用来访问的?切片也不例外。</p>
<h4 id="为何切片引用可以存储在栈上"><a class="header" href="#为何切片引用可以存储在栈上">为何切片引用可以存储在栈上</a></h4>
<p>切片引用是一个宽指针,存储在栈上,指向了堆上的切片数据,该引用包含了切片的起始位置和长度,而且最重要的是,类似于指针,引用的大小是固定的(起始位置和长度都是整形),因此它才可以存储在栈上。</p>
<h4 id="有没有可以存储在栈上的"><a class="header" href="#有没有可以存储在栈上的">有没有可以存储在栈上的</a></h4>
<p>有,使用固定长度的数组: <code>let a: [i8;4] = [1,2,3,4];</code>,注意看,数组的类型与切片是不同的,前者的类型带有长度:<code>[i8;4]</code>,而后者仅仅是 <code>[i8]</code></p>
<h2 id="切片引用"><a class="header" href="#切片引用">切片引用</a></h2>
<p>那么问题来了,该如何使用切片呢?</p>
<p>何以解忧,唯有引用。由于引用类型的大小在编译期是已知的,因此在 Rust 中,如果要使用切片,就必须要使用它的引用。</p>
<p><code>str</code> 切片的引用类型是 <code>&amp;str</code>,而 <code>[i32]</code> 的引用类型是 <code>&amp;[i32]</code>,相信聪明的读者已经看出来了,<code>&amp;str</code><code>&amp;[i32]</code> 都是我们非常常用的类型,例如:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let s1: &amp;str = &quot;banana&quot;;
let s2: &amp;str = &amp;String::from(&quot;banana&quot;);
let arr = [1, 2, 3, 4, 5];
let s3: &amp;[i32] = &amp;arr[1..3];
<span class="boring">}</span></code></pre></pre>
<p>这段代码就可以正常通过,原因在于这些切片引用的大小在编译器都是已知的。</p>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>我们常常说使用切片,实际上我们在用的是切片的引用,我们也在频繁说使用字符串,实际上我们在使用的也是字符串切片的引用。</p>
<p>总之,切片在 Rust 中是动态大小类型 DST是无法被我们直接使用的而我们在使用的都是切片的引用。</p>
<div class="table-wrapper"><table><thead><tr><th>切片</th><th>切片引用</th></tr></thead><tbody>
<tr><td>str 字符串切片</td><td>&amp;str 字符串切片的引用</td></tr>
<tr><td>[u8] 数组切片</td><td>&amp;[u8] 数组切片的引用</td></tr>
</tbody></table>
</div>
<p><strong>但是出于方便,我们往往不会说使用切片引用,而是直接说使用字符串切片或数组切片,实际上,这时指代的都是切片的引用!</strong></p>
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