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<main>
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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>
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<p>关于 <code>str</code> / <code>&str</code>，<code>[u8]</code> / <code>&[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>
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<blockquote>
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<p>在继续之前，查看<a href="https://course.rs/basic/compound-type/string-slice.html#切片slice">这里</a>了解何为切片</p>
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</blockquote>
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<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>
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<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>
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<p>Rust 语言特性内置的 <code>str</code> 和 <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>
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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>let string: str = "banana";
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<span class="boring">}</span></code></pre>
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<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>
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<pre><code class="language-shell">error[E0277]: the size for values of type `str` cannot be known at compilation time
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--> src/main.rs:4:9
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4 | let string: str = "banana";
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| ^^^^^^ doesn't have a size known at compile-time
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</code></pre>
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<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#动态大小类型-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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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型)，因此它才可以存储在栈上。</p>
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<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>
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<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>
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<h2 id="切片引用"><a class="header" href="#切片引用">切片引用</a></h2>
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<p>那么问题来了，该如何使用切片呢？</p>
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<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>
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<p><code>str</code> 切片的引用类型是 <code>&str</code>，而 <code>[i32]</code> 的引用类型是 <code>&[i32]</code>，相信聪明的读者已经看出来了，<code>&str</code> 和 <code>&[i32]</code> 都是我们非常常用的类型，例如:</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>let s1: &str = "banana";
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let s2: &str = &String::from("banana");
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let arr = [1, 2, 3, 4, 5];
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let s3: &[i32] = &arr[1..3];
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<span class="boring">}</span></code></pre>
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<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>
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<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
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<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>
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<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>
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<div class="table-wrapper">
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<table>
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<thead>
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<tr><th>切片</th><th>切片引用</th></tr>
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</thead>
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<tbody>
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<tr><td>str 字符串切片</td><td>&str 字符串切片的引用</td></tr>
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<tr><td>[u8] 数组切片</td><td>&[u8] 数组切片的引用</td></tr>
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</tbody>
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</table>
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</div>
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<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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