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<ol class="chapter"><li class="chapter-item affix "><a href="../../about-book.html">关于本书</a></li><li class="chapter-item affix "><a href="../../into-rust.html">进入 Rust 编程世界</a></li><li class="chapter-item affix "><a href="../../first-try/sth-you-should-not-do.html">避免从入门到放弃</a></li><li class="chapter-item affix "><a href="../../community.html">社区和锈书</a></li><li class="chapter-item affix "><li class="part-title">Rust 语言基础学习</li><li class="spacer"></li><li class="chapter-item "><a href="../../first-try/intro.html"><strong aria-hidden="true">1.</strong> 寻找牛刀,以便小试</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../first-try/installation.html"><strong aria-hidden="true">1.1.</strong> 安装 Rust 环境</a></li><li class="chapter-item "><a href="../../first-try/editor.html"><strong aria-hidden="true">1.2.</strong> 墙推 VSCode!</a></li><li class="chapter-item "><a href="../../first-try/cargo.html"><strong aria-hidden="true">1.3.</strong> 认识 Cargo</a></li><li class="chapter-item "><a href="../../first-try/hello-world.html"><strong aria-hidden="true">1.4.</strong> 不仅仅是 Hello world</a></li><li class="chapter-item "><a href="../../first-try/slowly-downloading.html"><strong aria-hidden="true">1.5.</strong> 下载依赖太慢了?</a></li></ol></li><li class="chapter-item expanded "><a href="../../basic/intro.html"><strong aria-hidden="true">2.</strong> Rust 基础入门</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/variable.html"><strong aria-hidden="true">2.1.</strong> 变量绑定与解构</a></li><li class="chapter-item "><a href="../../basic/base-type/index.html"><strong aria-hidden="true">2.2.</strong> 基本类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/base-type/numbers.html"><strong aria-hidden="true">2.2.1.</strong> 数值类型</a></li><li class="chapter-item "><a href="../../basic/base-type/char-bool.html"><strong aria-hidden="true">2.2.2.</strong> 字符、布尔、单元类型</a></li><li class="chapter-item "><a href="../../basic/base-type/statement-expression.html"><strong aria-hidden="true">2.2.3.</strong> 语句与表达式</a></li><li class="chapter-item "><a href="../../basic/base-type/function.html"><strong aria-hidden="true">2.2.4.</strong> 函数</a></li></ol></li><li class="chapter-item "><a href="../../basic/ownership/index.html"><strong aria-hidden="true">2.3.</strong> 所有权和借用</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/ownership/ownership.html"><strong aria-hidden="true">2.3.1.</strong> 所有权</a></li><li class="chapter-item "><a href="../../basic/ownership/borrowing.html"><strong aria-hidden="true">2.3.2.</strong> 引用与借用</a></li></ol></li><li class="chapter-item "><a href="../../basic/compound-type/intro.html"><strong aria-hidden="true">2.4.</strong> 复合类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/compound-type/string-slice.html"><strong aria-hidden="true">2.4.1.</strong> 字符串与切片</a></li><li class="chapter-item "><a href="../../basic/compound-type/tuple.html"><strong aria-hidden="true">2.4.2.</strong> 元组</a></li><li class="chapter-item "><a href="../../basic/compound-type/struct.html"><strong aria-hidden="true">2.4.3.</strong> 结构体</a></li><li class="chapter-item "><a href="../../basic/compound-type/enum.html"><strong aria-hidden="true">2.4.4.</strong> 枚举</a></li><li class="chapter-item "><a href="../../basic/compound-type/array.html"><strong aria-hidden="true">2.4.5.</strong> 数组</a></li></ol></li><li class="chapter-item "><a href="../../basic/flow-control.html"><strong aria-hidden="true">2.5.</strong> 流程控制</a></li><li class="chapter-item expanded "><a href="../../basic/match-pattern/intro.html">
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<main>
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<h1 id="解构-option"><a class="header" href="#解构-option">解构 Option</a></h1>
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<p>在枚举那章,提到过 <code>Option</code> 枚举,它用来解决 Rust 中变量是否有值的问题,定义如下:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Option<T> {
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Some(T),
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None,
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>简单解释就是:<strong>一个变量要么有值:<code>Some(T)</code>, 要么为空:<code>None</code></strong>。</p>
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<p>那么现在的问题就是该如何去使用这个 <code>Option</code> 枚举类型,根据我们上一节的经验,可以通过 <code>match</code> 来实现。</p>
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<blockquote>
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<p>因为 <code>Option</code>,<code>Some</code>,<code>None</code> 都包含在 <code>prelude</code> 中,因此你可以直接通过名称来使用它们,而无需以 <code>Option::Some</code> 这种形式去使用,总之,千万不要因为调用路径变短了,就忘记 <code>Some</code> 和 <code>None</code> 也是 <code>Option</code> 底下的枚举成员!</p>
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</blockquote>
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<h2 id="匹配-optiont"><a class="header" href="#匹配-optiont">匹配 <code>Option<T></code></a></h2>
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<p>使用 <code>Option<T></code>,是为了从 <code>Some</code> 中取出其内部的 <code>T</code> 值以及处理没有值的情况,为了演示这一点,下面一起来编写一个函数,它获取一个 <code>Option<i32></code>,如果其中含有一个值,将其加一;如果其中没有值,则函数返回 <code>None</code> 值:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>fn plus_one(x: Option<i32>) -> Option<i32> {
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match x {
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None => None,
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Some(i) => Some(i + 1),
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}
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}
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let five = Some(5);
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let six = plus_one(five);
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let none = plus_one(None);
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<span class="boring">}
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</span></code></pre></pre>
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<p><code>plus_one</code> 接受一个 <code>Option<i32></code> 类型的参数,同时返回一个 <code>Option<i32></code> 类型的值(这种形式的函数在标准库内随处所见),在该函数的内部处理中,如果传入的是一个 <code>None</code> ,则返回一个 <code>None</code> 且不做任何处理;如果传入的是一个 <code>Some(i32)</code>,则通过模式绑定,把其中的值绑定到变量 <code>i</code> 上,然后返回 <code>i+1</code> 的值,同时用 <code>Some</code> 进行包裹。</p>
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<p>为了进一步说明,假设 <code>plus_one</code> 函数接受的参数值 x 是 <code>Some(5)</code>,来看看具体的分支匹配情况:</p>
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<h4 id="传入参数-some5"><a class="header" href="#传入参数-some5">传入参数 <code>Some(5)</code></a></h4>
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<pre><code class="language-rust ignore">None => None,
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</code></pre>
|
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<p>首先是匹配 <code>None</code> 分支,因为值 <code>Some(5)</code> 并不匹配模式 <code>None</code>,所以继续匹配下一个分支。</p>
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<pre><code class="language-rust ignore">Some(i) => Some(i + 1),
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</code></pre>
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<p><code>Some(5)</code> 与 <code>Some(i)</code> 匹配吗?当然匹配!它们是相同的成员。<code>i</code> 绑定了 <code>Some</code> 中包含的值,因此 <code>i</code> 的值是 <code>5</code>。接着匹配分支的代码被执行,最后将 <code>i</code> 的值加一并返回一个含有值 <code>6</code> 的新 <code>Some</code>。</p>
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<h4 id="传入参数-none"><a class="header" href="#传入参数-none">传入参数 None</a></h4>
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<p>接着考虑下 <code>plus_one</code> 的第二个调用,这次传入的 <code>x</code> 是 <code>None</code>, 我们进入 <code>match</code> 并与第一个分支相比较。</p>
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<pre><code class="language-rust ignore">None => None,
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</code></pre>
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<p>匹配上了!接着程序继续执行该分支后的代码:返回表达式 <code>None</code> 的值,也就是返回一个 <code>None</code>,因为第一个分支就匹配到了,其他的分支将不再比较。</p>
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