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<main>
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<h1 id="字符布尔单元类型"><a class="header" href="#字符布尔单元类型">字符、布尔、单元类型</a></h1>
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<p>这三个类型所处的地位比较尴尬，你说它们重要吧，确实出现的身影不是很多，说它们不重要吧，有时候也是不可或缺，而且这三个类型都有一个共同点：简单，因此我们统一放在一起讲。</p>
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<h2 id="字符类型char"><a class="header" href="#字符类型char">字符类型(char)</a></h2>
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<p>字符，对于没有其它编程经验的新手来说可能不太好理解（没有编程经验敢来学 Rust 的绝对是好汉），但是你可以把它理解为英文中的字母，中文中的汉字。</p>
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<p>下面的代码展示了几个颇具异域风情的字符：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let c = 'z';
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let z = 'ℤ';
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let g = '国';
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let heart_eyed_cat = '😻';
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}</code></pre></pre>
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<p>如果大家是从有年代感的编程语言过来，可能会大喊一声：这 XX 叫字符？是的，在 Rust 语言中这些都是字符，Rust 的字符不仅仅是 <code>ASCII</code>，所有的 <code>Unicode</code> 值都可以作为 Rust 字符，包括单个的中文、日文、韩文、emoji 表情符号等等，都是合法的字符类型。<code>Unicode</code> 值的范围从 <code>U+0000 ~ U+D7FF</code> 和 <code>U+E000 ~ U+10FFFF</code>。不过“字符”并不是 <code>Unicode</code> 中的一个概念，所以人在直觉上对“字符”的理解和 Rust 的字符概念并不一致。</p>
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<p>由于 <code>Unicode</code> 都是 4 个字节编码，因此字符类型也是占用 4 个字节：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let x = '中';
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println!("字符'中'占用了{}字节的内存大小", size_of_val(&x));
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}</code></pre></pre>
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<p>输出如下:</p>
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<pre><code class="language-console">$ cargo run
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Compiling ...
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字符'中'占用了4字节的内存大小
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</code></pre>
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<blockquote>
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<p>注意，我们还没开始讲字符串，但是这里提前说一下，和一些语言不同，Rust 的字符只能用 <code>''</code> 来表示， <code>""</code> 是留给字符串的。</p>
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</blockquote>
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<h2 id="布尔bool"><a class="header" href="#布尔bool">布尔(bool)</a></h2>
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<p>Rust 中的布尔类型有两个可能的值：<code>true</code> 和 <code>false</code>，布尔值占用内存的大小为 <code>1</code> 个字节：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let t = true;
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let f: bool = false; // 使用类型标注,显式指定f的类型
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if f {
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println!("这是段毫无意义的代码");
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}
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}</code></pre></pre>
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<p>使用布尔类型的场景主要在于流程控制，例如上述代码的中的 <code>if</code> 就是其中之一。</p>
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<h2 id="单元类型unit"><a class="header" href="#单元类型unit">单元类型(unit)</a></h2>
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<p>单元类型就是 <code>()</code> ，对，你没看错，就是 <code>()</code> ，唯一的值也是 <code>()</code> ，一些读者读到这里可能就不愿意了，你也太敷衍了吧，管这叫类型？</p>
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<p>只能说，再不起眼的东西，都有其用途，在目前为止的学习过程中，大家已经看到过很多次 <code>fn main()</code> 函数的使用吧？那么这个函数返回什么呢？</p>
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<p>没错， <code>main</code> 函数就返回这个单元类型 <code>()</code>，你不能说 <code>main</code> 函数无返回值，因为没有返回值的函数在 Rust 中是有单独的定义的：<code>发散函数( diverging functions )</code>，顾名思义，无法收敛的函数。</p>
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<p>例如常见的 <code>println!()</code> 的返回值也是单元类型 <code>()</code>。</p>
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<p>再比如，你可以用 <code>()</code> 作为 <code>map</code> 的值，表示我们不关注具体的值，只关注 <code>key</code>。 这种用法和 Go 语言的 <em><strong>struct{}</strong></em> 类似，可以作为一个值用来占位，但是完全<strong>不占用</strong>任何内存。</p>
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<h2 id="课后练习"><a class="header" href="#课后练习">课后练习</a></h2>
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<blockquote>
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<p><a href="https://practice-zh.course.rs/basic-types/char-bool-unit.html">Rust By Practice</a>，支持代码在线编辑和运行，并提供详细的<a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/basic-types/char-bool.md">习题解答</a>。</p>
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