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<main>
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<h1 id="复合类型"><a class="header" href="#复合类型">复合类型</a></h1>
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<p>行百里者半九十，欢迎大家来到这里，虽然还不到中点，但是已经不远了。如果说之前学的基础数据类型是原子，那么本章将讲的数据类型可以认为是分子。</p>
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<p>本章的重点在复合类型上，顾名思义，复合类型是由其它类型组合而成的，最典型的就是结构体 <code>struct</code> 和枚举 <code>enum</code>。例如平面上的一个点 <code>point(x, y)</code>，它由两个数值类型的值 <code>x</code> 和 <code>y</code> 组合而来。我们无法单独去维护这两个数值，因为单独一个 <code>x</code> 或者 <code>y</code> 是含义不完整的，无法标识平面上的一个点，应该把它们看作一个整体去理解和处理。</p>
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<p>来看一段代码，它使用我们之前学过的内容来构建文件操作：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">#![allow(unused_variables)]
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type File = String;
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fn open(f: &mut File) -> bool {
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true
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}
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fn close(f: &mut File) -> bool {
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true
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}
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#[allow(dead_code)]
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fn read(f: &mut File, save_to: &mut Vec<u8>) -> ! {
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unimplemented!()
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}
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fn main() {
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let mut f1 = File::from("f1.txt");
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open(&mut f1);
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//read(&mut f1, &mut vec![]);
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close(&mut f1);
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}</code></pre></pre>
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<p>接下来我们的学习非常类似原型设计：有的方法只提供 API 接口，但是不提供具体实现。此外，有的变量在声明之后并未使用，因此在这个阶段我们需要排除一些编译器噪音（Rust 在编译的时候会扫描代码，变量声明后未使用会以 <code>warning</code> 警告的形式进行提示），引入 <code>#![allow(unused_variables)]</code> 属性标记，该标记会告诉编译器忽略未使用的变量，不要抛出 <code>warning</code> 警告，具体的常见编译器属性你可以在这里查阅：<a href="https://course.rs/profiling/compiler/attributes.html">编译器属性标记</a>。</p>
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<p><code>read</code> 函数也非常有趣，它返回一个 <code>!</code> 类型，这个表明该函数是一个发散函数，不会返回任何值，包括 <code>()</code>。<code>unimplemented!()</code> 告诉编译器该函数尚未实现，<code>unimplemented!()</code> 标记通常意味着我们期望快速完成主要代码，回头再通过搜索这些标记来完成次要代码，类似的标记还有 <code>todo!()</code>，当代码执行到这种未实现的地方时，程序会直接报错。你可以反注释 <code>read(&mut f1, &mut vec![]);</code> 这行，然后再观察下结果。</p>
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<p>同时，从代码设计角度来看，关于文件操作的类型和函数应该组织在一起，散落得到处都是，是难以管理和使用的。而且通过 <code>open(&mut f1)</code> 进行调用，也远没有使用 <code>f1.open()</code> 来调用好，这就体现出了只使用基本类型的局限性：<strong>无法从更高的抽象层次去简化代码</strong>。</p>
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<p>接下来，我们将引入一个高级数据结构 —— 结构体 <code>struct</code>，来看看复合类型是怎样更好的解决这类问题。 开始之前，先来看看 Rust 的重点也是难点：字符串 <code>String</code> 和 <code>&str</code>。</p>
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