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Rust入门教程（二）.html

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  19. </a></div> <!----></nav>  <ul class="sidebar-links"><li><section class="sidebar-group depth-0"><p class="sidebar-heading open"><span>Rust 入门教程（二）：结构体和枚举</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#一、结构体的使用" class="sidebar-link">一、结构体的使用</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#_1-定义和实例化-struct" class="sidebar-link">1. 定义和实例化 struct</a></li><li class="sidebar-sub-header"><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#_2-struct-例子" class="sidebar-link">2. struct 例子</a></li><li class="sidebar-sub-header"><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#_3-struct-方法" class="sidebar-link">3. struct 方法</a></li></ul></li><li><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#二、枚举与模式匹配" class="sidebar-link">二、枚举与模式匹配</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#_1-枚举的定义" class="sidebar-link">1. 枚举的定义</a></li><li class="sidebar-sub-header"><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#_2-option-枚举" class="sidebar-link">2. Option 枚举</a></li><li class="sidebar-sub-header"><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#_3-match" class="sidebar-link">3. match</a></li><li class="sidebar-sub-header"><a href="/rust_camp_tutorial/Rust%E6%96%87%E6%A1%A3/Rust%E5%85%A5%E9%97%A8%E6%95%99%E7%A8%8B%EF%BC%88%E4%BA%8C%EF%BC%89.html#_4-if-let" class="sidebar-link">4. if let</a></li></ul></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="rust-入门教程-二-结构体和枚举"><a href="#rust-入门教程-二-结构体和枚举" class="header-anchor">#</a> Rust 入门教程（二）：结构体和枚举</h1> <h2 id="一、结构体的使用"><a href="#一、结构体的使用" class="header-anchor">#</a> 一、结构体的使用</h2> <h3 id="_1-定义和实例化-struct"><a href="#_1-定义和实例化-struct" class="header-anchor">#</a> 1. 定义和实例化 struct</h3> <p>例子：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">User</span> <span class="token punctuation">{</span>
    20. 

  20.     username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    21. 

  21.     email<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    22. 

  22.     sign_in_count<span class="token punctuation">:</span> <span class="token keyword">usize</span><span class="token punctuation">,</span>
    23. 

  23.     active<span class="token punctuation">:</span> <span class="token keyword">bool</span><span class="token punctuation">,</span>
    24. 

  24. <span class="token punctuation">}</span>
    25. 

  25. </code></pre></div><p>需要注意的是，每个字段后面用逗号隔开，最后一个字段后面可以没有逗号。</p> <p>实例化例子：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> user1 <span class="token operator">=</span> <span class="token class-name">User</span> <span class="token punctuation">{</span>
    26. 

  26.         email<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;[email protected]&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    27. 

  27.         username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;cherry&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    28. 

  28.         active<span class="token punctuation">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    29. 

  29.         sign_in_count<span class="token punctuation">:</span> <span class="token number">244</span><span class="token punctuation">,</span>
    30. 

  30.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    31. 

  31. </code></pre></div><p>先创建 struct 实例，然后为每个字段指定值，无需按照声明的顺序指定。</p> <p>但是注意不能少指定字段。</p> <p>用点标记法取得结构体中的字段值，一旦 struct 的实例是可变的，那么实例中的所有字段都是可变的，不会同时既存在可变的字段又存在不可变的字段。</p> <p><strong>结构体作为函数的返回值</strong></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">struct_build</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">User</span> <span class="token punctuation">{</span>
    32. 

  32.     <span class="token class-name">User</span> <span class="token punctuation">{</span>
    33. 

  33.         email<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;[email protected]&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    34. 

  34.         username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;cherry&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    35. 

  35.         active<span class="token punctuation">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    36. 

  36.         sign_in_count<span class="token punctuation">:</span> <span class="token number">244</span><span class="token punctuation">,</span>
    37. 

  37.     <span class="token punctuation">}</span>
    38. 

  38. <span class="token punctuation">}</span>
    39. 

  39. </code></pre></div><p><strong>字段初始化简写</strong></p> <p>当字段名与字段值对应变量相同的时候，就可以使用字段初始化简写的方式：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">struct_build</span><span class="token punctuation">(</span>email<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span> username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">User</span> <span class="token punctuation">{</span>
    40. 

  40.     <span class="token class-name">User</span> <span class="token punctuation">{</span>
    41. 

  41.         email<span class="token punctuation">,</span>
    42. 

  42.         username<span class="token punctuation">,</span>
    43. 

  43.         active<span class="token punctuation">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
    44. 

  44.         sign_in_count<span class="token punctuation">:</span> <span class="token number">244</span><span class="token punctuation">,</span>
    45. 

  45.     <span class="token punctuation">}</span>
    46. 

  46. <span class="token punctuation">}</span>
    47. 

  47. </code></pre></div><p><strong>struct 更新语法</strong></p> <p>当想基于某个 struct 实例创建一个新的实例时（新的实例中某些字段可能和原先相同，某些不同），若不使用 struct 更新语法，则是这样写：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> user2 <span class="token operator">=</span> <span class="token class-name">User</span> <span class="token punctuation">{</span>
    48. 

  48.         email<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;[email protected]&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    49. 

  49.         username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Chris Paul&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    50. 

  50.         active<span class="token punctuation">:</span> user1<span class="token punctuation">.</span>active<span class="token punctuation">,</span>
    51. 

  51.         sign_in_count<span class="token punctuation">:</span> user1<span class="token punctuation">.</span>sign_in_count<span class="token punctuation">,</span>
    52. 

  52.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    53. 

  53. </code></pre></div><p>而使用 struct 更新语法，则是这样写：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> user3 <span class="token operator">=</span> <span class="token class-name">User</span> <span class="token punctuation">{</span>
    54. 

  54.         email<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;[email protected]&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    55. 

  55.         username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Chris Paul&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    56. 

  56.         <span class="token punctuation">..</span>user1
    57. 

  57.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    58. 

  58. </code></pre></div><p>用 <code>..user1</code> 表示该实例中未赋值的其他字段和实例 <code>user1</code> 中的值一致。</p> <p><strong>Tuple Struct</strong></p> <p>可以定义类似 Tuple 的 Struct，叫做 Tuple Struct。Tuple struct 整体有个名，但里面的元素没有名</p> <p>适用：想给整个 tuple 起名，并让它不同于其它 tuple，而且又不需要给每个元素起名</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">Color</span><span class="token punctuation">(</span><span class="token keyword">i32</span><span class="token punctuation">,</span> <span class="token keyword">i32</span><span class="token punctuation">,</span> <span class="token keyword">i32</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    59. 

  59. <span class="token keyword">struct</span> <span class="token type-definition class-name">Point</span><span class="token punctuation">(</span><span class="token keyword">i32</span><span class="token punctuation">,</span> <span class="token keyword">i32</span><span class="token punctuation">,</span> <span class="token keyword">i32</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    60. 

  60. <span class="token keyword">let</span> <span class="token function">black</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    61. 

  61. <span class="token keyword">let</span> origin <span class="token operator">=</span> <span class="token class-name">Point</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    62. 

  62. </code></pre></div><p>black 和 origin 是不同的类型，是不同 tuple struct 的实例</p> <p><strong>Unit-Like Struct(没有任何字段)</strong></p> <p>可以定义没有任何字段的 struct，叫做 <code>Unit-Like struct</code>，因为与 <code>()</code> 和单元类型类似，适用于需要在某个类型上实现某个trait，但是在里面又没有想要存储的数据</p> <p><strong>struct 数据所有权</strong></p> <p>再来看这个例子：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">User</span> <span class="token punctuation">{</span>
    63. 

  63.     username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    64. 

  64.     email<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    65. 

  65.     sign_in_count<span class="token punctuation">:</span> <span class="token keyword">usize</span><span class="token punctuation">,</span>
    66. 

  66.     active<span class="token punctuation">:</span> <span class="token keyword">bool</span><span class="token punctuation">,</span>
    67. 

  67. <span class="token punctuation">}</span>
    68. 

  68. </code></pre></div><p>这里的字段使用了 <code>String</code> 而不是 <code>&amp;str</code>，原因如下：</p> <ul><li>该 struct 实例拥有其所有的数据</li> <li>只要 struct 实例是有效的，那么里面的字段数据也是有效的 struct 里也可以存放引用，但这需要使用生命周期（以后讲）</li> <li>若字段为 <code>&amp;str</code>，当其有效作用域小于该实例的作用域，该字段被清理时，实例未清理，访问该字段属于悬垂引用（类似野指针）</li> <li>生命周期保证只要 struct 实例是有效的，那么里面的引用也是有效的</li> <li>如果 struct 里面存储引用，而不使用生命周期，就会报错</li></ul> <h3 id="_2-struct-例子"><a href="#_2-struct-例子" class="header-anchor">#</a> 2. struct 例子</h3> <p>一个简单的例子：计算长方形的面积</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    69. 

  69.     <span class="token keyword">let</span> width <span class="token operator">=</span> <span class="token number">25</span><span class="token punctuation">;</span>
    70. 

  70.     <span class="token keyword">let</span> length <span class="token operator">=</span> <span class="token number">12</span><span class="token punctuation">;</span>
    71. 

  71.     <span class="token keyword">let</span> area <span class="token operator">=</span> <span class="token function">area_of_rectangle</span><span class="token punctuation">(</span>width<span class="token punctuation">,</span> length<span class="token punctuation">)</span><span class="token punctuation">;</span>
    72. 

  72.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The Area of Rectangle is {}.&quot;</span><span class="token punctuation">,</span> area<span class="token punctuation">)</span><span class="token punctuation">;</span>
    73. 

  73. <span class="token punctuation">}</span>
    74. 

  74. 

  75. <span class="token keyword">fn</span> <span class="token function-definition function">area_of_rectangle</span><span class="token punctuation">(</span>width<span class="token punctuation">:</span> <span class="token keyword">usize</span><span class="token punctuation">,</span> length<span class="token punctuation">:</span> <span class="token keyword">usize</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">usize</span> <span class="token punctuation">{</span>
    76. 

  76.     width <span class="token operator">*</span> length
    77. 

  77. <span class="token punctuation">}</span>
    78. 

  78. </code></pre></div><p>上面这个例子很简单，但是长方形的长和宽没有联系起来，width 和 length 是两个分离的没有逻辑联系的变量，我们考虑用元组将其联系起来：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    79. 

  79.     <span class="token keyword">let</span> rect <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">,</span> <span class="token number">12</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    80. 

  80.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The Area of Rectangle is {}.&quot;</span><span class="token punctuation">,</span> <span class="token function">area_of_rectangle</span><span class="token punctuation">(</span>rect<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    81. 

  81. <span class="token punctuation">}</span>
    82. 

  82. 

  83. <span class="token keyword">fn</span> <span class="token function-definition function">area_of_rectangle</span><span class="token punctuation">(</span>rect<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token keyword">u32</span><span class="token punctuation">,</span> <span class="token keyword">u32</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u32</span> <span class="token punctuation">{</span>
    84. 

  84.     rect<span class="token number">.0</span> <span class="token operator">*</span> rect<span class="token number">.1</span>
    85. 

  85. <span class="token punctuation">}</span>
    86. 

  86. </code></pre></div><p>但是这样仿佛可读性变差了，我们再用结构体实现：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">Rectangle</span> <span class="token punctuation">{</span>
    87. 

  87.     width<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    88. 

  88.     length<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    89. 

  89. <span class="token punctuation">}</span>
    90. 

  90. 

  91. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    92. 

  92.     <span class="token keyword">let</span> rect <span class="token operator">=</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    93. 

  93.         width<span class="token punctuation">:</span> <span class="token number">35</span><span class="token punctuation">,</span>
    94. 

  94.         length<span class="token punctuation">:</span> <span class="token number">12</span><span class="token punctuation">,</span>
    95. 

  95.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    96. 

  96.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The Area of Rectangle is {}.&quot;</span><span class="token punctuation">,</span> <span class="token function">area_of_rectangle</span><span class="token punctuation">(</span><span class="token operator">&amp;</span>rect<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    97. 

  97. <span class="token punctuation">}</span>
    98. 

  98. 

  99. <span class="token keyword">fn</span> <span class="token function-definition function">area_of_rectangle</span><span class="token punctuation">(</span>rect<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token class-name">Rectangle</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u32</span> <span class="token punctuation">{</span>
    100. 

  100.     rect<span class="token punctuation">.</span>length <span class="token operator">*</span> rect<span class="token punctuation">.</span>width
    101. 

  101. <span class="token punctuation">}</span>
    102. 

  102. </code></pre></div><p>函数的参数使用结构体的借用，是为了不获得该实例的所有权，主函数在函数调用之后还可以继续使用该实例。</p> <p>此时我们输出实例 <code>rect</code>，会报这样的错误：<code>Rectangle doesn't implement std::fmt::Display</code>，即该结构体未实现 <code>Display</code> 这个 trait，而 Rust 中很多类型都是实现了这个 trait，才能将其在终端打印出来。因为结构体这种比标量类型更加复杂，打印的类型的可能性很多，因此需要用户自定义实现 <code>Display</code>。</p> <p>在报错的提示信息里，有个 note 提示我们可以使用 <code>{:?}</code> (或 <code>{:#?}</code>)来打印信息:</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;{:?}&quot;</span><span class="token punctuation">,</span> rect<span class="token punctuation">)</span><span class="token punctuation">;</span>
    103. 

  103. </code></pre></div><p>然而又出现了错误，这次的报错信息是：<code>Rectangle doesn't implement Debug</code>，显然 <code>Debug</code> 也是一种格式化方法，再看提示的 note：<code>add #[derive(Debug)] to Rectangle or manually impl Debug for Rectangle</code>，我们在结构体前添加 <code>#[derive(Debug)]</code>，使得该结构体派生与 <code>Debug</code> 这个 trait。</p> <p>最终完整的程序如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token attribute attr-name">#[derive(Debug)]</span>
    104. 

  104. <span class="token keyword">struct</span> <span class="token type-definition class-name">Rectangle</span> <span class="token punctuation">{</span>
    105. 

  105.     width<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    106. 

  106.     length<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    107. 

  107. <span class="token punctuation">}</span>
    108. 

  108. 

  109. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    110. 

  110.     <span class="token keyword">let</span> rect <span class="token operator">=</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    111. 

  111.         width<span class="token punctuation">:</span> <span class="token number">35</span><span class="token punctuation">,</span>
    112. 

  112.         length<span class="token punctuation">:</span> <span class="token number">12</span><span class="token punctuation">,</span>
    113. 

  113.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    114. 

  114. 

  115.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The Area of Rectangle is {}.&quot;</span><span class="token punctuation">,</span> <span class="token function">area_of_rectangle</span><span class="token punctuation">(</span><span class="token operator">&amp;</span>rect<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    116. 

  116.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;{:?}&quot;</span><span class="token punctuation">,</span> rect<span class="token punctuation">)</span><span class="token punctuation">;</span>
    117. 

  117. <span class="token punctuation">}</span>
    118. 

  118. 

  119. <span class="token keyword">fn</span> <span class="token function-definition function">area_of_rectangle</span><span class="token punctuation">(</span>rect<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token class-name">Rectangle</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u32</span> <span class="token punctuation">{</span>
    120. 

  120.     rect<span class="token punctuation">.</span>length <span class="token operator">*</span> rect<span class="token punctuation">.</span>width
    121. 

  121. <span class="token punctuation">}</span>
    122. 

  122. </code></pre></div><p>输出结果为：</p> <div class="language-rust extra-class"><pre class="language-rust"><code>➜  ~<span class="token operator">/</span><span class="token class-name">Code</span><span class="token operator">/</span>rust<span class="token operator">/</span>area_of_rectangle git<span class="token punctuation">:</span><span class="token punctuation">(</span>master<span class="token punctuation">)</span> ✗ cargo run
    123. 

  123.    <span class="token class-name">Compiling</span> area_of_rectangle v0<span class="token number">.1</span><span class="token punctuation">.</span><span class="token number">0</span> <span class="token punctuation">(</span><span class="token operator">/</span>home<span class="token operator">/</span>cherry<span class="token operator">/</span><span class="token class-name">Code</span><span class="token operator">/</span>rust<span class="token operator">/</span>area_of_rectangle<span class="token punctuation">)</span>
    124. 

  124.     <span class="token class-name">Finished</span> dev <span class="token punctuation">[</span>unoptimized <span class="token operator">+</span> debuginfo<span class="token punctuation">]</span> <span class="token function">target</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token keyword">in</span> <span class="token number">0</span><span class="token punctuation">.</span>25s
    125. 

  125.      <span class="token class-name">Running</span> `target<span class="token operator">/</span>debug<span class="token operator">/</span>area_of_rectangle`
    126. 

  126. <span class="token class-name">The</span> <span class="token class-name">Area</span> of <span class="token class-name">Rectangle</span> is <span class="token number">420</span><span class="token punctuation">.</span>
    127. 

  127. <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span> width<span class="token punctuation">:</span> <span class="token number">35</span><span class="token punctuation">,</span> length<span class="token punctuation">:</span> <span class="token number">12</span> <span class="token punctuation">}</span>
    128. 

  128. </code></pre></div><p>若在输出格式中间加入一个 <code>#</code>，结构体输出将更加清晰：<code>println!(&quot;{:?}&quot;, rect);</code>，输出结果为：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    129. 

  129.     width<span class="token punctuation">:</span> <span class="token number">35</span><span class="token punctuation">,</span>
    130. 

  130.     length<span class="token punctuation">:</span> <span class="token number">12</span><span class="token punctuation">,</span>
    131. 

  131. <span class="token punctuation">}</span>
    132. 

  132. </code></pre></div><h3 id="_3-struct-方法"><a href="#_3-struct-方法" class="header-anchor">#</a> 3. struct 方法</h3> <p>方法和函数类似: fn关键字、名称、参数、返回值</p> <p>方法与函数不同之处:</p> <ul><li>方法是在 struct(或 enum、trait 对象）的上下文中定义</li> <li>第一个参数是 self，表示方法被调用的 struct 实例</li></ul> <p>上一节我们定义了计算长方形面积的函数，但是该函数只能计算长方形的函数，无法计算其他形状的面积，因此我们希望将函数与长方形这一结构体关联起来，例子如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">Rectangle</span> <span class="token punctuation">{</span>
    133. 

  133.     width<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    134. 

  134.     length<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    135. 

  135. <span class="token punctuation">}</span>
    136. 

  136. 

  137. <span class="token keyword">impl</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    138. 

  138.     <span class="token keyword">fn</span> <span class="token function-definition function">area_of_rectangle</span><span class="token punctuation">(</span><span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u32</span> <span class="token punctuation">{</span>
    139. 

  139.         <span class="token keyword">self</span><span class="token punctuation">.</span>length <span class="token operator">*</span> <span class="token keyword">self</span><span class="token punctuation">.</span>width
    140. 

  140.     <span class="token punctuation">}</span>
    141. 

  141. <span class="token punctuation">}</span>
    142. 

  142. 

  143. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    144. 

  144.     <span class="token keyword">let</span> rect <span class="token operator">=</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    145. 

  145.         width<span class="token punctuation">:</span> <span class="token number">35</span><span class="token punctuation">,</span>
    146. 

  146.         length<span class="token punctuation">:</span> <span class="token number">12</span><span class="token punctuation">,</span>
    147. 

  147.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    148. 

  148. 

  149.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The Area of Rectangle is {}.&quot;</span><span class="token punctuation">,</span> rect<span class="token punctuation">.</span><span class="token function">area_of_rectangle</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    150. 

  150. <span class="token punctuation">}</span>
    151. 

  151. </code></pre></div><p>在impl块里定义方法，方法的第一个参数可以是 <code>&amp;self</code>，也可以<strong>获得其所有权</strong>或<strong>可变借用</strong>，和其他参数一样。这样写可以有更良好的代码组织。</p> <p><strong>方法调用的运算符</strong></p> <ul><li>C/C++ 中 <code>object-&gt;something()</code> 和 <code>(*object).something()</code> 一样，但是 Rust 没有 <code>→</code> 运算符</li> <li>Rust 会自动引用或解引用一在调用方法时就会发生这种行为</li> <li>在调用方法时，Rust 根据情况自动添加 <code>&amp;</code>、<code>&amp;mut</code> 或 <code>*</code>，以便 object 可以匹配方法的签名</li> <li>下面这两种写法效果相同
    152. 

  152. <ul><li><code>p1.distance(&amp;p2);</code></li> <li><code>(&amp;p1).distance(&amp;p2);</code></li></ul></li></ul> <p><strong>方法参数</strong></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">impl</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    153. 

  153.     <span class="token keyword">fn</span> <span class="token function-definition function">area_of_rectangle</span><span class="token punctuation">(</span><span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u32</span> <span class="token punctuation">{</span>
    154. 

  154.         <span class="token keyword">self</span><span class="token punctuation">.</span>length <span class="token operator">*</span> <span class="token keyword">self</span><span class="token punctuation">.</span>width
    155. 

  155.     <span class="token punctuation">}</span>
    156. 

  156. 

  157.     <span class="token keyword">fn</span> <span class="token function-definition function">can_hold</span><span class="token punctuation">(</span><span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">,</span> other<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token class-name">Rectangle</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">bool</span> <span class="token punctuation">{</span>
    158. 

  158.         <span class="token keyword">self</span><span class="token punctuation">.</span>length <span class="token operator">&gt;</span> other<span class="token punctuation">.</span>length <span class="token operator">&amp;&amp;</span> <span class="token keyword">self</span><span class="token punctuation">.</span>width <span class="token operator">&gt;</span> other<span class="token punctuation">.</span>width 
    159. 

  159.     <span class="token punctuation">}</span>
    160. 

  160. <span class="token punctuation">}</span>
    161. 

  161. </code></pre></div><p><strong>关联函数</strong></p> <p>可以在 impl 块里定义不把 self 作为第一个参数的函数，它们叫关联函数（不叫方法）</p> <p>例如：<code>String::from()</code></p> <p>关联函数通常用于构造器，例子如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">Rectangle</span> <span class="token punctuation">{</span>
    162. 

  162.     width<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    163. 

  163.     length<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
    164. 

  164. <span class="token punctuation">}</span>
    165. 

  165. 

  166. 

  167. <span class="token keyword">impl</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    168. 

  168.     <span class="token keyword">fn</span> <span class="token function-definition function">square</span><span class="token punctuation">(</span>size<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    169. 

  169.         <span class="token class-name">Rectangle</span> <span class="token punctuation">{</span>
    170. 

  170.             width<span class="token punctuation">:</span> size<span class="token punctuation">,</span>
    171. 

  171.             length<span class="token punctuation">:</span> size<span class="token punctuation">,</span>
    172. 

  172.         <span class="token punctuation">}</span>
    173. 

  173.     <span class="token punctuation">}</span>
    174. 

  174. <span class="token punctuation">}</span>
    175. 

  175. 

  176. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    177. 

  177.     <span class="token keyword">let</span> s <span class="token operator">=</span> <span class="token class-name">Rectangle</span><span class="token punctuation">::</span><span class="token function">square</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    178. 

  178. <span class="token punctuation">}</span>
    179. 

  179. </code></pre></div><p><code>::</code> 符号</p> <ul><li>关联函数</li> <li>模块创建的命名空间</li></ul> <h2 id="二、枚举与模式匹配"><a href="#二、枚举与模式匹配" class="header-anchor">#</a> 二、枚举与模式匹配</h2> <h3 id="_1-枚举的定义"><a href="#_1-枚举的定义" class="header-anchor">#</a> 1. 枚举的定义</h3> <p>枚举允许我们列举所有可能的类型来定义一个类型</p> <p>例如 IP 地址，目前只有 IPv4 和 IPv6 两种类型，我们可以定义这样的枚举类型并使用：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">enum</span> <span class="token type-definition class-name">IpAddrKind</span> <span class="token punctuation">{</span>
    180. 

  180.     <span class="token constant">V4</span><span class="token punctuation">,</span>
    181. 

  181.     <span class="token constant">V6</span><span class="token punctuation">,</span>
    182. 

  182. <span class="token punctuation">}</span>
    183. 

  183. 

  184. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    185. 

  185.     <span class="token keyword">let</span> four <span class="token operator">=</span> <span class="token class-name">IpAddrKind</span><span class="token punctuation">::</span><span class="token constant">V4</span><span class="token punctuation">;</span>
    186. 

  186.     <span class="token keyword">let</span> six <span class="token operator">=</span> <span class="token class-name">IpAddrKind</span><span class="token punctuation">::</span><span class="token constant">V6</span><span class="token punctuation">;</span>
    187. 

  187.     <span class="token function">route</span><span class="token punctuation">(</span>four<span class="token punctuation">)</span><span class="token punctuation">;</span>
    188. 

  188.     <span class="token function">route</span><span class="token punctuation">(</span>six<span class="token punctuation">)</span><span class="token punctuation">;</span>
    189. 

  189.     <span class="token function">route</span><span class="token punctuation">(</span><span class="token class-name">IpAddrKind</span><span class="token punctuation">::</span><span class="token constant">V6</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    190. 

  190. <span class="token punctuation">}</span>
    191. 

  191. 

  192. <span class="token keyword">fn</span> <span class="token function-definition function">route</span><span class="token punctuation">(</span>ip_kind<span class="token punctuation">:</span> <span class="token class-name">IpAddrKind</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    193. 

  193. </code></pre></div><p>枚举的变体都位于标识符的命名空间下，使用两个冒号 <code>::</code> 进行分隔</p> <p>枚举类型是一种自定义的类型，因此它可以作为结构体里字段的类型，例子如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">enum</span> <span class="token type-definition class-name">IpAddrKind</span> <span class="token punctuation">{</span>
    194. 

  194.     <span class="token constant">V4</span><span class="token punctuation">,</span>
    195. 

  195.     <span class="token constant">V6</span><span class="token punctuation">,</span>
    196. 

  196. <span class="token punctuation">}</span>
    197. 

  197. 

  198. <span class="token keyword">struct</span> <span class="token type-definition class-name">IpAddr</span> <span class="token punctuation">{</span>
    199. 

  199.     kind<span class="token punctuation">:</span> <span class="token class-name">IpAddrKind</span><span class="token punctuation">,</span>
    200. 

  200.     address<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    201. 

  201. <span class="token punctuation">}</span>
    202. 

  202. 

  203. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    204. 

  204.     <span class="token keyword">let</span> home <span class="token operator">=</span> <span class="token class-name">IpAddr</span> <span class="token punctuation">{</span>
    205. 

  205.         kind<span class="token punctuation">:</span> <span class="token class-name">IpAddrKind</span><span class="token punctuation">::</span><span class="token constant">V4</span><span class="token punctuation">,</span>
    206. 

  206.         address<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;192.168.3.1&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    207. 

  207.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    208. 

  208. <span class="token punctuation">}</span>
    209. 

  209. </code></pre></div><p><strong>将数据附加到枚举的变体中</strong></p> <p>上述的枚举类型我们可以改为：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">enum</span> <span class="token type-definition class-name">IpAddr</span> <span class="token punctuation">{</span>
    210. 

  210.     <span class="token constant">V4</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    211. 

  211.     <span class="token constant">V6</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    212. 

  212. <span class="token punctuation">}</span>
    213. 

  213. </code></pre></div><p>优点是：不需要使用 struct，<strong>每个变体可以拥有不同的类型以及相关联的数据量</strong>，例如</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">enum</span> <span class="token type-definition class-name">IpAddr</span> <span class="token punctuation">{</span>
    214. 

  214.     <span class="token constant">V4</span><span class="token punctuation">(</span><span class="token keyword">u8</span><span class="token punctuation">,</span> <span class="token keyword">u8</span><span class="token punctuation">,</span> <span class="token keyword">u8</span><span class="token punctuation">,</span> <span class="token keyword">u8</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    215. 

  215.     <span class="token constant">V6</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    216. 

  216. <span class="token punctuation">}</span>
    217. 

  217. 

  218. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    219. 

  219.     <span class="token keyword">let</span> home <span class="token operator">=</span> <span class="token class-name">IpAddrKind</span><span class="token punctuation">::</span><span class="token constant">V4</span><span class="token punctuation">(</span><span class="token number">192</span><span class="token punctuation">,</span> <span class="token number">168</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    220. 

  220.     <span class="token keyword">let</span> loopback <span class="token operator">=</span> <span class="token class-name">IpAddrKind</span><span class="token punctuation">::</span><span class="token constant">V6</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;::1&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    221. 

  221. <span class="token punctuation">}</span>
    222. 

  222. </code></pre></div><p><strong>标准库中的 IpAddr</strong></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">lpv4Addr</span> <span class="token punctuation">{</span>
    223. 

  223.     <span class="token comment">// --snip--</span>
    224. 

  224. <span class="token punctuation">}</span>
    225. 

  225. <span class="token keyword">struct</span> <span class="token type-definition class-name">lpv6Addr</span> <span class="token punctuation">{</span>
    226. 

  226.     <span class="token comment">// --snip--</span>
    227. 

  227. <span class="token punctuation">}</span>
    228. 

  228. <span class="token keyword">enum</span> <span class="token type-definition class-name">lpAddr</span> <span class="token punctuation">{</span>
    229. 

  229.     <span class="token constant">V4</span><span class="token punctuation">(</span>lpv4Addr<span class="token punctuation">)</span><span class="token punctuation">,</span>
    230. 

  230.     <span class="token constant">V6</span><span class="token punctuation">(</span>lpv6Addr<span class="token punctuation">)</span><span class="token punctuation">,</span>
    231. 

  231. <span class="token punctuation">}</span>
    232. 

  232. </code></pre></div><p><strong>为枚举定义方法</strong></p> <p>也使用 <code>impl</code> 这个关键字</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">enum</span> <span class="token type-definition class-name">Message</span> <span class="token punctuation">{</span>
    233. 

  233.     <span class="token class-name">Quit</span><span class="token punctuation">,</span>
    234. 

  234.     <span class="token class-name">Move</span> <span class="token punctuation">{</span>x<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">}</span><span class="token punctuation">,</span>
    235. 

  235.     <span class="token class-name">Write</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    236. 

  236.     <span class="token class-name">ChangeColor</span><span class="token punctuation">(</span><span class="token keyword">i32</span><span class="token punctuation">,</span> <span class="token keyword">i32</span><span class="token punctuation">,</span> <span class="token keyword">i32</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    237. 

  237. <span class="token punctuation">}</span>
    238. 

  238. 

  239. <span class="token keyword">impl</span> <span class="token class-name">Message</span> <span class="token punctuation">{</span>
    240. 

  240.     <span class="token keyword">fn</span> <span class="token function-definition function">call</span><span class="token punctuation">(</span><span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    241. 

  241. <span class="token punctuation">}</span>
    242. 

  242. 

  243. <span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    244. 

  244.     <span class="token keyword">let</span> q <span class="token operator">=</span> <span class="token class-name">Message</span><span class="token punctuation">::</span><span class="token class-name">Quit</span><span class="token punctuation">;</span>
    245. 

  245.     <span class="token keyword">let</span> m <span class="token operator">=</span> <span class="token class-name">Message</span><span class="token punctuation">::</span><span class="token class-name">Move</span><span class="token punctuation">{</span>x<span class="token punctuation">:</span> <span class="token number">10</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token number">12</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    246. 

  246.     <span class="token keyword">let</span> w <span class="token operator">=</span> <span class="token class-name">Message</span><span class="token punctuation">::</span><span class="token class-name">Write</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Hello&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    247. 

  247.     <span class="token keyword">let</span> c <span class="token operator">=</span> <span class="token class-name">Message</span><span class="token punctuation">::</span><span class="token class-name">ChangeColor</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">255</span><span class="token punctuation">,</span> <span class="token number">255</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    248. 

  248. <span class="token punctuation">}</span>
    249. 

  249. </code></pre></div><h3 id="_2-option-枚举"><a href="#_2-option-枚举" class="header-anchor">#</a> 2. Option 枚举</h3> <ul><li>定义于标准库中</li> <li>在 Prelude（预导入模块）中</li> <li>描述了某个值可能存在（某种类型）或不存在的情况</li></ul> <p><strong>Rust 中没有 NULL</strong></p> <p>其它语言中:</p> <ul><li>Null是一个值，它表示“没有值”</li> <li>一个变量可以处于两种状态：空值（null）、非空</li> <li>Null 引用：Billion Dollar Mistake</li></ul> <p>Null 的问题在于:当你尝试像使用非Null值那样使用Null值的时候，就会引起某种错误，但是 Null 的概念还是有用的：<strong>因某种原因而变为无效或缺失的值</strong></p> <p>Rust 中类似与 NULL 的概念的枚举：<code>Option&lt;T&gt;</code></p> <p>标准库中的定义：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">enum</span> <span class="token type-definition class-name">Option</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    250. 

  250.     <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token class-name">T</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    251. 

  251.     <span class="token class-name">None</span><span class="token punctuation">,</span>
    252. 

  252. <span class="token punctuation">}</span>
    253. 

  253. </code></pre></div><p>它包含在预导入模块（Prelude）中，可以直接使用 <code>Option&lt;T&gt;</code>, <code>Some(T)</code>, <code>None</code>。例子如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    254. 

  254.     <span class="token keyword">let</span> some_num <span class="token operator">=</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    255. 

  255.     <span class="token keyword">let</span> some_string <span class="token operator">=</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token string">&quot;The String&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    256. 

  256.     <span class="token keyword">let</span> absent_num<span class="token punctuation">:</span> <span class="token class-name">Option</span><span class="token operator">&lt;</span><span class="token keyword">i32</span><span class="token operator">&gt;</span> <span class="token operator">=</span> <span class="token class-name">None</span><span class="token punctuation">;</span>
    257. 

  257. <span class="token punctuation">}</span>
    258. 

  258. </code></pre></div><p>其中 <code>Some(3)</code> 编译器可以推断出来 <code>T</code> 类型为 <code>usize</code>，而 <code>None</code> 的话编译器无法推断出来，因此需要显式指定 <code>Option&lt;i32&gt;</code></p> <p>这种设计比 NULL 好在哪？</p> <p>因为 <code>Option&lt;T&gt;</code> 和 <code>T</code> 是不同的类型，不能将 <code>Option&lt;T&gt;</code> 当成 <code>T</code> 使用，例子如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test02</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    259. 

  259.     <span class="token keyword">let</span> x<span class="token punctuation">:</span> <span class="token keyword">i8</span> <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">;</span>
    260. 

  260.     <span class="token keyword">let</span> y<span class="token punctuation">:</span> <span class="token class-name">Option</span><span class="token operator">&lt;</span><span class="token keyword">i8</span><span class="token operator">&gt;</span> <span class="token operator">=</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    261. 

  261.     <span class="token keyword">let</span> sum <span class="token operator">=</span> x <span class="token operator">+</span> y<span class="token punctuation">;</span>
    262. 

  262. <span class="token punctuation">}</span>
    263. 

  263. </code></pre></div><p>这样会报错，提示 <code>cannot add Option&lt;i8&gt; to i8</code>，表示两者不是同一个类型，若想使用 <code>Option&lt;T&gt;</code> 中的 <code>T</code>，则必须将其手动转换为 <code>T</code>，这种设计方式可以避免代码中 NULL 值泛滥的情况。</p> <h3 id="_3-match"><a href="#_3-match" class="header-anchor">#</a> 3. match</h3> <p><strong>强大的控制流运算符 match</strong></p> <p>允许一个值与一系列模式进行匹配，并执行匹配的模式对应的代码。模式可以是字面值、变量名、通配符...</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">enum</span> <span class="token type-definition class-name">Coin</span> <span class="token punctuation">{</span>
    264. 

  264.     <span class="token class-name">Penny</span><span class="token punctuation">,</span>
    265. 

  265.     <span class="token class-name">Nickel</span><span class="token punctuation">,</span>
    266. 

  266.     <span class="token class-name">Dime</span><span class="token punctuation">,</span>
    267. 

  267.     <span class="token class-name">Quarter</span><span class="token punctuation">,</span>
    268. 

  268. <span class="token punctuation">}</span>
    269. 

  269. 

  270. <span class="token keyword">fn</span> <span class="token function-definition function">value_in_cents</span><span class="token punctuation">(</span>coin<span class="token punctuation">:</span> <span class="token class-name">Coin</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u8</span> <span class="token punctuation">{</span>
    271. 

  271.     <span class="token keyword">match</span> coin <span class="token punctuation">{</span>
    272. 

  272.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Penny</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    273. 

  273.             <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Penny!&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    274. 

  274.             <span class="token number">1</span>
    275. 

  275.         <span class="token punctuation">}</span><span class="token punctuation">,</span>
    276. 

  276.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Nickel</span> <span class="token operator">=&gt;</span> <span class="token number">5</span><span class="token punctuation">,</span>
    277. 

  277.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Dime</span> <span class="token operator">=&gt;</span> <span class="token number">10</span><span class="token punctuation">,</span>
    278. 

  278.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Quarter</span> <span class="token operator">=&gt;</span> <span class="token number">25</span><span class="token punctuation">,</span>
    279. 

  279.     <span class="token punctuation">}</span>
    280. 

  280. <span class="token punctuation">}</span>
    281. 

  281. </code></pre></div><p><strong>绑定值的模式</strong></p> <p>匹配的分支可以绑定到被匹配对象的部分值，因此可以从 enum 变体中提取值，例子如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token attribute attr-name">#[derive(Debug)]</span>
    282. 

  282. <span class="token keyword">enum</span> <span class="token type-definition class-name">USState</span> <span class="token punctuation">{</span>
    283. 

  283.     <span class="token class-name">California</span><span class="token punctuation">,</span>
    284. 

  284.     <span class="token class-name">Texas</span><span class="token punctuation">,</span>
    285. 

  285. <span class="token punctuation">}</span>
    286. 

  286. 

  287. <span class="token keyword">enum</span> <span class="token type-definition class-name">Coin</span> <span class="token punctuation">{</span>
    288. 

  288.     <span class="token class-name">Penny</span><span class="token punctuation">,</span>
    289. 

  289.     <span class="token class-name">Nickel</span><span class="token punctuation">,</span>
    290. 

  290.     <span class="token class-name">Dime</span><span class="token punctuation">,</span>
    291. 

  291.     <span class="token class-name">Quarter</span><span class="token punctuation">(</span><span class="token class-name">USState</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    292. 

  292. <span class="token punctuation">}</span>
    293. 

  293. 

  294. <span class="token keyword">fn</span> <span class="token function-definition function">value_in_cents</span><span class="token punctuation">(</span>coin<span class="token punctuation">:</span> <span class="token class-name">Coin</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u8</span> <span class="token punctuation">{</span>
    295. 

  295.     <span class="token keyword">match</span> coin <span class="token punctuation">{</span>
    296. 

  296.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Penny</span> <span class="token operator">=&gt;</span> <span class="token number">1</span><span class="token punctuation">,</span>
    297. 

  297.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Nickel</span> <span class="token operator">=&gt;</span> <span class="token number">5</span><span class="token punctuation">,</span>
    298. 

  298.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Dime</span> <span class="token operator">=&gt;</span> <span class="token number">10</span><span class="token punctuation">,</span>  
    299. 

  299.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Quarter</span><span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    300. 

  300.             <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;State quarter from {:?}&quot;</span><span class="token punctuation">,</span> state<span class="token punctuation">)</span><span class="token punctuation">;</span>
    301. 

  301.             <span class="token number">25</span>
    302. 

  302.         <span class="token punctuation">}</span><span class="token punctuation">,</span>
    303. 

  303. 

  304.         <span class="token comment">/* Coin::Quarter(state) 也可以这样展开写 */</span>
    305. 

  305.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Quarter</span><span class="token punctuation">(</span><span class="token class-name">USState</span><span class="token punctuation">::</span><span class="token class-name">Texas</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    306. 

  306.             <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;State quarter from {:?}&quot;</span><span class="token punctuation">,</span> <span class="token class-name">USState</span><span class="token punctuation">::</span><span class="token class-name">Texas</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    307. 

  307.             <span class="token number">25</span>
    308. 

  308.         <span class="token punctuation">}</span><span class="token punctuation">,</span>
    309. 

  309.         <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Quarter</span><span class="token punctuation">(</span><span class="token class-name">USState</span><span class="token punctuation">::</span><span class="token class-name">California</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    310. 

  310.             <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;State quarter from {:?}&quot;</span><span class="token punctuation">,</span> <span class="token class-name">USState</span><span class="token punctuation">::</span><span class="token class-name">California</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    311. 

  311.             <span class="token number">25</span>
    312. 

  312.         <span class="token punctuation">}</span>
    313. 

  313.     <span class="token punctuation">}</span>
    314. 

  314. <span class="token punctuation">}</span>
    315. 

  315. 

  316. <span class="token keyword">fn</span> <span class="token function-definition function">test03</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    317. 

  317.     <span class="token keyword">let</span> c <span class="token operator">=</span> <span class="token class-name">Coin</span><span class="token punctuation">::</span><span class="token class-name">Quarter</span><span class="token punctuation">(</span><span class="token class-name">USState</span><span class="token punctuation">::</span><span class="token class-name">California</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    318. 

  318.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;{}&quot;</span><span class="token punctuation">,</span> <span class="token function">value_in_cents</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    319. 

  319. <span class="token punctuation">}</span>
    320. 

  320. </code></pre></div><p><strong>匹配 Option<T></T></strong></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test04</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    321. 

  321.     <span class="token keyword">let</span> five <span class="token operator">=</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    322. 

  322.     <span class="token keyword">let</span> six <span class="token operator">=</span> <span class="token function">plus_one</span><span class="token punctuation">(</span>five<span class="token punctuation">)</span><span class="token punctuation">;</span>
    323. 

  323.     <span class="token keyword">let</span> none <span class="token operator">=</span> <span class="token function">plus_one</span><span class="token punctuation">(</span><span class="token class-name">None</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    324. 

  324. <span class="token punctuation">}</span>
    325. 

  325. 

  326. <span class="token keyword">fn</span> <span class="token function-definition function">plus_one</span><span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token class-name">Option</span><span class="token operator">&lt;</span><span class="token keyword">i32</span><span class="token operator">&gt;</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">Option</span><span class="token operator">&lt;</span><span class="token keyword">i32</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    327. 

  327.     <span class="token keyword">match</span> x <span class="token punctuation">{</span>
    328. 

  328.         <span class="token class-name">None</span> <span class="token operator">=&gt;</span> <span class="token class-name">None</span><span class="token punctuation">,</span>
    329. 

  329.         <span class="token class-name">Some</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token class-name">Some</span><span class="token punctuation">(</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span>
    330. 

  330.     <span class="token punctuation">}</span>
    331. 

  331. <span class="token punctuation">}</span>
    332. 

  332. </code></pre></div><p>注意：match 匹配必须穷举所有的可能，可以使用 <code>_</code> 通配符替代其他没有列出的值</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test05</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    333. 

  333.     <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token number">0u8</span><span class="token punctuation">;</span>
    334. 

  334.     <span class="token keyword">match</span> x <span class="token punctuation">{</span>
    335. 

  335.         <span class="token number">1</span> <span class="token operator">=&gt;</span> <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;one&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    336. 

  336.         <span class="token number">3</span> <span class="token operator">=&gt;</span> <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;three&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    337. 

  337.         <span class="token number">5</span> <span class="token operator">=&gt;</span> <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;five&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    338. 

  338.         _ <span class="token operator">=&gt;</span> <span class="token punctuation">(</span><span class="token punctuation">)</span>
    339. 

  339.     <span class="token punctuation">}</span>
    340. 

  340. <span class="token punctuation">}</span>
    341. 

  341. </code></pre></div><h3 id="_4-if-let"><a href="#_4-if-let" class="header-anchor">#</a> 4. if let</h3> <p><code>if let</code> 是一种比 <code>match</code> 简单的控制流，他处理只关心一种匹配而忽略其他匹配的情况，它有更少的代码，更少的缩进，更少的模板代码，但是放弃了穷举的可能，可以把 <code>if let</code> 看作是 <code>match</code> 的语法糖。</p> <p><code>if let</code> 的格式如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">if</span> <span class="token keyword">let</span> pattern <span class="token operator">=</span> value <span class="token punctuation">{</span>
    342. 

  342.     <span class="token comment">//TODO</span>
    343. 

  343. <span class="token punctuation">}</span>
    344. 

  344. </code></pre></div><p>他也可以搭配 <code>else</code> 例子如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test06</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    345. 

  345.     <span class="token keyword">let</span> v <span class="token operator">=</span> <span class="token number">8u8</span><span class="token punctuation">;</span>
    346. 

  346. 

  347.     <span class="token keyword">match</span> v <span class="token punctuation">{</span>
    348. 

  348.         <span class="token number">3</span> <span class="token operator">=&gt;</span> <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Three!&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    349. 

  349.         _ <span class="token operator">=&gt;</span> <span class="token punctuation">(</span><span class="token punctuation">)</span>
    350. 

  350.     <span class="token punctuation">}</span>
    351. 

  351. 

  352.     <span class="token keyword">if</span> <span class="token keyword">let</span> <span class="token number">3</span> <span class="token operator">=</span> v <span class="token punctuation">{</span>
    353. 

  353.         <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Three&quot;</span><span class="token punctuation">)</span>
    354. 

  354.     <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token keyword">let</span> <span class="token number">5</span> <span class="token operator">=</span> v <span class="token punctuation">{</span>
    355. 

  355.         <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Five!&quot;</span><span class="token punctuation">)</span>
    356. 

  356.     <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    357. 

  357.         <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Others!&quot;</span><span class="token punctuation">)</span>
    358. 

  358.     <span class="token punctuation">}</span>
    359. 

  359. <span class="token punctuation">}</span>
    360. 

  360. </code></pre></div></div> <footer class="page-edit"><!----> <!----></footer> <!----> </main></div><div class="global-ui"></div></div>
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