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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（六）：泛型和特性</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%E5%85%AD%EF%BC%89.html#_1-泛型" class="sidebar-link">1. 泛型</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%E5%85%AD%EF%BC%89.html#_1-1-泛型介绍" class="sidebar-link">1.1 泛型介绍</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%E5%85%AD%EF%BC%89.html#_1-2-在函数定义中使用泛型" class="sidebar-link">1.2 在函数定义中使用泛型</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%E5%85%AD%EF%BC%89.html#_1-3-结构体中的泛型" class="sidebar-link">1.3 结构体中的泛型</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%E5%85%AD%EF%BC%89.html#_1-4-enum定义中的泛型" class="sidebar-link">1.4 Enum定义中的泛型</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%E5%85%AD%EF%BC%89.html#_1-5-方法定义中使用泛型" class="sidebar-link">1.5 方法定义中使用泛型</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%E5%85%AD%EF%BC%89.html#_1-6-泛型代码的性能" class="sidebar-link">1.6 泛型代码的性能</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%E5%85%AD%EF%BC%89.html#_2-trait" class="sidebar-link">2. trait</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%E5%85%AD%EF%BC%89.html#_2-1-定义一个-trait" class="sidebar-link">2.1 定义一个 Trait</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%E5%85%AD%EF%BC%89.html#_2-2-在类型上实现-trait" class="sidebar-link">2.2 在类型上实现 trait</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%E5%85%AD%EF%BC%89.html#_2-3-实现-trait-的约束" class="sidebar-link">2.3 实现 trait 的约束</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%E5%85%AD%EF%BC%89.html#附" class="sidebar-link">附</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%E5%85%AD%EF%BC%89.html#_2-4-实现-trait-作为参数" class="sidebar-link">2.4 实现 Trait 作为参数</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%E5%85%AD%EF%BC%89.html#_2-5-实现-trait-作为返回类型" class="sidebar-link">2.5 实现 Trait 作为返回类型</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%E5%85%AD%EF%BC%89.html#_2-6-使用-trait-bound-实现之前泛型-的例子" class="sidebar-link">2.6 使用 trait bound 实现之前泛型 的例子</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%E5%85%AD%EF%BC%89.html#_2-7-使用-trait-bound-有条件的实现方法" class="sidebar-link">2.7 使用 Trait Bound 有条件的实现方法</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-六-泛型和特性"><a href="#rust入门教程-六-泛型和特性" class="header-anchor">#</a> Rust入门教程（六）：泛型和特性</h1> <blockquote><p>泛型是一个编程语言不可或缺的机制。
    20. 

  20. C++ 语言中用&quot;模板&quot;来实现泛型，而 C 语言中没有泛型的机制，这也导致 C 语言难以构建类型复杂的工程。
    21. 

  21. 泛型机制是编程语言用于表达类型抽象的机制，一般用于功能确定、数据类型待定的类，如链表、映射表等。</p></blockquote> <h2 id="_1-泛型"><a href="#_1-泛型" class="header-anchor">#</a> 1. 泛型</h2> <h3 id="_1-1-泛型介绍"><a href="#_1-1-泛型介绍" class="header-anchor">#</a> 1.1 泛型介绍</h3> <ul><li>泛型可以提高代码复用能力，也就是处理重复代码的问题</li> <li>泛型是具体类型或其它属性的抽象代替:
    22. 

  22. <ul><li>你编写的代码不是最终的代码，而是一种模板，里面有一些“占位符”</li> <li>编译器在编译时将<strong>占位符</strong>替换为具体的类型</li> <li>例如：<code>fn largest&lt;T&gt;(list: &amp;[T]) -&gt;T {...}</code></li></ul></li> <li>类型参数
    23. 

  23. <ul><li>很短，通常一个字母</li> <li>CamelCase</li> <li>T: type 的缩写</li></ul></li></ul> <h3 id="_1-2-在函数定义中使用泛型"><a href="#_1-2-在函数定义中使用泛型" class="header-anchor">#</a> 1.2 在函数定义中使用泛型</h3> <p>泛型函数</p> <ul><li>参数类型</li> <li>返回类型</li></ul> <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>
    24. 

  24.     <span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span><span class="token number">10</span><span class="token punctuation">,</span> <span class="token number">80</span><span class="token punctuation">,</span> <span class="token number">2022</span><span class="token punctuation">,</span> <span class="token number">36</span><span class="token punctuation">,</span> <span class="token number">47</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    25. 

  25.     <span class="token keyword">let</span> largest <span class="token operator">=</span> <span class="token function">largest</span><span class="token punctuation">(</span><span class="token operator">&amp;</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span>
    26. 

  26.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The largest ele is {}&quot;</span><span class="token punctuation">,</span> largest<span class="token punctuation">)</span><span class="token punctuation">;</span>
    27. 

  27. <span class="token punctuation">}</span>
    28. 

  28. 

  29. <span class="token keyword">fn</span> <span class="token function-definition function">largest</span><span class="token punctuation">(</span>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token keyword">i32</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">i32</span> <span class="token punctuation">{</span>
    30. 

  30.     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    31. 

  31.     <span class="token keyword">for</span> <span class="token operator">&amp;</span>item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    32. 

  32.         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> largest <span class="token punctuation">{</span>
    33. 

  33.             largest <span class="token operator">=</span> item<span class="token punctuation">;</span>
    34. 

  34.         <span class="token punctuation">}</span>
    35. 

  35.     <span class="token punctuation">}</span>
    36. 

  36.     largest
    37. 

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

  38. </code></pre></div><p>上面这段代码是求一个集合中最大的元素，我们定义的集合是一个 <code>i32</code> 类型，但是这时如果我们要传入 <code>f32</code> 或者字符型，还用同样的逻辑判断函数的话，是会报错的，这时我们就需要用到泛型。</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">largest</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>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">T</span> <span class="token punctuation">{</span>
    39. 

  39.     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    40. 

  40.     <span class="token keyword">for</span> <span class="token operator">&amp;</span>item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    41. 

  41.         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> largest <span class="token punctuation">{</span>
    42. 

  42.             largest <span class="token operator">=</span> item<span class="token punctuation">;</span>
    43. 

  43.         <span class="token punctuation">}</span>
    44. 

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

  45.     largest
    46. 

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

  47. </code></pre></div><p>我们声明了一个泛型 <code>T</code>，但是这样是会编译报错的，因为不是所有类型 T 都可以进行大小比较，只有实现了下面的 <code>std::cmp::PartialOrd</code> 的 trait 才能进行大小比较，所以要对 T 进行约束。</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>pattern git<span class="token punctuation">:</span><span class="token punctuation">(</span>master<span class="token punctuation">)</span> ✗ cargo run
    48. 

  48.    <span class="token class-name">Compiling</span> pattern 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>pattern<span class="token punctuation">)</span>
    49. 

  49. error<span class="token punctuation">[</span><span class="token constant">E0369</span><span class="token punctuation">]</span><span class="token punctuation">:</span> binary operation `<span class="token operator">&gt;</span>` cannot be applied to <span class="token keyword">type</span> `<span class="token class-name">T</span>`
    50. 

  50.   <span class="token operator">-</span><span class="token punctuation">-&gt;</span> src<span class="token operator">/</span>main<span class="token punctuation">.</span>rs<span class="token punctuation">:</span><span class="token number">10</span><span class="token punctuation">:</span><span class="token number">17</span>
    51. 

  51.    <span class="token operator">|</span>
    52. 

  52. <span class="token number">10</span> <span class="token operator">|</span>         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> largest <span class="token punctuation">{</span>
    53. 

  53.    <span class="token operator">|</span>            <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span> <span class="token operator">^</span> <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span> <span class="token class-name">T</span>
    54. 

  54.    <span class="token operator">|</span>            <span class="token operator">|</span>
    55. 

  55.    <span class="token operator">|</span>            <span class="token class-name">T</span>
    56. 

  56.    <span class="token operator">|</span>
    57. 

  57. help<span class="token punctuation">:</span> consider restricting <span class="token keyword">type</span> <span class="token type-definition class-name">parameter</span> `<span class="token class-name">T</span>`
    58. 

  58.    <span class="token operator">|</span>
    59. 

  59. <span class="token number">7</span>  <span class="token operator">|</span> <span class="token keyword">fn</span> <span class="token function-definition function">largest</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token namespace">std<span class="token punctuation">::</span>cmp<span class="token punctuation">::</span></span><span class="token class-name">PartialOrd</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">T</span> <span class="token punctuation">{</span>
    60. 

  60.    <span class="token operator">|</span>             <span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span>
    61. 

  61. 

  62. <span class="token class-name">For</span> more information about this error<span class="token punctuation">,</span> <span class="token keyword">try</span> `rustc <span class="token operator">-</span><span class="token operator">-</span>explain <span class="token constant">E0369</span>`<span class="token punctuation">.</span>
    63. 

  63. error<span class="token punctuation">:</span> could not compile `pattern` due to previous error
    64. 

  64. </code></pre></div><p>但是把 <code>std::cmp::PartialOrd</code> 这个 trait 加上又会报其他错误，这里在后面会进行介绍。</p> <h3 id="_1-3-结构体中的泛型"><a href="#_1-3-结构体中的泛型" class="header-anchor">#</a> 1.3 结构体中的泛型</h3> <p>可以使用多个泛型的类型参数，但是也不要有太多的类型，否则代码可读性将会下降。例如：</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">Point</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    65. 

  65.     x<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">,</span>
    66. 

  66.     y<span class="token punctuation">:</span> <span class="token class-name">U</span><span class="token punctuation">,</span>
    67. 

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

  68. 

  69. <span class="token keyword">fn</span> <span class="token function-definition function">test01</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    70. 

  70.     <span class="token keyword">let</span> integer <span class="token operator">=</span> <span class="token class-name">Point</span><span class="token punctuation">{</span>x<span class="token punctuation">:</span> <span class="token number">2022</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token number">6.1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>   
    71. 

  71. <span class="token punctuation">}</span>
    72. 

  72. </code></pre></div><h3 id="_1-4-enum定义中的泛型"><a href="#_1-4-enum定义中的泛型" class="header-anchor">#</a> 1.4 Enum定义中的泛型</h3> <p>可以让枚举的变体持有泛型数据类型，例如：Option<T>, Result&lt;T, E&gt;</T></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>
    73. 

  73.     <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>
    74. 

  74.     <span class="token class-name">None</span>
    75. 

  75. <span class="token punctuation">}</span>
    76. 

  76. 

  77. <span class="token keyword">enum</span> <span class="token type-definition class-name">Result</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">E</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    78. 

  78.     <span class="token class-name">Ok</span><span class="token punctuation">(</span><span class="token class-name">T</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    79. 

  79.     <span class="token class-name">Err</span><span class="token punctuation">(</span><span class="token class-name">E</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    80. 

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

  81. </code></pre></div><h3 id="_1-5-方法定义中使用泛型"><a href="#_1-5-方法定义中使用泛型" class="header-anchor">#</a> 1.5 方法定义中使用泛型</h3> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test01</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    82. 

  82.     <span class="token keyword">let</span> integer <span class="token operator">=</span> <span class="token class-name">Point</span><span class="token punctuation">{</span>x<span class="token punctuation">:</span> <span class="token number">2022</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token number">61</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    83. 

  83.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;{}&quot;</span><span class="token punctuation">,</span> integer<span class="token punctuation">.</span><span class="token function">x</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    84. 

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

  85. 

  86. <span class="token keyword">impl</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token operator">&gt;</span> <span class="token class-name">Point</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>
    87. 

  87.     <span class="token keyword">fn</span> <span class="token function-definition function">x</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 operator">&amp;</span><span class="token class-name">T</span> <span class="token punctuation">{</span>
    88. 

  88.         <span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">.</span>x
    89. 

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

  90. <span class="token punctuation">}</span>
    91. 

  91. </code></pre></div><p>注意</p> <ul><li>把 T 放在 impl 关键字后，表示在类型 T 上实现方法
    92. 

  92. <ul><li>例如：<code>impl&lt;T&gt; Point&lt;T&gt;</code></li></ul></li> <li>只针对具体类型实现方法（其余类型没实现方法）
    93. 

  93. <ul><li>例如: <code>impl Point&lt;f32&gt;</code></li></ul></li> <li>struct 中的泛型参数可以和方法的泛型参数不同</li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">impl</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token operator">&gt;</span> <span class="token class-name">Point</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    94. 

  94.     <span class="token keyword">fn</span> <span class="token function-definition function">x</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 operator">&amp;</span><span class="token class-name">T</span> <span class="token punctuation">{</span>
    95. 

  95.         <span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">.</span>x
    96. 

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

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

  98. 

  99. <span class="token keyword">impl</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token operator">&gt;</span> <span class="token class-name">Point</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    100. 

  100.     <span class="token keyword">fn</span> <span class="token function-definition function">mixup</span><span class="token operator">&lt;</span><span class="token class-name">V</span><span class="token punctuation">,</span> <span class="token class-name">W</span><span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token keyword">self</span><span class="token punctuation">,</span> other<span class="token punctuation">:</span> <span class="token class-name">Point</span><span class="token operator">&lt;</span><span class="token class-name">V</span><span class="token punctuation">,</span><span class="token class-name">W</span><span class="token operator">&gt;</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">Point</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">W</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    101. 

  101.         <span class="token class-name">Point</span> <span class="token punctuation">{</span>
    102. 

  102.             x<span class="token punctuation">:</span> <span class="token keyword">self</span><span class="token punctuation">.</span>x<span class="token punctuation">,</span>
    103. 

  103.             y<span class="token punctuation">:</span> other<span class="token punctuation">.</span>y  
    104. 

  104.         <span class="token punctuation">}</span>
    105. 

  105.     <span class="token punctuation">}</span>
    106. 

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

  107. 

  108. <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>
    109. 

  109.     <span class="token keyword">let</span> p1 <span class="token operator">=</span> <span class="token class-name">Point</span><span class="token punctuation">{</span>x<span class="token punctuation">:</span> <span class="token number">61</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token number">85</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    110. 

  110.     <span class="token keyword">let</span> p2 <span class="token operator">=</span> <span class="token class-name">Point</span><span class="token punctuation">{</span>x<span class="token punctuation">:</span> <span class="token string">&quot;Hello&quot;</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token string">&quot;Rust&quot;</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    111. 

  111.     <span class="token keyword">let</span> p3 <span class="token operator">=</span> p1<span class="token punctuation">.</span><span class="token function">mixup</span><span class="token punctuation">(</span>p2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    112. 

  112.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;p3.x = {}, p3.y = {}&quot;</span><span class="token punctuation">,</span> p3<span class="token punctuation">.</span>x<span class="token punctuation">,</span> p3<span class="token punctuation">.</span>y<span class="token punctuation">)</span><span class="token punctuation">;</span>
    113. 

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

  114. </code></pre></div><p>上面实现的结构体方法实际上是将第一个 Point 中的 x 和第二个 Point 的 y 结合起来形成一个新的 Point。</p> <h3 id="_1-6-泛型代码的性能"><a href="#_1-6-泛型代码的性能" class="header-anchor">#</a> 1.6 泛型代码的性能</h3> <p>使用泛型的代码和使用具体类型的代码运行速度是一样的</p> <ul><li>单态化（monomorphization）
    115. 

  115. <ul><li>在编译时将泛型替换为具体类型的过程</li></ul></li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> ingeter <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>
    116. 

  116. <span class="token keyword">let</span> float <span class="token operator">=</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">5.0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    117. 

  117. 

  118. <span class="token keyword">enum</span> <span class="token type-definition class-name">Option_i32</span> <span class="token punctuation">{</span>
    119. 

  119.     <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token keyword">i32</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    120. 

  120.     <span class="token class-name">None</span>
    121. 

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

  122. 

  123. <span class="token keyword">enum</span> <span class="token type-definition class-name">Option_f32</span> <span class="token punctuation">{</span>
    124. 

  124.     <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token keyword">f32</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    125. 

  125.     <span class="token class-name">None</span>
    126. 

  126. <span class="token punctuation">}</span>
    127. 

  127. 

  128. <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>
    129. 

  129.     <span class="token keyword">let</span> integer <span class="token operator">=</span> <span class="token class-name">Option_i32</span><span class="token punctuation">::</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>
    130. 

  130.     <span class="token keyword">let</span> float <span class="token operator">=</span> <span class="token class-name">Option_f64</span><span class="token punctuation">::</span><span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">5.0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    131. 

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

  132. </code></pre></div><h2 id="_2-trait"><a href="#_2-trait" class="header-anchor">#</a> 2. trait</h2> <ul><li>Trait 告诉 Rust 编译器
    133. 

  133. <ul><li>某种类型具有哪些并且可以与其它类型共享的功能</li></ul></li> <li>Trait：抽象的定义共享行为</li> <li>Trait bounds（约束）：泛型类型参数指定为实现了特定行为的类型</li> <li>Trait与其它语言的接口（interface）类似，但有些区别</li></ul> <h3 id="_2-1-定义一个-trait"><a href="#_2-1-定义一个-trait" class="header-anchor">#</a> 2.1 定义一个 Trait</h3> <p>Trait的定义：把方法签名放在一起，来定义实现某种目的所必需的一组行为。</p> <ul><li>关键字：trait</li> <li>只有方法签名，没有具体实现</li> <li>trait 可以有多个方法：每个方法签名占一行，以 <code>;</code> 结尾</li> <li>实现该 trait 的类型必须提供具体的方法实现</li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">trait</span> <span class="token type-definition class-name">Summary</span> <span class="token punctuation">{</span>
    134. 

  134.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span><span class="token punctuation">;</span>
    135. 

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

  136. </code></pre></div><h3 id="_2-2-在类型上实现-trait"><a href="#_2-2-在类型上实现-trait" class="header-anchor">#</a> 2.2 在类型上实现 trait</h3> <ul><li>在类型上实现 trait。与为类型实现方法类似</li> <li>不同之处：<code>impl Xxxx for Tweet {...}</code></li> <li>在 impl 的块里，需要对 Trait 里的方法签名进行具体的实现</li></ul> <p>文件 <code>lib.rs</code>：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">trait</span> <span class="token type-definition class-name">Summary</span> <span class="token punctuation">{</span>
    137. 

  137.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span><span class="token punctuation">;</span>
    138. 

  138. <span class="token punctuation">}</span>
    139. 

  139. 

  140. <span class="token keyword">pub</span> <span class="token keyword">struct</span> <span class="token type-definition class-name">NewsArticle</span> <span class="token punctuation">{</span>
    141. 

  141.     <span class="token keyword">pub</span> headline<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    142. 

  142.     <span class="token keyword">pub</span> location<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    143. 

  143.     <span class="token keyword">pub</span> author<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    144. 

  144.     <span class="token keyword">pub</span> content<span class="token punctuation">:</span> <span class="token class-name">String</span>
    145. 

  145. <span class="token punctuation">}</span>
    146. 

  146. 

  147. <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token keyword">for</span> <span class="token class-name">NewsArticle</span> <span class="token punctuation">{</span>
    148. 

  148.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span> <span class="token punctuation">{</span>
    149. 

  149.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;{}, by {} ({})&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>headline<span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>author<span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>location<span class="token punctuation">)</span>
    150. 

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

  151. <span class="token punctuation">}</span>
    152. 

  152. 

  153. <span class="token keyword">pub</span> <span class="token keyword">struct</span> <span class="token type-definition class-name">Tweet</span> <span class="token punctuation">{</span>
    154. 

  154.     <span class="token keyword">pub</span> username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    155. 

  155.     <span class="token keyword">pub</span> content<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    156. 

  156.     <span class="token keyword">pub</span> reply<span class="token punctuation">:</span> <span class="token keyword">bool</span><span class="token punctuation">,</span>
    157. 

  157.     <span class="token keyword">pub</span> retweet<span class="token punctuation">:</span> <span class="token keyword">bool</span>
    158. 

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

  159. 

  160. <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token keyword">for</span> <span class="token class-name">Tweet</span> <span class="token punctuation">{</span>
    161. 

  161.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span> <span class="token punctuation">{</span>
    162. 

  162.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;{}: {}&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>username<span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>content<span class="token punctuation">)</span>
    163. 

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

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

  165. </code></pre></div><p>文件 <code>main.rs</code>：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">use</span> <span class="token namespace">trait_demo<span class="token punctuation">::</span></span><span class="token class-name">Summary</span><span class="token punctuation">;</span>
    166. 

  166. <span class="token keyword">use</span> <span class="token namespace">trait_demo<span class="token punctuation">::</span></span><span class="token class-name">Tweet</span><span class="token punctuation">;</span>
    167. 

  167. 

  168. <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>
    169. 

  169.     <span class="token keyword">let</span> tweet <span class="token operator">=</span> <span class="token class-name">Tweet</span> <span class="token punctuation">{</span>
    170. 

  170.         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_ICT&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    171. 

  171.         content<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;People in Shanghai are free today...&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    172. 

  172.         reply<span class="token punctuation">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    173. 

  173.         retweet<span class="token punctuation">:</span> <span class="token boolean">false</span>
    174. 

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

  175. 

  176.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Get 1 new tweet: {}&quot;</span><span class="token punctuation">,</span> tweet<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    177. 

  177. 

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

  179. </code></pre></div><p>实现的功能很简单，不做具体解释了。</p> <h3 id="_2-3-实现-trait-的约束"><a href="#_2-3-实现-trait-的约束" class="header-anchor">#</a> 2.3 实现 trait 的约束</h3> <ul><li>可以在某个类型上实现某个 trait 的前提条件是
    180. 

  180. <ul><li>这个类型或这个 trait 是在本地 crate 里定义的</li></ul></li> <li>无法为外部类型来实现外部的trait
    181. 

  181. <ul><li>这个限制是程序属性的一部分（也就是一致性）</li> <li>更具体地说是<strong>孤儿规则</strong>：之所以这样命名是因为父类型不存在</li> <li>此规则确保其他人的代码不能破坏您的代码，反之亦然</li> <li>如果没有这个规则，两个 crate 可以为同一类型实现同一个 trait，Rust 就不知道应该使用哪个实现了</li></ul></li></ul> <p><strong>默认实现</strong></p> <p>默认实现的方法可以调用 trait 中的其他方法，即使这些方法没有默认实现，但是注意，无法从方法的重写实现中调用默认实现。</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">trait</span> <span class="token type-definition class-name">Summary</span> <span class="token punctuation">{</span>
    182. 

  182.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span> <span class="token punctuation">{</span>
    183. 

  183.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;(Read more from {} ...)&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span><span class="token function">summarize_author</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    184. 

  184.     <span class="token punctuation">}</span>
    185. 

  185. 

  186.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize_author</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 class-name">String</span><span class="token punctuation">;</span>
    187. 

  187. <span class="token punctuation">}</span>
    188. 

  188. </code></pre></div><p>在 trait 中可以有方法的默认实现，在默认实现的基础上，类型可以对该 trait 进行重载。同样，在 trait 中默认实现的方法可以实现 trait 中其他方法。</p> <h3 id="附"><a href="#附" class="header-anchor">#</a> 附</h3> <p>刚刚 trait 例子的完整代码如下：</p> <p><code>lib.rs</code>：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">trait</span> <span class="token type-definition class-name">Summary</span> <span class="token punctuation">{</span>
    189. 

  189.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span> <span class="token punctuation">{</span>
    190. 

  190.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;(Read more from {} ...)&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span><span class="token function">summarize_author</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    191. 

  191.     <span class="token punctuation">}</span>
    192. 

  192. 

  193.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize_author</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 class-name">String</span><span class="token punctuation">;</span>
    194. 

  194. <span class="token punctuation">}</span>
    195. 

  195. 

  196. <span class="token keyword">pub</span> <span class="token keyword">struct</span> <span class="token type-definition class-name">NewsArticle</span> <span class="token punctuation">{</span>
    197. 

  197.     <span class="token keyword">pub</span> headline<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    198. 

  198.     <span class="token keyword">pub</span> location<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    199. 

  199.     <span class="token keyword">pub</span> author<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    200. 

  200.     <span class="token keyword">pub</span> content<span class="token punctuation">:</span> <span class="token class-name">String</span>
    201. 

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

  202. 

  203. <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token keyword">for</span> <span class="token class-name">NewsArticle</span> <span class="token punctuation">{</span>
    204. 

  204.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span> <span class="token punctuation">{</span>
    205. 

  205.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;{}, by {} ({})&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>headline<span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>author<span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>location<span class="token punctuation">)</span>
    206. 

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

  207. 

  208.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize_author</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 class-name">String</span> <span class="token punctuation">{</span>
    209. 

  209.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;@{}&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>author<span class="token punctuation">)</span>
    210. 

  210.     <span class="token punctuation">}</span>
    211. 

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

  212. 

  213. <span class="token keyword">pub</span> <span class="token keyword">struct</span> <span class="token type-definition class-name">Tweet</span> <span class="token punctuation">{</span>
    214. 

  214.     <span class="token keyword">pub</span> username<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    215. 

  215.     <span class="token keyword">pub</span> content<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
    216. 

  216.     <span class="token keyword">pub</span> reply<span class="token punctuation">:</span> <span class="token keyword">bool</span><span class="token punctuation">,</span>
    217. 

  217.     <span class="token keyword">pub</span> retweet<span class="token punctuation">:</span> <span class="token keyword">bool</span>
    218. 

  218. <span class="token punctuation">}</span>
    219. 

  219. 

  220. <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token keyword">for</span> <span class="token class-name">Tweet</span> <span class="token punctuation">{</span>
    221. 

  221.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize</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 class-name">String</span> <span class="token punctuation">{</span>
    222. 

  222.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;{}: {}&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>username<span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>content<span class="token punctuation">)</span>
    223. 

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

  224. 

  225.     <span class="token keyword">fn</span> <span class="token function-definition function">summarize_author</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 class-name">String</span> <span class="token punctuation">{</span>
    226. 

  226.         <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;@{}&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>username<span class="token punctuation">)</span>
    227. 

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

  228. <span class="token punctuation">}</span>
    229. 

  229. </code></pre></div><p><code>main.rs</code>：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">use</span> <span class="token namespace">trait_demo<span class="token punctuation">::</span></span><span class="token class-name">Summary</span><span class="token punctuation">;</span>
    230. 

  230. <span class="token keyword">use</span> <span class="token namespace">trait_demo<span class="token punctuation">::</span></span><span class="token class-name">Tweet</span><span class="token punctuation">;</span>
    231. 

  231. <span class="token keyword">use</span> <span class="token namespace">trait_demo<span class="token punctuation">::</span></span><span class="token class-name">NewsArticle</span><span class="token punctuation">;</span>
    232. 

  232. 

  233. <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>
    234. 

  234.     <span class="token keyword">let</span> tweet <span class="token operator">=</span> <span class="token class-name">Tweet</span> <span class="token punctuation">{</span>
    235. 

  235.         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_ICT&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    236. 

  236.         content<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;People in Shanghai are free today...&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    237. 

  237.         reply<span class="token punctuation">:</span> <span class="token boolean">false</span><span class="token punctuation">,</span>
    238. 

  238.         retweet<span class="token punctuation">:</span> <span class="token boolean">false</span>
    239. 

  239.     <span class="token punctuation">}</span><span class="token punctuation">;</span>
    240. 

  240. 

  241.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Get 1 new tweet: {}&quot;</span><span class="token punctuation">,</span> tweet<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    242. 

  242. 

  243.     <span class="token keyword">let</span> news <span class="token operator">=</span> <span class="token class-name">NewsArticle</span> <span class="token punctuation">{</span>
    244. 

  244.         headline<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;WWDC will be held in June 7th&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    245. 

  245.         location<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;USA&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    246. 

  246.         author<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;Tim Cook&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    247. 

  247.         content<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;The Apple will take us a lot of devices.&quot;</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
    248. 

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

  249. 

  250.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;You receive a news: {}&quot;</span><span class="token punctuation">,</span> news<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    251. 

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

  252. </code></pre></div><p>最终输出结果为：</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>trait_demo git<span class="token punctuation">:</span><span class="token punctuation">(</span>master<span class="token punctuation">)</span> ✗ cargo run
    253. 

  253.    <span class="token class-name">Compiling</span> trait_demo 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>trait_demo<span class="token punctuation">)</span>
    254. 

  254.     <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>33s
    255. 

  255.      <span class="token class-name">Running</span> `target<span class="token operator">/</span>debug<span class="token operator">/</span>trait_demo`
    256. 

  256. <span class="token class-name">Get</span> <span class="token number">1</span> new tweet<span class="token punctuation">:</span> <span class="token class-name">Cherry_ICT</span><span class="token punctuation">:</span> <span class="token class-name">People</span> <span class="token keyword">in</span> <span class="token class-name">Shanghai</span> are free today<span class="token punctuation">...</span>
    257. 

  257. <span class="token class-name">You</span> receive a news<span class="token punctuation">:</span> <span class="token constant">WWDC</span> will be held <span class="token keyword">in</span> <span class="token class-name">June</span> 7th<span class="token punctuation">,</span> by <span class="token class-name">Tim</span> <span class="token class-name">Cook</span> <span class="token punctuation">(</span><span class="token constant">USA</span><span class="token punctuation">)</span>
    258. 

  258. </code></pre></div><h3 id="_2-4-实现-trait-作为参数"><a href="#_2-4-实现-trait-作为参数" class="header-anchor">#</a> 2.4 实现 Trait 作为参数</h3> <ul><li>impl Trait 语法：适用于简单情况</li> <li>Trait bound 语法：可用于复杂情况
    259. 

  259. <ul><li>impl trait 语法实际上是 trait bound 语法的语法糖</li></ul></li> <li>使用 <code>+</code> 指定多个 trait bound</li> <li>Trait bound 使用 where 子句
    260. 

  260. <ul><li>在方法签名后指定 where 子句</li></ul></li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">trait</span> <span class="token type-definition class-name">Summary</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    261. 

  261. 

  262. <span class="token keyword">pub</span> <span class="token keyword">struct</span> <span class="token type-definition class-name">NewsArticle</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    263. 

  263. 

  264. <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token keyword">for</span> <span class="token class-name">NewsArticle</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    265. 

  265. 

  266. <span class="token keyword">pub</span> <span class="token keyword">struct</span> <span class="token type-definition class-name">Tweet</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    267. 

  267. 

  268. <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token keyword">for</span> <span class="token class-name">Tweet</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    269. 

  269. 

  270. <span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify</span><span class="token punctuation">(</span>item<span class="token punctuation">:</span> <span class="token keyword">impl</span> <span class="token class-name">Summary</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    271. 

  271.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Breaking news! {}&quot;</span><span class="token punctuation">,</span> item<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    272. 

  272. <span class="token punctuation">}</span>
    273. 

  273. </code></pre></div><p>这是采用 impl Trait 的语法，这里的 notify 方法要求传入的参数可以是 <code>NewsArticle</code> 类型或者是 <code>Tweet</code> 类型，也就是要求参数要实现 <code>Summary</code> 这个 trait，从而使用 summarize 这个方法。</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">Summary</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>item<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    274. 

  274.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Breaking news! {}&quot;</span><span class="token punctuation">,</span> item<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    275. 

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

  276. </code></pre></div><p>这是采用 Trait bound 的写法，下面这个例子讲展示出这种写法的优势：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">Summary</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>item1<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">,</span> item2<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    277. 

  277.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Breaking news! {}&quot;</span><span class="token punctuation">,</span> item<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    278. 

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

  279. </code></pre></div><p>当有多个参数时，采用这种写法可以使得代码相对简洁一些。</p> <p>使用 <code>+</code> 指定多个 trait bound：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify1</span><span class="token punctuation">(</span>item<span class="token punctuation">:</span> <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token operator">+</span> <span class="token class-name">Display</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    280. 

  280.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Breaking news! {}&quot;</span><span class="token punctuation">,</span> item<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    281. 

  281. <span class="token punctuation">}</span>
    282. 

  282. 

  283. <span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">Summary</span> <span class="token operator">+</span> <span class="token class-name">Display</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>item<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    284. 

  284.     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;Breaking news! {}&quot;</span><span class="token punctuation">,</span> item<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    285. 

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

  286. </code></pre></div><p>然而如果一个函数中参数过多，那么整个函数声明就会变得非常长，不太直观，可读性差，这里可以使用 where 子句来指定 trait 的约束：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify2</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">Summary</span> <span class="token operator">+</span> <span class="token class-name">Display</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token punctuation">:</span> <span class="token class-name">Clone</span> <span class="token operator">+</span> <span class="token class-name">Debug</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>a<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">,</span> b<span class="token punctuation">:</span> <span class="token class-name">U</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">String</span> <span class="token punctuation">{</span>
    287. 

  287.     <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;Breaking news! {}&quot;</span><span class="token punctuation">,</span> a<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    288. 

  288. <span class="token punctuation">}</span>
    289. 

  289. </code></pre></div><p>这个例子中函数签名太长，不够直观，采用 where 子句可以使得更加直观：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify3</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>a<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">,</span> b<span class="token punctuation">:</span> <span class="token class-name">U</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">String</span>
    290. 

  290. <span class="token keyword">where</span>
    291. 

  291.     <span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">Summary</span> <span class="token operator">+</span> <span class="token class-name">Display</span><span class="token punctuation">,</span>
    292. 

  292.     <span class="token class-name">U</span><span class="token punctuation">:</span> <span class="token class-name">Clone</span> <span class="token operator">+</span> <span class="token class-name">Debug</span><span class="token punctuation">,</span>
    293. 

  293. <span class="token punctuation">{</span>
    294. 

  294.     <span class="token macro property">format!</span><span class="token punctuation">(</span><span class="token string">&quot;Breaking news! {}&quot;</span><span class="token punctuation">,</span> a<span class="token punctuation">.</span><span class="token function">summarize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
    295. 

  295. <span class="token punctuation">}</span>
    296. 

  296. </code></pre></div><h3 id="_2-5-实现-trait-作为返回类型"><a href="#_2-5-实现-trait-作为返回类型" class="header-anchor">#</a> 2.5 实现 Trait 作为返回类型</h3> <ul><li>impl trait 语法
    297. 

  297. <ul><li>注意：impl Trait 只能返回确定的同一种类型，返回可能不同类型的代码会报错</li></ul></li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">fn</span> <span class="token function-definition function">notify4</span><span class="token punctuation">(</span>flag<span class="token punctuation">:</span> <span class="token keyword">bool</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">impl</span> <span class="token class-name">Summary</span> <span class="token punctuation">{</span>
    298. 

  298.     <span class="token keyword">if</span> flag <span class="token punctuation">{</span>
    299. 

  299.         <span class="token class-name">NewsArticle</span> <span class="token punctuation">{</span><span class="token punctuation">...</span><span class="token punctuation">}</span>
    300. 

  300.     <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    301. 

  301.         <span class="token class-name">Tweet</span> <span class="token punctuation">{</span><span class="token punctuation">...</span><span class="token punctuation">}</span>
    302. 

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

  303. <span class="token punctuation">}</span>
    304. 

  304. </code></pre></div><p>这样的话这个函数便没有了确定的返回类型，这样便会报错。</p> <h3 id="_2-6-使用-trait-bound-实现之前泛型-的例子"><a href="#_2-6-使用-trait-bound-实现之前泛型-的例子" class="header-anchor">#</a> 2.6 使用 trait bound 实现之前泛型 的例子</h3> <p>我们再来看一下<a href="#12-%E5%9C%A8%E5%87%BD%E6%95%B0%E5%AE%9A%E4%B9%89%E4%B8%AD%E4%BD%BF%E7%94%A8%E6%B3%9B%E5%9E%8B">之前的代码</a>。解决如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">largest</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">PartialOrd</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">T</span> <span class="token punctuation">{</span>
    305. 

  305.     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    306. 

  306.     <span class="token keyword">for</span> <span class="token operator">&amp;</span>item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    307. 

  307.         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> largest <span class="token punctuation">{</span>
    308. 

  308.             largest <span class="token operator">=</span> item<span class="token punctuation">;</span>
    309. 

  309.         <span class="token punctuation">}</span>
    310. 

  310.     <span class="token punctuation">}</span>
    311. 

  311.     largest
    312. 

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

  313. </code></pre></div><p>之前我们说过，实际上比较大小的运算符是实现了 <code>std::cmp::PartialOrd</code> 这样一个 trait，因此我们需要指定实现这个 trait 的泛型才能进行大小比较。</p> <p>但是这样改完后又会出现一个问题：</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>pattern git<span class="token punctuation">:</span><span class="token punctuation">(</span>master<span class="token punctuation">)</span> ✗ cargo run 
    314. 

  314.    <span class="token class-name">Compiling</span> pattern 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>pattern<span class="token punctuation">)</span>
    315. 

  315. error<span class="token punctuation">[</span><span class="token constant">E0508</span><span class="token punctuation">]</span><span class="token punctuation">:</span> cannot <span class="token keyword">move</span> out of <span class="token keyword">type</span> `<span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span>`<span class="token punctuation">,</span> a non<span class="token operator">-</span>copy slice
    316. 

  316.   <span class="token operator">-</span><span class="token punctuation">-&gt;</span> src<span class="token operator">/</span>main<span class="token punctuation">.</span>rs<span class="token punctuation">:</span><span class="token number">10</span><span class="token punctuation">:</span><span class="token number">19</span>
    317. 

  317.    <span class="token operator">|</span>
    318. 

  318. <span class="token number">10</span> <span class="token operator">|</span> <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    319. 

  319.    <span class="token operator">|</span>                   <span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span>
    320. 

  320.    <span class="token operator">|</span>                   <span class="token operator">|</span>
    321. 

  321.    <span class="token operator">|</span>                   cannot <span class="token keyword">move</span> out of here
    322. 

  322.    <span class="token operator">|</span>                   <span class="token keyword">move</span> occurs because `list<span class="token punctuation">[</span>_<span class="token punctuation">]</span>` has <span class="token keyword">type</span> `<span class="token class-name">T</span>`<span class="token punctuation">,</span> which does not implement the `<span class="token class-name">Copy</span>` <span class="token keyword">trait</span>
    323. 

  323.    <span class="token operator">|</span>                   help<span class="token punctuation">:</span> consider borrowing here<span class="token punctuation">:</span> `<span class="token operator">&amp;</span>list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span>`
    324. 

  324. 

  325. error<span class="token punctuation">[</span><span class="token constant">E0507</span><span class="token punctuation">]</span><span class="token punctuation">:</span> cannot <span class="token keyword">move</span> out of a shared reference
    326. 

  326.   <span class="token operator">-</span><span class="token punctuation">-&gt;</span> src<span class="token operator">/</span>main<span class="token punctuation">.</span>rs<span class="token punctuation">:</span><span class="token number">11</span><span class="token punctuation">:</span><span class="token number">18</span>
    327. 

  327.    <span class="token operator">|</span>
    328. 

  328. <span class="token number">11</span> <span class="token operator">|</span>     <span class="token keyword">for</span> <span class="token operator">&amp;</span>item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    329. 

  329.    <span class="token operator">|</span>         <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span>    <span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span>
    330. 

  330.    <span class="token operator">|</span>         <span class="token operator">||</span>
    331. 

  331.    <span class="token operator">|</span>         <span class="token operator">|</span>data moved here
    332. 

  332.    <span class="token operator">|</span>         <span class="token operator">|</span><span class="token keyword">move</span> occurs because `item` has <span class="token keyword">type</span> `<span class="token class-name">T</span>`<span class="token punctuation">,</span> which does not implement the `<span class="token class-name">Copy</span>` <span class="token keyword">trait</span>
    333. 

  333.    <span class="token operator">|</span>         help<span class="token punctuation">:</span> consider removing the `<span class="token operator">&amp;</span>`<span class="token punctuation">:</span> `item`
    334. 

  334. 

  335. <span class="token class-name">Some</span> errors have detailed explanations<span class="token punctuation">:</span> <span class="token constant">E0507</span><span class="token punctuation">,</span> <span class="token constant">E0508</span><span class="token punctuation">.</span>
    336. 

  336. <span class="token class-name">For</span> more information about an error<span class="token punctuation">,</span> <span class="token keyword">try</span> `rustc <span class="token operator">-</span><span class="token operator">-</span>explain <span class="token constant">E0507</span>`<span class="token punctuation">.</span>
    337. 

  337. error<span class="token punctuation">:</span> could not compile `pattern` due to <span class="token number">2</span> previous errors
    338. 

  338. </code></pre></div><p>报错原因是：无法从 list 中移除 T，因为没有实现 Copy trait，建议采用借用</p> <p>因为上面两个 vector 中的元素分别为整型和字符型，这两种类型有确定的大小并且都是存储在栈中，因此都实现了 Copy trait，于是在 T 的 trait 约束中再加上 Copy 即可：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">largest</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">PartialOrd</span> <span class="token operator">+</span> <span class="token class-name">Copy</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">T</span> <span class="token punctuation">{</span>
    339. 

  339.     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    340. 

  340.     <span class="token keyword">for</span> <span class="token operator">&amp;</span>item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    341. 

  341.         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> largest <span class="token punctuation">{</span>
    342. 

  342.             largest <span class="token operator">=</span> item<span class="token punctuation">;</span>
    343. 

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

  344.     <span class="token punctuation">}</span>
    345. 

  345.     largest
    346. 

  346. <span class="token punctuation">}</span>
    347. 

  347. </code></pre></div><p>但是如果将 vec 中元素类型改为 String，那么又会报错，因为 String 是存储在堆中，没有实现 Copy trait，但是实现了 Clone trait</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> str_list <span class="token operator">=</span> <span class="token macro property">vec!</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 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;World&quot;</span><span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    348. 

  348. <span class="token keyword">let</span> largest <span class="token operator">=</span> <span class="token function">get_max_ele</span><span class="token punctuation">(</span><span class="token operator">&amp;</span>str_list<span class="token punctuation">)</span><span class="token punctuation">;</span>
    349. 

  349. </code></pre></div><p>我们将 T 加上 Clone 约束，去掉 Copy 约束：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">get_max_ele</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">PartialOrd</span> <span class="token operator">+</span> <span class="token class-name">Clone</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">T</span> <span class="token punctuation">{</span>
    350. 

  350.     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    351. 

  351.     <span class="token keyword">for</span> <span class="token operator">&amp;</span>item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    352. 

  352.         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> largest <span class="token punctuation">{</span>
    353. 

  353.             largest <span class="token operator">=</span> item<span class="token punctuation">;</span>
    354. 

  354.         <span class="token punctuation">}</span>
    355. 

  355.     <span class="token punctuation">}</span>
    356. 

  356.     largest
    357. 

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

  358. </code></pre></div><p>这样又会出现错误：</p> <div class="language-rust extra-class"><pre class="language-rust"><code>error<span class="token punctuation">[</span><span class="token constant">E0508</span><span class="token punctuation">]</span><span class="token punctuation">:</span> cannot <span class="token keyword">move</span> out of <span class="token keyword">type</span> `<span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span>`<span class="token punctuation">,</span> a non<span class="token operator">-</span>copy slice
    359. 

  359.   <span class="token operator">-</span><span class="token punctuation">-&gt;</span> src<span class="token operator">/</span>main<span class="token punctuation">.</span>rs<span class="token punctuation">:</span><span class="token number">21</span><span class="token punctuation">:</span><span class="token number">23</span>
    360. 

  360.    <span class="token operator">|</span>
    361. 

  361. <span class="token number">21</span> <span class="token operator">|</span>     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    362. 

  362.    <span class="token operator">|</span>                       <span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span>
    363. 

  363.    <span class="token operator">|</span>                       <span class="token operator">|</span>
    364. 

  364.    <span class="token operator">|</span>                       cannot <span class="token keyword">move</span> out of here
    365. 

  365.    <span class="token operator">|</span>                       <span class="token keyword">move</span> occurs because `list<span class="token punctuation">[</span>_<span class="token punctuation">]</span>` has <span class="token keyword">type</span> `<span class="token class-name">T</span>`<span class="token punctuation">,</span> which does not implement the `<span class="token class-name">Copy</span>` <span class="token keyword">trait</span>
    366. 

  366.    <span class="token operator">|</span>                       help<span class="token punctuation">:</span> consider borrowing here<span class="token punctuation">:</span> `<span class="token operator">&amp;</span>list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span>`
    367. 

  367. </code></pre></div><p>是因为这里 list[0] 是字符串切片，是一个借用，没有所有权，因此一个借用给一个变量赋值，这个借用对应的类型必须要实现 Copy trait。因此在 list 前面加上引用，并且将 item 也设为引用，最后返回 &amp;T。</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">get_max_ele</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">PartialOrd</span> <span class="token operator">+</span> <span class="token class-name">Clone</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token class-name">T</span> <span class="token punctuation">{</span>
    368. 

  368.     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> <span class="token operator">&amp;</span>list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    369. 

  369.     <span class="token keyword">for</span> item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    370. 

  370.         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> largest <span class="token punctuation">{</span>
    371. 

  371.             largest <span class="token operator">=</span> item<span class="token punctuation">;</span>
    372. 

  372.         <span class="token punctuation">}</span>
    373. 

  373.     <span class="token punctuation">}</span>
    374. 

  374.     largest
    375. 

  375. <span class="token punctuation">}</span>
    376. 

  376. </code></pre></div><p>若要最后还是返回 T，则可以使用 clone 方法：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">get_max_ele</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">PartialOrd</span> <span class="token operator">+</span> <span class="token class-name">Clone</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>list<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token punctuation">[</span><span class="token class-name">T</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">T</span> <span class="token punctuation">{</span>
    377. 

  377.     <span class="token keyword">let</span> <span class="token keyword">mut</span> largest <span class="token operator">=</span> list<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">clone</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    378. 

  378.     <span class="token keyword">for</span> item <span class="token keyword">in</span> list <span class="token punctuation">{</span>
    379. 

  379.         <span class="token keyword">if</span> item <span class="token operator">&gt;</span> <span class="token operator">&amp;</span>largest <span class="token punctuation">{</span>
    380. 

  380.             largest <span class="token operator">=</span> item<span class="token punctuation">.</span><span class="token function">clone</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    381. 

  381.         <span class="token punctuation">}</span>
    382. 

  382.     <span class="token punctuation">}</span>
    383. 

  383.     largest
    384. 

  384. <span class="token punctuation">}</span>
    385. 

  385. </code></pre></div><h3 id="_2-7-使用-trait-bound-有条件的实现方法"><a href="#_2-7-使用-trait-bound-有条件的实现方法" class="header-anchor">#</a> 2.7 使用 Trait Bound 有条件的实现方法</h3> <p>在使用泛型类型参数的 impl 块上使用 Trait bound，我们可以有条件的为实现了特定 Trait 的类型来实现方法</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">Pair</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>
    386. 

  386.     x<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">,</span>
    387. 

  387.     y<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">,</span>
    388. 

  388. <span class="token punctuation">}</span>
    389. 

  389. 

  390. <span class="token keyword">impl</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token operator">&gt;</span> <span class="token class-name">Pair</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>
    391. 

  391.     <span class="token keyword">fn</span> <span class="token function-definition function">new</span><span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">Self</span> <span class="token punctuation">{</span>
    392. 

  392.         <span class="token keyword">Self</span> <span class="token punctuation">{</span> x<span class="token punctuation">,</span> y <span class="token punctuation">}</span>
    393. 

  393.     <span class="token punctuation">}</span>
    394. 

  394. <span class="token punctuation">}</span>
    395. 

  395. 

  396. <span class="token keyword">impl</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">Display</span> <span class="token operator">+</span> <span class="token class-name">PartialOrd</span><span class="token operator">&gt;</span> <span class="token class-name">Pair</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>
    397. 

  397.     <span class="token keyword">fn</span> <span class="token function-definition function">cmd_display</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>
    398. 

  398.         <span class="token keyword">if</span> <span class="token keyword">self</span><span class="token punctuation">.</span>x <span class="token operator">&gt;=</span> <span class="token keyword">self</span><span class="token punctuation">.</span>y <span class="token punctuation">{</span>
    399. 

  399.             <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The largest member is x = {}&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    400. 

  400.         <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    401. 

  401.             <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The largest member is y = {}&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>y<span class="token punctuation">)</span><span class="token punctuation">;</span>
    402. 

  402.         <span class="token punctuation">}</span>
    403. 

  403.     <span class="token punctuation">}</span>
    404. 

  404. <span class="token punctuation">}</span>
    405. 

  405. </code></pre></div><ul><li>也可以为实现了其它 Trait 的任意类型有条件的实现某个 Trait</li> <li>为满足 Trait Bound 的所有类型上实现 Trait 叫做覆盖实现 （blanket implementations）</li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">impl</span><span class="token operator">&lt;</span><span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token namespace">fmt<span class="token punctuation">::</span></span><span class="token class-name">Display</span><span class="token operator">&gt;</span> <span class="token class-name">Tostring</span> <span class="token keyword">for</span> <span class="token class-name">T</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>
    406. 

  406. </code></pre></div><p>含义为：为实现了 Display trait 的类型实现 ToString trait，而 ToString 中实现了 to_string 方法。</p> <p>例如 <code>let s = 3.to_string();</code></p></div> <footer class="page-edit"><!----> <!----></footer> <!----> </main></div><div class="global-ui"></div></div>
    407. 

  407.     <script src="/rust_camp_tutorial/assets/js/app.d7ab8f65.js" defer></script><script src="/rust_camp_tutorial/assets/js/2.3dc1b8de.js" defer></script><script src="/rust_camp_tutorial/assets/js/1.7f771cfb.js" defer></script><script src="/rust_camp_tutorial/assets/js/31.e1aa8cbc.js" defer></script>
    408. 

  408.   </body>
    409. 

  409. </html>
    410.