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+            
+                Rust NewType 模式
+            
+            
+        </div>
+        <span class="post-date">
+            May 3, 2024
+        </span>
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+        
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+    <div class="post-content">
+    <p>New Type模式是一种软件设计模式，用于在已有类型的基础上创建一个新的类型。在Rust中，这通常是通过<strong>定义一个结构体，其中只包含一个单一成员。这个结构体（New Type）对外提供了一个新的、独立的类型，用于对原始类型增加额外的语义或限制。</strong></p>
+<h2 id="加强类型安全"><a href="#加强类型安全" class="headerlink" title="加强类型安全"></a>加强类型安全</h2><figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Meters</span>(<span class="type">f64</span>);</span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Feet</span>(<span class="type">f64</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> <span class="variable">length_in_meters</span> = <span class="title function_ invoke__">Meters</span>(<span class="number">100.0</span>);</span><br><span class="line"><span class="keyword">let</span> <span class="variable">length_in_feet</span> = <span class="title function_ invoke__">Feet</span>(<span class="number">328.084</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 编译器会防止以下代码执行，因为类型不匹配</span></span><br><span class="line"><span class="comment">// let wrong_length = Meters(length_in_feet); // 编译错误</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 正确的构造</span></span><br><span class="line"><span class="keyword">fn</span> <span class="title function_">add_lengths</span>(length1: Meters, length2: Meters) <span class="punctuation">-&gt;</span> Meters &#123;</span><br><span class="line">    <span class="title function_ invoke__">Meters</span>(length1.<span class="number">0</span> + length2.<span class="number">0</span>)</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
+
+<p>这个例子使用 newtype 模式避免将原始类型f64用于不同的量度，从而增强了类型的安全性。</p>
+<h2 id="实现特定-trait"><a href="#实现特定-trait" class="headerlink" title="实现特定 trait"></a>实现特定 trait</h2><figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Kilometers</span>(<span class="type">f64</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">impl</span> <span class="title class_">Kilometers</span> &#123;</span><br><span class="line">    <span class="keyword">fn</span> <span class="title function_">to_miles</span>(&amp;<span class="keyword">self</span>) <span class="punctuation">-&gt;</span> <span class="type">f64</span> &#123;</span><br><span class="line">        <span class="keyword">self</span>.<span class="number">0</span> * <span class="number">0.621371</span></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> <span class="variable">distance</span> = <span class="title function_ invoke__">Kilometers</span>(<span class="number">10.0</span>);</span><br><span class="line"><span class="built_in">println!</span>(<span class="string">&quot;The distance in miles is &#123;&#125;&quot;</span>, distance.<span class="title function_ invoke__">to_miles</span>());</span><br></pre></td></tr></table></figure>
+
+<p>这里，Kilometers有一个方法to_miles，该方法是不会影响其他f64数据的。如果我们有另一个表示温度的f64类型，就不会意外调用到与距离相关的方法。</p>
+<p>New Type模式同样适用于对<code>Box&lt;dyn SomeTrait&gt;</code>类型的包装，这可以在需要动态分派（动态调用实现了某个接口的不同类型的对象的方法）的时候提供便利。通过创建一个New Type来包装这样的<code>Box&lt;dyn SomeTrait&gt;</code>类型，可以提供自定义的方法或实现更多的trait，同时也可以让API更加清晰和易于使用。</p>
+<h2 id="零成本抽象"><a href="#零成本抽象" class="headerlink" title="零成本抽象"></a>零成本抽象</h2><p>在Rust中，New Type模式不仅是类型安全的，还是一种零成本抽象。这是因为Rust编译器在编译时期会进行足够的优化，以确保New Type的使用没有运行时开销。 Rust的零成本抽象原则确保了抽象不会引入额外的运行时成本。例如，当你使用Meters这样的New Type时，Rust确保：</p>
+<ol>
+<li>无额外内存开销：Meters只包含一个f64，在内存中的表现和单独的f64是一样的。</li>
+<li>无额外运行时开销：使用Meters时，性能和直接使用f64完全相同。编译器会移除任何关于New Type的包装和解包的代码。</li>
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