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C++ 的 Trait
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May 4, 2024
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<blockquote>
<p>C++ 的 traits 技术，是一种约定俗称的技术方案，用来为同一类数据（包括自定义数据类型和内置数据类型）提供统一的类型名（traits），这样可以统一的操作函数，例如 advance(), swap(), encode()/decode() 等。</p>
</blockquote>
<p>例如，拥有义类型Foo, Bar，以及编译器自带类型 int, double, string，我们想要为这些不同的类型提供统一的编码函数 decode() 。</p>
<p>除了使用 trait 技术之外，函数重载和模板函数 + 内置字段也可以实现，前者每增加一种数据类型就需要重新实现一个函数，而同一类数据（int, unsinged int）可以使用同样的编码方法。我们想要的是针对同一种数据类型，只编写一个函数。后者对于系统自定义变量 int, double 而言，是无法在其内部定义 type 的。</p>
<p>traits 技术的关键在于，使用另外的模板类 type_traits 来保存不同数据类型的 type，这样就可以兼容自定义数据类型和内置数据类型:</p>
<figure class="highlight c++"><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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 定义数据 type 类</span></span><br><span class="line"><span class="keyword">enum</span> <span class="title class_">Type</span> {</span><br><span class="line"> TYPE_1,</span><br><span class="line"> TYPE_2,</span><br><span class="line"> TYPE_3</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于自定义类型，在类内部定义 type，然后在 traits 类中定义同样的 type:</p>
<figure class="highlight c++"><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="comment">// 自定义数据类型</span></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Foo</span> {</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> Type type = TYPE_1;</span><br><span class="line">};</span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Bar</span> {</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> Type type = TYPE_2;</span><br><span class="line">};</span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">type_traits</span> {</span><br><span class="line"> Type type = T::type;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于内置数据类型，使用模板类的特化为自定义类型生成独有的 type_traits:</p>
<figure class="highlight c++"><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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 内置数据类型</span></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> <span class="type">int</span>></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">type_traits</span> {</span><br><span class="line"> Type type = Type::TYPE_1;</span><br><span class="line">}</span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> <span class="type">double</span>></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">type_traits</span> {</span><br><span class="line"> Type type = Type::TYPE_3;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这样就可以为不同数据类型生成统一的模板函数</p>
<figure class="highlight c++"><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="comment">// 统一的编码函数</span></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> T></span><br><span class="line"><span class="type">void</span> decode<<span class="type">const</span> T& data, <span class="type">char</span>* buf) {</span><br><span class="line"> <span class="keyword">if</span>(type_traits<T>::type == Type::TYPE_1) {</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span>(type_traits<T>::type == Type::TYPE_2) {</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>总结</p>
<ul>
<li>traits 技术的关键在于使用第三方模板类 traits，利用模板特化的功能, 实现对自定义数据和编译器内置数据的统一</li>
<li>这个例子使用了枚举变量来表示数据类型，而实际操作中通常使用不同的类来表示不同的类型，这样可以在编写模板函数时更好的优化。</li>
<li>tratis 技术常见于标准库的实现中，但对日常开发中降低代码冗余也有很好的借鉴意义</li>
<li>C++20 提供了Concept 的特性，使用Concept 可以使得实现类似的功能更加方便</li>
</ul>
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