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<li><a class="icon" target="_blank" rel="noopener" href="http://digg.com/submit?url=https://tblgsn.github.io/stlfunctor20211004.html&title=C++ 如何编写出一个完美的 functor"><i class="fab fa-digg " aria-hidden="true"></i></a></li>
<li><a class="icon" target="_blank" rel="noopener" href="http://www.tumblr.com/share/link?url=https://tblgsn.github.io/stlfunctor20211004.html&name=C++ 如何编写出一个完美的 functor&description=<p>在 STL 中 functor 扮演着重要的角色。在 STL 中 functor 默默的为 algorithms 提供支持，因此 functor 通常与 algorithm 同时出现,更为准确的来说： “functor (包括functor adapter) 总是伴随着 algorithm 同时出现。”</p>
<p>一个常见的使用场景如下:(这段代码会打印出容器 v 中的所有元素)</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">// 自己定义的一般函数</span></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">myfun</span>&#123;</span><br><span class="line"> <span class="function"><span class="type">bool</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> <span class="type">int</span> i)</span> </span>&#123;</span><br><span class="line"> std::cout &lt;&lt; i &lt;&lt;std::endl;</span><br><span class="line"> &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//填充一个 vector v，然后使用 for_each 算法</span></span><br><span class="line"></span><br><span class="line">for_each(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(), myfunction);</span><br></pre></td></tr></table></figure>"><i class="fab fa-tumblr " aria-hidden="true"></i></a></li>
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<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#1-STL-%E4%B8%AD%E7%9A%84-functor-%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90"><span class="toc-number">1.</span> <span class="toc-text">1. STL 中的 functor 源码解析</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#2-const-%E7%9A%84%E9%87%8D%E8%A6%81%E6%80%A7%E4%BB%A5%E5%8F%8A%E9%80%9A%E8%BF%87%E4%BC%A0%E5%80%BC"><span class="toc-number">2.</span> <span class="toc-text">2. const 的重要性以及通过传值</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#3-%E5%A6%82%E4%BD%95%E8%AE%A9%E6%88%91%E4%BB%AC%E8%87%AA%E5%B7%B1%E7%BC%96%E5%86%99%E7%9A%84%E4%B8%80%E8%88%AC%E5%87%BD%E6%95%B0%E8%9E%8D%E5%85%A5%E5%88%B0-STL-%E5%BA%93"><span class="toc-number">3.</span> <span class="toc-text">3. 如何让我们自己编写的一般函数融入到 STL 库</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%80%BB%E7%BB%93%E2%80%94%E6%88%90%E4%B8%BA%E4%B8%80%E4%B8%AA-functor-%E7%9A%84%E5%85%B3%E9%94%AE"><span class="toc-number">4.</span> <span class="toc-text">总结—成为一个 functor 的关键</span></a></li></ol>
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C++ 如何编写出一个完美的 functor
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<p>在 STL 中 functor 扮演着重要的角色。在 STL 中 functor 默默的为 algorithms 提供支持，因此 functor 通常与 algorithm 同时出现,更为准确的来说： “functor (包括functor adapter) 总是伴随着 algorithm 同时出现。”</p>
<p>一个常见的使用场景如下:(这段代码会打印出容器 v 中的所有元素)</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">// 自己定义的一般函数</span></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">myfun</span>{</span><br><span class="line"> <span class="function"><span class="type">bool</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> <span class="type">int</span> i)</span> </span>{</span><br><span class="line"> std::cout << i <<std::endl;</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">//填充一个 vector v，然后使用 for_each 算法</span></span><br><span class="line"></span><br><span class="line">for_each(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(), myfunction);</span><br></pre></td></tr></table></figure>
<span id="more"></span>
<p>右侧除了说明 for_each 调用方式外，还包含了 for_each 算法的源代码。 for_each 函数会遍历当前的容器,通过 取值运算符(‘*’)不断的取出容器中的值，并作为参数传递给函数对象f, 函数对象指向的是我们定义的 <strong>myfun</strong> “functor”。这相当于调用 <strong>myfun</strong>中的 operator()方法。</p>
<blockquote>
<p>另外，还有一个很重要的一点是 function object 总是被设计成 按值传递，这意味着通常是通过 copy 操作完成赋值的.</p>
</blockquote>
<p><img src="/img/functor-myfun.png" alt="用户自己定义的 'functor'"></p>
<p>在接触 functor 时，另一个跳不过的部件也十分的重要，它们就是 funtion adaptors ，他们将 functor 改头换面，以提供给用户更容易使用的接口。通常，function adaptors 在封装 functor 时,会向 functor 提问(ask)。functor 必须作出回答，这样 function 才能完成封装任务。 </p>
<p><img src="/img/STL-functor.png" alt="STL中的functor 与 function adaptors"></p>
<h2 id="1-STL-中的-functor-源码解析"><a href="#1-STL-中的-functor-源码解析" class="headerlink" title="1. STL 中的 functor 源码解析"></a>1. STL 中的 functor 源码解析</h2><p>一个典型的 STL 中的 functor 源代码如下:</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="keyword">class</span> <span class="title class_">T</span>></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">negate</span> : <span class="keyword">public</span> unary_function<T, T> {</span><br><span class="line"> <span class="function">T <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> T& x)</span> <span class="type">const</span> </span>{ <span class="keyword">return</span> -x; }</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>negate 会将调用取负操作符(可能被重载过)，并返回取负之后的值。注意返回的类型是 <em>T</em>,而不是 *T&*，你可能经常性的返回 reference 以获得更高的效率,然而在这里这样做很危险。</p>
<blockquote>
<p>这一点在 《Effective C++ 》Item 21 中被提及。</p>
</blockquote>
<p>此外，这里有几点需要注意: </p>
<ul>
<li>使用了两个 const,这使得 该 functor 只与输入的参数有关。对于相同的参数，其返回的值应该是相同的，对于不同的值返回的类型。</li>
<li>这一点是上面的延伸 ———— functor 不应该维护状态。</li>
<li>negate 继承了一个奇怪的东西?<br>下面我们慢慢回答 “如何书写出一个完美的 functor”。</li>
</ul>
<h2 id="2-const-的重要性以及通过传值"><a href="#2-const-的重要性以及通过传值" class="headerlink" title="2. const 的重要性以及通过传值"></a>2. const 的重要性以及通过传值</h2><blockquote>
<p>在 《effective C++》中作者曾经建议：”多使用 const 关键字”。<br>猜想一下,下面的这段代码的运行结果会是什么样子:</p>
</blockquote>
<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><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Predicate</span> {</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> <span class="comment">//借助成员初值列(member initialization list) 可以更高效率的初始化</span></span><br><span class="line"> <span class="built_in">Predicate</span>():<span class="built_in">timeCalled</span>(<span class="number">0</span>){};</span><br><span class="line"> <span class="function"><span class="type">bool</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> <span class="type">int</span> i)</span> </span>{</span><br><span class="line"> <span class="comment">// 当timeCalled 等于 3 时，返回 true</span></span><br><span class="line"> <span class="keyword">return</span> ++timeCalled == <span class="number">3</span>;</span><br><span class="line"> }</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="type">int</span> timeCalled;</span><br><span class="line">};</span><br><span class="line"><span class="comment">//填充 6 个元素</span></span><br><span class="line">v.<span class="built_in">erase</span>(<span class="built_in">remove_if</span>(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(),<span class="built_in">Predicate</span>()),</span><br><span class="line"> v.<span class="built_in">end</span>());</span><br></pre></td></tr></table></figure>
<p>我们尝试删除 vector 中第三个元素,然而上面的这段代码实际上会删除两个元素(一个是第三个元素，一个是第六个元素)！！<br>我们必须通过阅读 remove_if 的源代码才能理解程序的底层到底发生了什么。</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="keyword">template</span> <<span class="keyword">class</span> <span class="title class_">ForwardIterator</span>, <span class="keyword">class</span> <span class="title class_">Predicate</span>></span><br><span class="line"><span class="function">ForwardIterator <span class="title">remove_if</span><span class="params">(ForwardIterator first, ForwardIterator last, Predicate pred)</span> </span>{</span><br><span class="line"> first = <span class="built_in">find_if</span>(first, last, pred);</span><br><span class="line"> ForwardIterator next = first;</span><br><span class="line"> <span class="keyword">return</span> first == last ? first : <span class="built_in">remove_copy_if</span>(++next, last, first, pred);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<blockquote>
<p>当遇见第三个元素时，first 不等于 last，所以会调用 remove_copy_if 函数然而由于 pred 是通过传值传递的，所以这个时候中的 pred 中的 timeCalled 的值是 0！</p>
</blockquote>
<p>你可能会认为是 STL 设计的不合理，所以才导致程序出现了 bug? 当我们尝试通过引用(reference)来传递 pred 对象时(这需要修改remove_if、find_if 、remove_copy_if三者), 此时程序能够顺利的完成给定的任务。<br>然而我们再来查看另一个例子:</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="keyword">template</span> <<span class="keyword">typename</span> Iterator, <span class="keyword">typename</span> Predication></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">myfun</span><span class="params">(Iterator begin, Iterator end, Predication& f)</span></span>{ <span class="comment">//通过引用方式传递</span></span><br><span class="line"> <span class="keyword">for</span>(; begin != end; ++ begin) {</span><br><span class="line"> <span class="keyword">if</span>(<span class="built_in">f</span>(*begin)) {</span><br><span class="line"> std::cout << <span class="string">"访问第三个元素"</span><<std::endl;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">vector<<span class="type">int</span>> v = {<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>,<span class="number">4</span>,<span class="number">5</span>,<span class="number">6</span>,<span class="number">7</span>,<span class="number">8</span>,<span class="number">9</span>};</span><br><span class="line">Predicate p;</span><br><span class="line"><span class="built_in">myfun</span>(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(), p);</span><br></pre></td></tr></table></figure>
<p>上面的代码会在遇见容器v中的第三个元素时，打印出”访问第三个元素”。此时你可能会觉得当我们定义 algorithm 中的 Predication 对象，就应该使用按 reference 传递的方式。<br>然而，当我们简单的连续调用两次 myfun 之后，你会发现程序只会打印一次”访问第三个元素”。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">myfun</span>(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(), p);</span><br><span class="line"><span class="built_in">myfun</span>(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(), p);</span><br></pre></td></tr></table></figure>
<blockquote>
<p>对于相同的输入，我们的 function 每次的表现不一样？<br>这时如果你戏剧性的将 myfun中的函数对象f 改为按值传递，程序就可以顺利的打印两次”访问第三个元素”了;</p>
</blockquote>
<p>怎么回事，有时按值传递 Predication 对象，程序会顺利的运行。有时我们按 reference 传递 Predication 对象 程序也能够顺利的运行。</p>
<p>要想做到行为统一,真正的秘诀在于使用 const, 以及按值传递 Predication。</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="keyword">class</span> <span class="title class_">Predicate</span> {</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> <span class="built_in">Predicate</span>():<span class="built_in">timeCalled</span>(<span class="number">0</span>){};</span><br><span class="line"> <span class="function"><span class="type">bool</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> <span class="type">int</span> i)</span> <span class="type">const</span> </span>{ <span class="comment">//此时程序不能通过编译</span></span><br><span class="line"> <span class="keyword">return</span> ++timeCalled == <span class="number">3</span>;</span><br><span class="line"> }</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="type">int</span> timeCalled;</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>第二个 const 阻止了我们修改函数的状态，这样我们就可以避免了对于相同的输入函数会给出不同的输出结果。<br>而按值传递 Predication 除了这以外，还能让我们的代码保持与 STL 库的一致性。</p>
<h2 id="3-如何让我们自己编写的一般函数融入到-STL-库"><a href="#3-如何让我们自己编写的一般函数融入到-STL-库" class="headerlink" title="3. 如何让我们自己编写的一般函数融入到 STL 库"></a>3. 如何让我们自己编写的一般函数融入到 STL 库</h2><p>当我们按照上面的”建议”编写出自己的函数之后，并不是一切都万事大吉了。</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></pre></td><td class="code"><pre><span class="line"><span class="comment">//bool interesting(const int i );</span></span><br><span class="line"></span><br><span class="line">vector<<span class="type">int</span>> ve = {<span class="number">0</span>,<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>,<span class="number">4</span>,<span class="number">5</span>,<span class="number">6</span>,<span class="number">7</span>,<span class="number">8</span>,<span class="number">9</span>};</span><br><span class="line"><span class="built_in">find_if</span>(ve.<span class="built_in">begin</span>(), ve.<span class="built_in">end</span>(),</span><br><span class="line"> interesting);<span class="comment">//interesting 是一个符合之前规定的 function</span></span><br></pre></td></tr></table></figure>
<p>你突然想的到上面的补集，也就是得到不感兴趣(not interesting)的数。而你对于 STL 函数十分的熟悉，你会想到使用 not1函数来得到 interesting 的相反数。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">find_if</span>(ve.<span class="built_in">begin</span>(), ve.<span class="built_in">end</span>(),</span><br><span class="line"> <span class="built_in">not1</span>(interesting)); <span class="comment">//遗憾的是这样的代码并不能运行</span></span><br></pre></td></tr></table></figure>
<p>因为 not1 实际上应该是一种 functor adapter,它需要向 funtctor <strong>提问</strong>才能顺利的运行。</p>
<ul>
<li>ptr_fun<br>要想让程序顺利的运行，一种方式是使用 ptr_fun 函数。<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">find_if</span>(ve.<span class="built_in">begin</span>(), ve.<span class="built_in">end</span>(), <span class="built_in">not1</span>(<span class="built_in">ptr_fun</span>(interesting)));</span><br></pre></td></tr></table></figure>
<blockquote>
<p>ptr_fun 本质上也是一种 function adapter。</p>
</blockquote>
</li>
<li>继承 unary_function 或 binary_function<br>要想让你自己编写的函数真正的成为一个functor,除了使用 ptr_fun 之外，你还可以继承下面的两个仿函数。<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">//一元仿函数</span></span><br><span class="line"><span class="keyword">template</span> <<span class="keyword">class</span> <span class="title class_">Arg</span>, <span class="keyword">class</span> <span class="title class_">Result</span>></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">unary_function</span> {</span><br><span class="line"> <span class="keyword">typedef</span> Arg argument_type; <span class="comment">//参数类型</span></span><br><span class="line"> <span class="keyword">typedef</span> Result result_type;<span class="comment">//返回值类型</span></span><br><span class="line">};</span><br><span class="line"><span class="comment">//二元仿函数</span></span><br><span class="line"><span class="keyword">template</span> <<span class="keyword">class</span> <span class="title class_">Arg1</span>, <span class="keyword">class</span> <span class="title class_">Arg2</span>, <span class="keyword">class</span> <span class="title class_">Result</span>></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">binary_function</span> {</span><br><span class="line"> <span class="keyword">typedef</span> Arg1 first_argument_type;</span><br><span class="line"> <span class="keyword">typedef</span> Arg2 second_argument_type;</span><br><span class="line"> <span class="keyword">typedef</span> Result result_type;</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
这两个一个分别对应 一元函数和二元函数。当你的 function class 继承他们之一时 struct 中的 typedef 也就被继承下来了。此时我们的 function 就可以回答 not1(functor adaptors 的提问了)。<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></pre></td><td class="code"><pre><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_">myfun</span>: <span class="keyword">public</span> std::unary_function<T, <span class="type">bool</span>>{</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="type">bool</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> <span class="type">int</span> i )</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>; <span class="comment">//都返回 true</span></span><br><span class="line"> }</span><br><span class="line">};</span><br><span class="line"><span class="built_in">find_if</span>(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(), <span class="built_in">not1</span>(<span class="built_in">myfun</span><<span class="type">int</span>>()));</span><br></pre></td></tr></table></figure></li>
</ul>
<h2 id="总结—成为一个-functor-的关键"><a href="#总结—成为一个-functor-的关键" class="headerlink" title="总结—成为一个 functor 的关键"></a>总结—成为一个 functor 的关键</h2><pre><code>- 一个优秀的 functor 的输出应该只与输入有关，让 functor 维持一个状态并不总是一件好事
- 让你的 function 融入到 STL 的 functor 体系中，会更便于使用。
</code></pre>
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<ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#1-STL-%E4%B8%AD%E7%9A%84-functor-%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90"><span class="toc-number">1.</span> <span class="toc-text">1. STL 中的 functor 源码解析</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#2-const-%E7%9A%84%E9%87%8D%E8%A6%81%E6%80%A7%E4%BB%A5%E5%8F%8A%E9%80%9A%E8%BF%87%E4%BC%A0%E5%80%BC"><span class="toc-number">2.</span> <span class="toc-text">2. const 的重要性以及通过传值</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#3-%E5%A6%82%E4%BD%95%E8%AE%A9%E6%88%91%E4%BB%AC%E8%87%AA%E5%B7%B1%E7%BC%96%E5%86%99%E7%9A%84%E4%B8%80%E8%88%AC%E5%87%BD%E6%95%B0%E8%9E%8D%E5%85%A5%E5%88%B0-STL-%E5%BA%93"><span class="toc-number">3.</span> <span class="toc-text">3. 如何让我们自己编写的一般函数融入到 STL 库</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%80%BB%E7%BB%93%E2%80%94%E6%88%90%E4%B8%BA%E4%B8%80%E4%B8%AA-functor-%E7%9A%84%E5%85%B3%E9%94%AE"><span class="toc-number">4.</span> <span class="toc-text">总结—成为一个 functor 的关键</span></a></li></ol>
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<li><a class="icon" target="_blank" rel="noopener" href="http://digg.com/submit?url=https://tblgsn.github.io/stlfunctor20211004.html&title=C++ 如何编写出一个完美的 functor"><i class="fab fa-digg fa-lg" aria-hidden="true"></i></a></li>
<li><a class="icon" target="_blank" rel="noopener" href="http://www.tumblr.com/share/link?url=https://tblgsn.github.io/stlfunctor20211004.html&name=C++ 如何编写出一个完美的 functor&description=<p>在 STL 中 functor 扮演着重要的角色。在 STL 中 functor 默默的为 algorithms 提供支持，因此 functor 通常与 algorithm 同时出现,更为准确的来说： “functor (包括functor adapter) 总是伴随着 algorithm 同时出现。”</p>
<p>一个常见的使用场景如下:(这段代码会打印出容器 v 中的所有元素)</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">// 自己定义的一般函数</span></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">myfun</span>&#123;</span><br><span class="line"> <span class="function"><span class="type">bool</span> <span class="title">operator</span><span class="params">()</span><span class="params">(<span class="type">const</span> <span class="type">int</span> i)</span> </span>&#123;</span><br><span class="line"> std::cout &lt;&lt; i &lt;&lt;std::endl;</span><br><span class="line"> &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//填充一个 vector v，然后使用 for_each 算法</span></span><br><span class="line"></span><br><span class="line">for_each(v.<span class="built_in">begin</span>(), v.<span class="built_in">end</span>(), myfunction);</span><br></pre></td></tr></table></figure>"><i class="fab fa-tumblr fa-lg" aria-hidden="true"></i></a></li>
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