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<li>这样可以避免野指针的出现</li>
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<li>释放不在使用的内存资源，避免内存的泄漏.<br>智能指针使得为我们能够更加安全有效的使用（共享）同一块堆空间."><i class="fab fa-tumblr " aria-hidden="true"></i></a></li>
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<ol class="toc"><li class="toc-item toc-level-1"><a class="toc-link" href="#shared-ptr%E7%9A%84%E5%AE%9E%E7%8E%B0"><span class="toc-number">1.</span> <span class="toc-text">shared_ptr的实现</span></a></li><li class="toc-item toc-level-1"><a class="toc-link" href="#%E5%A6%82%E4%BD%95%E4%BD%BF%E7%94%A8%E6%99%BA%E8%83%BD%E6%8C%87%E9%92%88"><span class="toc-number">2.</span> <span class="toc-text">如何使用智能指针</span></a><ol class="toc-child"><li class="toc-item toc-level-2"><a class="toc-link" href="#shared-ptr%E7%9A%84%E6%9E%84%E9%80%A0%E6%96%B9%E6%B3%95"><span class="toc-number">2.1.</span> <span class="toc-text">shared_ptr的构造方法</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#shared-ptr%E6%99%BA%E8%83%BD%E6%8C%87%E9%92%88%E7%9A%84%E9%87%8A%E6%94%BE%E5%92%8C%E5%86%85%E5%AD%98%E7%9A%84%E9%87%8A%E6%94%BE"><span class="toc-number">2.2.</span> <span class="toc-text">shared_ptr智能指针的释放和内存的释放</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%85%B6%E4%BB%96%E6%96%B9%E6%B3%95"><span class="toc-number">2.3.</span> <span class="toc-text">其他方法</span></a></li></ol></li><li class="toc-item toc-level-1"><a class="toc-link" href="#shared-ptr-unique-ptr-std-weak-ptr%E4%B9%8B%E9%97%B4%E7%9A%84%E5%8C%BA%E5%88%AB"><span class="toc-number">3.</span> <span class="toc-text">shared_ptr,unique_ptr, std::weak_ptr之间的区别</span></a></li><li class="toc-item toc-level-1"><a class="toc-link" href="#%E5%8F%82%E8%80%83"><span class="toc-number">4.</span> <span class="toc-text">参考</span></a></li></ol>
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C++ Smart Pointer(智能指针)
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<a class="p-category" href="/tags/C/" rel="tag">C++</a>, <a class="p-category" href="/tags/%E6%99%BA%E8%83%BD%E6%8C%87%E9%92%88/" rel="tag">智能指针</a>
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<p>早在C++98/03标准的时候，C++就引入了智能指针(Smart Pointer)的概念来帮助程序员对于堆内存进行自动回收。</p>
<p>引入的目地主要是：对指针的行为进行一定的规范。这有很多的好处，比如：</p>
<ul>
<li>这样可以避免野指针的出现</li>
<li>这样可以避免错误的对于同一块地址进行两次释放</li>
<li>释放不在使用的内存资源，避免内存的泄漏.<br>智能指针使得为我们能够更加安全有效的使用（共享）同一块堆空间.<span id="more"></span></li>
</ul>
<h1 id="shared-ptr的实现"><a href="#shared-ptr的实现" class="headerlink" title="shared_ptr的实现"></a>shared_ptr的实现</h1><p>C++ 智能指针是采用的 引用计数（Reference counting） 的方式实现的。更简单的说，智能指针会负责维护一个整形值用于标记当前空间的引用值。当有新的对象使用同一块堆内存时，整形值加一。当使用该块堆内存的对象被释放掉时，整形值减一。当这个值减少为0时，则表示除了不在会有对象使用这块内存，该堆空间会被释放掉(而不用程序员担心什么时候释放)。</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></pre></td><td class="code"><pre><span class="line"><span class="comment">// std::shared_ptr采用的是模板类的方式定义的，它继承自std::_shared_ptr 这个类(shared_ptr_base.h文件中).</span></span><br><span class="line"> <span class="keyword">template</span><<span class="keyword">typename</span> _Tp></span><br><span class="line"> <span class="keyword">class</span> <span class="title class_">shared_ptr</span> : <span class="keyword">public</span> __shared_ptr<_Tp></span><br></pre></td></tr></table></figure>
<p>shared_ptr继承自_shared_ptr，在这个类中维持了两个对象，一个是element_type类型的指针_M_ptr,一个是__weak_count<_Lp>类型的计数器_M_refcount。通过对这两个对象的维护，实现智能指针的目的.<br>例如,use_count()函数借助_M_refcount直接返回的引用数。 </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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">long</span> <span class="title">use_count</span><span class="params">()</span> <span class="type">const</span> <span class="keyword">noexcept</span></span>{ </span><br><span class="line"> <span class="keyword">return</span> _M_refcount._M_get_use_count(); </span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h1 id="如何使用智能指针"><a href="#如何使用智能指针" class="headerlink" title="如何使用智能指针"></a>如何使用智能指针</h1><p>C++11中提供了三种智能指针：std::shared_ptr, std::unique_ptr, std::weak_ptr，使用时需添加头文件<memory></p>
<blockquote>
<p>C++98/03 标准中，曾经支持使用 auto_ptr 智能指针来实现堆内存的自动回收,但后来在C++17被废除.</p>
</blockquote>
<p>下面以shared_ptr为例，介绍C++中智能指针的使用.</p>
<h2 id="shared-ptr的构造方法"><a href="#shared-ptr的构造方法" class="headerlink" title="shared_ptr的构造方法"></a>shared_ptr的构造方法</h2><p>shared_ptr 这个类为我们提供了十几种创建共享指针的构造方法.</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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 我们能够通过以下的方式来创建 shared_ptr 指针</span></span><br><span class="line">std::shared_ptr<<span class="type">int</span>> p1;</span><br><span class="line"><span class="function">std::shared_ptr<<span class="type">int</span>> <span class="title">p1</span><span class="params">(<span class="literal">nullptr</span>)</span></span>;</span><br><span class="line"><span class="function">std::shared_ptr<<span class="type">int</span>> <span class="title">p1</span><span class="params">(<span class="keyword">new</span> <span class="type">int</span>[<span class="number">10</span>])</span></span>;</span><br><span class="line"><span class="comment">//我们还可以借助make_shared<T>函数</span></span><br><span class="line">std::shared_ptr<<span class="type">int</span>> p3 = std::<span class="built_in">make_shared</span><<span class="type">int</span>>(<span class="number">10</span>);</span><br><span class="line"><span class="comment">//拷贝构造函数</span></span><br><span class="line"><span class="function">std::shared_ptr<<span class="type">int</span>> <span class="title">p4</span><span class="params">(p3)</span></span>;</span><br><span class="line">>>运行之后，计数加<span class="number">1</span>,p3 和 p4 指向同一块堆内存.</span><br><span class="line"><span class="comment">//移动构造函数</span></span><br><span class="line"><span class="function">std::shared_ptr<<span class="type">int</span>> <span class="title">p5</span><span class="params">(std::move(p4))</span></span>;</span><br><span class="line">>运行之后，计数不变p5变为指向内存的智能指针，p4变成空智能指针</span><br></pre></td></tr></table></figure>
<h2 id="shared-ptr智能指针的释放和内存的释放"><a href="#shared-ptr智能指针的释放和内存的释放" class="headerlink" title="shared_ptr智能指针的释放和内存的释放"></a>shared_ptr智能指针的释放和内存的释放</h2><p>以下的两种情况会导致内存的释放：</p>
<ul>
<li>最后一个拥有该块堆内存的shared_ptr指针被销毁时.</li>
<li>最后一个拥有该对象的shared_ptr通过操作符=或reset()被分配另一个指针时。</li>
</ul>
<p>智能指针除了为我们提供了默认的内存释放规则，还允许我们自定义内存释放的规则.</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">//指定 default_delete 作为释放规则</span></span><br><span class="line"><span class="function">std::shared_ptr<<span class="type">int</span>> <span class="title">p6</span><span class="params">(<span class="keyword">new</span> <span class="type">int</span>[<span class="number">10</span>], std::default_delete<<span class="type">int</span>[]>())</span></span>;</span><br><span class="line"><span class="comment">//自定义释放规则</span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">deleteInt</span><span class="params">(<span class="type">int</span>*p)</span> </span>{</span><br><span class="line"> <span class="keyword">delete</span> []p;</span><br><span class="line">}</span><br><span class="line"><span class="comment">//初始化智能指针，并自定义释放规则</span></span><br><span class="line"><span class="function">std::shared_ptr<<span class="type">int</span>> <span class="title">p7</span><span class="params">(<span class="keyword">new</span> <span class="type">int</span>[<span class="number">10</span>], deleteInt)</span></span>;</span><br><span class="line"></span><br><span class="line"><span class="function">std::shared_ptr<<span class="type">int</span>> <span class="title">p8</span><span class="params">(<span class="keyword">new</span> <span class="type">int</span>[<span class="number">10</span>],[](<span class="type">int</span>* p){<span class="keyword">delete</span> []p; })</span></span></span><br></pre></td></tr></table></figure>
<h2 id="其他方法"><a href="#其他方法" class="headerlink" title="其他方法"></a>其他方法</h2><p>shared_ptr 是继承的__shared_ptr 这个类.<br>这个基类为我们提供了以下的一些方法：</p>
<table>
<thead>
<tr>
<th align="center">方法名</th>
<th align="center">说明</th>
<th align="center">返回类型</th>
</tr>
</thead>
<tbody><tr>
<td align="center">get()</td>
<td align="center">获得当前智能智能指向元素的普通指针</td>
<td align="center">element_type*</td>
</tr>
<tr>
<td align="center">user_count()</td>
<td align="center">当前堆内存空间的引用量</td>
<td align="center">long</td>
</tr>
<tr>
<td align="center">unique()</td>
<td align="center">当前堆内存是否还有其他引用</td>
<td align="center">bool</td>
</tr>
<tr>
<td align="center">swap()</td>
<td align="center">交换两个智能指针</td>
<td align="center">void</td>
</tr>
<tr>
<td align="center">reset()</td>
<td align="center">没有参数时，当前智能指针变为空智能指针,有参数时当前智能指针指向给定的新内存空间，原内存引用值减1，当前内存引用值置1</td>
<td align="center">void</td>
</tr>
</tbody></table>
<h1 id="shared-ptr-unique-ptr-std-weak-ptr之间的区别"><a href="#shared-ptr-unique-ptr-std-weak-ptr之间的区别" class="headerlink" title="shared_ptr,unique_ptr, std::weak_ptr之间的区别"></a>shared_ptr,unique_ptr, std::weak_ptr之间的区别</h1><p>shared_ptr能够实现多个shared_ptr类型的指针“<strong>共享</strong>”使用同一块堆内存，当有一个指针放弃该堆内存的“使用权”时，也不会影响其他指向该堆内存的shared_ptr空间.只有当引用计算变为0时，才会释放掉这一块堆内存空间.</p>
<p>unique_ptr 通过指针<strong>占有</strong>并管理另一对象，并在 unique_ptr 离开作用域时释放该对象的智能指针。、</p>
<p>std::weak_ptr 用来表示对于std::shared_ptr 管理的对象的<strong>弱引用</strong>. 用来表达<strong>临时所有权</strong>的概念，当某个对象只有存在时才需要被访问，而且随时可能被他人删除时,std::weak_ptr 用来跟踪该对象。此外值得注意的是:weak_ptr 类型指针并不会影响所指堆内存空间的引用计数。 </p>
<h1 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h1><blockquote>
<p><a target="_blank" rel="noopener" href="http://c.biancheng.net/view/7898.html">http://c.biancheng.net/view/7898.html</a><br><a target="_blank" rel="noopener" href="https://en.cppreference.com/w/cpp/memory/shared_ptr">https://en.cppreference.com/w/cpp/memory/shared_ptr</a></p>
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<ol class="toc"><li class="toc-item toc-level-1"><a class="toc-link" href="#shared-ptr%E7%9A%84%E5%AE%9E%E7%8E%B0"><span class="toc-number">1.</span> <span class="toc-text">shared_ptr的实现</span></a></li><li class="toc-item toc-level-1"><a class="toc-link" href="#%E5%A6%82%E4%BD%95%E4%BD%BF%E7%94%A8%E6%99%BA%E8%83%BD%E6%8C%87%E9%92%88"><span class="toc-number">2.</span> <span class="toc-text">如何使用智能指针</span></a><ol class="toc-child"><li class="toc-item toc-level-2"><a class="toc-link" href="#shared-ptr%E7%9A%84%E6%9E%84%E9%80%A0%E6%96%B9%E6%B3%95"><span class="toc-number">2.1.</span> <span class="toc-text">shared_ptr的构造方法</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#shared-ptr%E6%99%BA%E8%83%BD%E6%8C%87%E9%92%88%E7%9A%84%E9%87%8A%E6%94%BE%E5%92%8C%E5%86%85%E5%AD%98%E7%9A%84%E9%87%8A%E6%94%BE"><span class="toc-number">2.2.</span> <span class="toc-text">shared_ptr智能指针的释放和内存的释放</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%85%B6%E4%BB%96%E6%96%B9%E6%B3%95"><span class="toc-number">2.3.</span> <span class="toc-text">其他方法</span></a></li></ol></li><li class="toc-item toc-level-1"><a class="toc-link" href="#shared-ptr-unique-ptr-std-weak-ptr%E4%B9%8B%E9%97%B4%E7%9A%84%E5%8C%BA%E5%88%AB"><span class="toc-number">3.</span> <span class="toc-text">shared_ptr,unique_ptr, std::weak_ptr之间的区别</span></a></li><li class="toc-item toc-level-1"><a class="toc-link" href="#%E5%8F%82%E8%80%83"><span class="toc-number">4.</span> <span class="toc-text">参考</span></a></li></ol>
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<p>引入的目地主要是：对指针的行为进行一定的规范。这有很多的好处，比如：</p>
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<li>这样可以避免野指针的出现</li>
<li>这样可以避免错误的对于同一块地址进行两次释放</li>
<li>释放不在使用的内存资源，避免内存的泄漏.<br>智能指针使得为我们能够更加安全有效的使用（共享）同一块堆空间."><i class="fab fa-tumblr fa-lg" aria-hidden="true"></i></a></li>
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