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&lt;p&gt;额，怎么开始呢？ 所有的程序本质上是 data structure 和 algorithm。vector 作为 STL 中序列式容器的一员，自然也遵循着这一原理。&lt;/p&gt;"><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="#vector-%E7%9A%84%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84"><span class="toc-number">1.</span> <span class="toc-text">vector 的数据结构</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#vector-%E7%9A%84%E5%86%85%E5%AD%98%E7%AE%A1%E7%90%86%E7%AD%96%E7%95%A5%E7%AD%96%E7%95%A5"><span class="toc-number">2.</span> <span class="toc-text">vector 的内存管理策略策略</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E4%BD%BF%E7%94%A8-vector-%E7%9A%84%E6%8A%80%E5%B7%A7"><span class="toc-number">3.</span> <span class="toc-text">使用 vector 的技巧</span></a></li></ol>
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        vector 源码解析
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        <span class="p-name" itemprop="name">@TBLGSn</span>
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        <time datetime="2021-08-31T16:50:00.000Z" class="dt-published" itemprop="datePublished">2021-08-31</time>
        
      
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    <p>作为C++的第一个容器，vector在设计上有着许多值得我们学习的地方，这篇博文旨在记录 vector 的源码学习经历。当然关于 STL 源码解析，一本侯捷的《STL 源码剖析》就足够了，在这本书中作者详细的介绍了 C plus plus 的 STL “组件”的每一个函数。因此这篇文章不应该照搬书中的内容，如果只是单纯的照搬书中的知识，那么这篇文章就没有任何存在的价值。 </p>
<p>额，怎么开始呢？ 所有的程序本质上是 data structure 和 algorithm。vector 作为 STL 中序列式容器的一员，自然也遵循着这一原理。</p>
<span id="more"></span>

<hr>
<h2 id="vector-的数据结构"><a href="#vector-的数据结构" class="headerlink" title="vector 的数据结构"></a>vector 的数据结构</h2><p>vector 中的数据结构实现十分简单，你可以想象成三个指针,分别指向 ① 当前内存空间的首部(第一个元素) ② 当前内存空间的尾部 ③ 最后一个元素。<br><img src="/img/sourcecode_vector_datastructure.png" alt="vector 数据结构"></p>
<h2 id="vector-的内存管理策略策略"><a href="#vector-的内存管理策略策略" class="headerlink" title="vector 的内存管理策略策略"></a>vector 的内存管理策略策略</h2><p>vector 相较于 array能够自我的对内存空间进行控制，自由的改变自身容量的大小。但这就意味着，vector 需要为我们动态的维护内存空间的大小,如果当前的容器已经装不下新的元素时，那么会申请一块更大的内存空间,并将原来的元素 copy 到新地址，并释放掉旧的地址空间。</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">// vector 中的插入单个元素形如：</span></span><br><span class="line"><span class="keyword">template</span> &lt;<span class="keyword">class</span> <span class="title class_">T</span>, <span class="keyword">class</span> <span class="title class_">Alloc</span>&gt;</span><br><span class="line"><span class="type">void</span> vector&lt;T, Alloc&gt;::<span class="built_in">insert_aux</span>(iterator position, <span class="type">const</span> T&amp; x)&#123;</span><br><span class="line">	<span class="keyword">if</span>(finish != end_of_storage) &#123; <span class="comment">//还有剩余空间</span></span><br><span class="line">		<span class="comment">//将position 之后的元素向后移动 </span></span><br><span class="line">	&#125; <span class="keyword">else</span> &#123; <span class="comment">//已经没有剩余的空间了</span></span><br><span class="line">		<span class="type">const</span> size_type old_size = <span class="built_in">size</span>();</span><br><span class="line">    	<span class="type">const</span> size_type len = old_size != <span class="number">0</span> ? <span class="number">2</span> * old_size : <span class="number">1</span>; <span class="comment">//申请两倍的新空间</span></span><br><span class="line">		iterator new_start = data_allocator::<span class="built_in">allocate</span>(len);<span class="comment">//申请空间</span></span><br><span class="line">		<span class="comment">//构造新空间copy, 插入元素,释放旧空间等等</span></span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>可以看见,每当当前的空间满了的时候，vector 需要做很多的<strong>额外的工作</strong>才能在正确的位置上插入新的元素。 因此避免重新申请(reallocation)内存空间对于高效的 vector 的使用十分重要。</p>
<p>下面将简要的证明,每次申请新的内存的空间为什么总是旧空间的两倍。</p>
<ul>
<li>我们假设一开始的空间为 1，插入10个元素,如果空间满,这时我们重新申请新的空间大小为之前的两倍</li>
</ul>
<table>
<thead>
<tr>
<th align="center">插入的元素</th>
<th align="center">1</th>
<th align="center">2</th>
<th align="center">3</th>
<th align="center">4</th>
<th align="center">5</th>
<th align="center">6</th>
<th align="center">7</th>
<th align="center">8</th>
<th align="center">9</th>
<th align="center">10</th>
</tr>
</thead>
<tbody><tr>
<td align="center">插入新元素的代价</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
<td align="center">1</td>
</tr>
<tr>
<td align="center">移动元素的代价</td>
<td align="center"></td>
<td align="center">1</td>
<td align="center">2</td>
<td align="center"></td>
<td align="center">4</td>
<td align="center"></td>
<td align="center"></td>
<td align="center"></td>
<td align="center">8</td>
<td align="center"></td>
</tr>
</tbody></table>
<p>不难看出 n 次插入过程的整体花费包含 n 次插入新元素的花销，以及 $\lfloor \lg (n-1) \rfloor$ 旧元素的花销。因此，整体的花销代价为：$ n +  \sum_{i&#x3D;0}^{ \lfloor \lg (n-1) \rfloor}{2^i} $ 。这个几何级数$\leqslant  2n$ ，所以最终的代价为 $O(n)$,平摊到 n 次插入操作,每次的代价为 O(1)</p>
<h2 id="使用-vector-的技巧"><a href="#使用-vector-的技巧" class="headerlink" title="使用 vector 的技巧"></a>使用 vector 的技巧</h2><p>通过阅读 vector 的源码，不难发现 vector 在设计上，尽可能的为我们提供高效率。但这并不代表我们就可以高枕无忧了。要想更加有效的使用 STL ,<strong>编码技巧</strong>是并不可少的。<br>对于 vector 的使用，我们除了注意不要频繁的申请新的空间(因为这样会付出极大的代价)，我们还需要对于额外的内存空间进行控制。vector 使用每次扩大为之前的两倍的策略使得最终插入操作到的代价为 O(1),这是时间上的优化，但 vector 不能从空间上进行优化，用户在使用 vector 时，应当注意空间上的浪费。</p>
<p>我们应该在使用在 vector 之前提供<strong>足够</strong>的空间来避免之后的重新申请内存空间。除默认构造函数外，vector 还为我们提供了诸多其他的构造函数，这使得我们能一开始就指定容器的容量大小</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></pre></td><td class="code"><pre><span class="line"><span class="comment">//构造函数</span></span><br><span class="line"><span class="built_in">vector</span>() : <span class="built_in">start</span>(<span class="number">0</span>), <span class="built_in">finish</span>(<span class="number">0</span>), <span class="built_in">end_of_storage</span>(<span class="number">0</span>) &#123;&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">vector</span>(size_type n, <span class="type">const</span> T&amp; value) &#123; </span><br><span class="line">	<span class="built_in">fill_initialize</span>(n, value); <span class="comment">//申请空间为n,并全部初始化为 value</span></span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//其他构造函数....</span></span><br></pre></td></tr></table></figure>
<p>但事先成功的预测容器大小绝非易事。我们要做的是尽可能的提供足够的空间的同时，避免空间上的浪费。<br>另一个注意的点是：vector 提供的 resize() 函数并不能用来更改内存空间的大小。使用这个函数需要你提供参数 new_size 来指出你需要保留的容器的大小。如果给定的参数大于当前的容器的大小，那么该函数就会进行<strong>填充操作</strong>。而如果给定的参数小于容器当前的大小，那么该函数就会进行<strong>擦除操作</strong>。</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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="type">void</span> <span class="title">resize</span><span class="params">(size_type new_size)</span> </span>&#123; <span class="built_in">resize</span>(new_size, <span class="built_in">T</span>()); &#125;</span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">resize</span><span class="params">(size_type new_size, <span class="type">const</span> T&amp; x)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (new_size &lt; <span class="built_in">size</span>()) <span class="comment">// 擦除操作</span></span><br><span class="line">      <span class="built_in">erase</span>(<span class="built_in">begin</span>() + new_size, <span class="built_in">end</span>());</span><br><span class="line">    <span class="keyword">else</span> <span class="comment">//填充操作</span></span><br><span class="line">      <span class="built_in">insert</span>(<span class="built_in">end</span>(), new_size - <span class="built_in">size</span>(), x);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>vector 为我们提供了 size()函数，通过这个函数我们能给知道当前元素的个数.因此，通过以下的代码我们就能将当前的元素的个数为 5。</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="function">vector&lt;<span class="type">int</span>&gt; <span class="title">v</span><span class="params">(<span class="number">10</span>,<span class="number">0</span>)</span></span>;</span><br><span class="line">v.<span class="built_in">resize</span>(<span class="number">5</span>); <span class="comment">//保留5个元素</span></span><br></pre></td></tr></table></figure>
<p>但是请注意这时 vector 的容量依旧是10，因为 vector 并不会更改 end_of_storage 的值，vector 会做的是调用多余元素的析构函数(如果有的话)，释放这部分空间。</p>
<p>要想更改当前容器的容量大小，在《Effective STL》 一书中Item 17，作者为什么提供了一种策略 —— 使用 **”the swap trick”**。</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="keyword">class</span> <span class="title class_">Widget</span>&#123;&#125;;</span><br><span class="line">vector&lt;Widget&gt; widgets;</span><br><span class="line"><span class="comment">//...插入元素等操作</span></span><br><span class="line"></span><br><span class="line"><span class="built_in">vector</span>&lt;Widget&gt;(widgets).<span class="built_in">swap</span>(widgets);<span class="comment">//删除当前容器的备用空间</span></span><br></pre></td></tr></table></figure>
<p>vector<Widget>(widgets) 会创建一个匿名的 vector 对象，而构造过程调用的是 vector 的copy构造函数。</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></pre></td><td class="code"><pre><span class="line"><span class="comment">//vector 的 copy构造函数</span></span><br><span class="line"><span class="built_in">vector</span>(<span class="type">const</span> vector&lt;T, Alloc&gt;&amp; x) &#123;</span><br><span class="line">	<span class="comment">//这里只会申请 x.end() - x.begin() 个空间大小，也就是当前元素的个数,并完成 copy 元素操作</span></span><br><span class="line">	start = <span class="built_in">allocate_and_copy</span>(x.<span class="built_in">end</span>() - x.<span class="built_in">begin</span>(), x.<span class="built_in">begin</span>(), x.<span class="built_in">end</span>());</span><br><span class="line">	finish = start + (x.<span class="built_in">end</span>() - x.<span class="built_in">begin</span>());</span><br><span class="line">	end_of_storage = finish;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这样得到的匿名对象就没有额外的备用空间。之后在调用swap()函数交换匿名对象和源对象的三个状态迭代器(start、finish、end_of_storage)。</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">//只会交换 迭代器状态 ，因为 copy 元素操作由 copy构造函数完成了</span></span><br><span class="line">  <span class="function"><span class="type">void</span> <span class="title">swap</span><span class="params">(vector&lt;T, Alloc&gt;&amp; x)</span> </span>&#123;</span><br><span class="line">    __STD::<span class="built_in">swap</span>(start, x.start);</span><br><span class="line">    __STD::<span class="built_in">swap</span>(finish, x.finish);</span><br><span class="line">    __STD::<span class="built_in">swap</span>(end_of_storage, x.end_of_storage);</span><br><span class="line">  &#125;</span><br></pre></td></tr></table></figure>
<p>随后匿名对象被销毁，销毁的是原 start 指向的空间。</p>
<p>使用copy构造函数创建临时对象是必须的，如果我们调用默认构造函数来构造对象，那么我们会删除掉所有的元素。</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">vector&lt;Widget&gt; widgets;</span><br><span class="line"><span class="built_in">vector</span>&lt;Widget&gt;().<span class="built_in">swap</span>(widgets); <span class="comment">//这会clear 掉 widgets的元素，并将容量设置为0</span></span><br></pre></td></tr></table></figure>


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