深入理解PHP之数组遍历顺序

经常会有人问我, PHP的数组, 如果用foreach来访问, 遍历的顺序是固定的么? 以什么顺序遍历呢?
比如:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;">
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><?php </span></span></li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_string" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(168, 255, 96);">'laruence'</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_string" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(168, 255, 96);">'huixinchen'</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_string" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(168, 255, 96);">'yahoo'</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span>    <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2007</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_string" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(168, 255, 96);">'baidu'</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span>    <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2008</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">foreach</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span><span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span> <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">as</span> <span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$key</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=&gt;</span> <span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$val</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span> <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">{</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">//结果是什么?</span></li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">}</span></li>
</ol>

又比如:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;">
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><?php </span></span></li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_string" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(168, 255, 96);">'huixinchen'</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">1</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span>  <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2007</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">0</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span>  <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2008</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">foreach</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span><span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span> <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">as</span> <span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$key</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=&gt;</span> <span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$val</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span> <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">{</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">//现在结果又是什么?</span></li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">}</span></li>
</ol>

要完全了解清楚这个问题, 我想首先应该要大家了解PHP数组的内部实现结构………PHP的数组在PHP中, 数组是用一种HASH结构(HashTable)来实现的, PHP使用了一些机制, 使得可以在O(1)的时间复杂度下实现数组的增删, 并同时支持线性遍历和随机访问.之前的文章中也讨论过, PHP的HASH算法, 基于此, 我们做进一步的延伸.认识HashTable之前, 首先让我们看看HashTable的结构定义, 我加了注释方便大家理解:

<ol style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;">
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">typedef</span> <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">struct</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_classname" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">_hashtable</span> <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">{</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">uint</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>nTableSize<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>        <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 散列表大小, Hash值的区间 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">uint</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>nTableMask<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>        <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 等于nTableSize -1, 用于快速定位,相当于是取模 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">uint</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>nNumOfElements<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>    <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* HashTable中实际元素的个数 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">ulong</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);"> </span><span style="background-color: rgb(51, 51, 51);">nNextFreeElement</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">;</span><span style="background-color: rgb(51, 51, 51);"> </span><span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);"><span style="background-color: rgb(51, 51, 51);">/* 下个空闲可用位置的数字索引,</span><span style="background-color: rgb(0, 0, 153);">执行$arr[]='xxx'时使用这个值</span><span style="background-color: rgb(51, 51, 51);"> */</span></span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">Bucket</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pInternalPointer<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>   <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 内部位置指针, 会被reset, current这些遍历函数使用 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">Bucket</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pListHead<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>      <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 头元素, 用于线性遍历 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">Bucket</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pListTail<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>      <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 尾元素, 用于线性遍历 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">Bucket</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);"> </span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">**</span><span style="background-color: rgb(51, 51, 51);">arBuckets</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">;</span><span style="background-color: rgb(51, 51, 51);">     </span><span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);"><span style="background-color: rgb(51, 51, 51);">/* 实际的存储容器</span><span style="background-color: rgb(0, 0, 153);">（<span class="sh_usertype" style="color: rgb(246, 243, 232); font-family: inherit; font-style: inherit; font-weight: inherit; line-height: 18px; white-space: pre; margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; vertical-align: baseline;">Bucket</span><span class="sh_normal" style="color: rgb(246, 243, 232); font-family: inherit; font-style: inherit; font-weight: inherit; line-height: 18px; white-space: pre; margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; vertical-align: baseline;"> </span><span class="sh_symbol" style="color: rgb(246, 243, 232); font-family: inherit; font-style: inherit; font-weight: inherit; line-height: 18px; white-space: pre; margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; vertical-align: baseline;">**</span><span style="color: rgb(246, 243, 232); font-family: Monaco, Consolas, Courier, monospace; line-height: 18px; white-space: pre;">arBuckets</span>定义一个指针数组的另一种方式）</span><span style="background-color: rgb(51, 51, 51);"> */</span></span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">dtor_func_t</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>pDestructor<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span><span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 元素的析构函数(指针) */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">zend_bool</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>persistent<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_type" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 255, 182);">unsigned</span> <span class="sh_type" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 255, 182);">char</span> nApplyCount<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span> <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 循环遍历保护 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">zend_bool</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>bApplyProtection<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_preproc" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">#if</span> ZEND_DEBUG</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_type" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 255, 182);">int</span> inconsistent<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);"><span class="sh_preproc" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">#endif</span></li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">}</span> HashTable<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
</ol>

关于nApplyCount的意义, 我们可以通过一个例子来了解:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;">
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><?php </span></span></li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">    <span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">array</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">1</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">,</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">,</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">3</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">,</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">4</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">,</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">5</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">,);</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">    <span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[]</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">&amp;</span><span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </li>
<li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">    <span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">var_export</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span><span class="sh_variable" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(198, 197, 254);">$arr</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">);</span> <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">//Fatal error: Nesting level too deep - recursive dependency?</span>
</li>
</ol>

这个字段就是为了防治循环引用导致的无限循环而设立的.查看上面的结构, 可以看出, 对于HashTable, 关键元素就是arBuckets了, 这个是实际存储的容器, 让我们来看看它的结构定义:

<ol style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;">
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">typedef</span> <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">struct</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_classname" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">bucket</span> <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">{</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">ulong</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>h<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>                        <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 数字索引/hash值 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">uint</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>nKeyLength<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>                <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 字符索引的长度 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">
<span class="sh_type" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-family: inherit; vertical-align: baseline; color: rgb(255, 255, 182); background-color: rgb(51, 51, 51);">void</span><span style="background-color: rgb(51, 51, 51);"> </span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">*</span><span style="background-color: rgb(51, 51, 51);">pData</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">;</span><span style="background-color: rgb(51, 51, 51);">                    </span><span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);"><span style="background-color: rgb(51, 51, 51);">/* 数据，</span><span style="background-color: rgb(0, 0, 153);">指向zval结构</span><span style="background-color: rgb(51, 51, 51);"> */</span></span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_type" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 255, 182);">void</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pDataPtr<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>                 <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 数据指针 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">struct</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_classname" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">bucket</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pListNext<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>               <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 下一个元素, 用于线性遍历 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">struct</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_classname" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">bucket</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pListLast<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>       <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 上一个元素, 用于线性遍历 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">struct</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_classname" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">bucket</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pNext<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>                   <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 处于同一个拉链中的下一个元素 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">struct</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_classname" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">bucket</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>pLast<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>                   <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 处于同一拉链中的上一个元素 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_type" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 255, 182);">char</span> arKey<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">1</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">];</span> <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* 节省内存,方便初始化的技巧 */</span>
</li>
<li style="margin: 0px; padding: 0px; border: 0px; border-image-source: initial; border-image-slice: initial; border-image-width: initial; border-image-outset: initial; border-image-repeat: initial; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; background-color: rgb(51, 51, 51);">
<span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">}</span> Bucket<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span>
</li>
</ol>

我们注意到, 最后一个元素, 这个是flexible array技巧, 可以节省内存,和方便初始化的一种做法, 有兴趣的朋友可以google flexible array.There are a couple of things it could refer to. The only place I've seen that precise wording used much is C99 though.A flexible array member is the official C99 name for what used to (usually) be called the "struct hack". The basic idea is that you define a struct something like this:struct x { int a; // whatever members you want here. size_t size; int x[]; // no size, last member only }; This is used primarily (or exclusively) with dynamic allocation. When you want to allocate an object of this type, you