📄 item_080.htm
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<div class=Section1>
<h3><span lang=EN-US>80. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>与其它方法相比，要尽量使用</span><span lang=EN-US>push_back</span><span
style='font-family:宋体;mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>来扩大容器。</span></h3>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>摘要</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>如果可以，全部使用</span><span lang=EN-US>push_back</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>：如果不需要关心插入的位置，最好是使用</span><span lang=EN-US>push_back</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>来把元素加到序列中。其它的方法既可能很慢，也不够清晰。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>讨论</span><a name=ch75index09></a><a name=ch75index08></a><a
name=ch75lev1sec2></a><span lang=EN-US><o:p></o:p></span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>可以用</span><span lang=EN-US>insert</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>在序列的不同位置插入元素，也可以用不同的方法把元素添加到序列中去，其中包括：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:225.0pt'><span
lang=EN-US>vector<int> vec;<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// vec</i></span><i style='mso-bidi-font-style:
normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>为空</span></i></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:225.0pt'><span
lang=EN-US>vec.resize(vec.size() + 1, 1);<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// vec</i></span><i style='mso-bidi-font-style:
normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>包含</span><span lang=EN-US>{ 1 }<o:p></o:p></span></i></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:225.0pt'><span
lang=EN-US>vec.insert(vec.end(), 2);<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// vec</i></span><i style='mso-bidi-font-style:
normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>包含</span><span lang=EN-US>{ 1, 2 }</span></i></p>
<p class=MsoNormal style='margin-left:36.0pt;tab-stops:225.0pt'><span
lang=EN-US>vec.push_back(3);<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// vec</i></span><i style='mso-bidi-font-style:
normal'><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>包含</span><span lang=EN-US>{ 1, 2, 3 }<o:p></o:p></span></i></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>在以上所有形式中，只有</span><span lang=EN-US>push_back</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>占用分摊的常数时间（</span><span lang=EN-US>amortized constant time</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）。其它形式的性能较差，甚至可能达到二次方。不用说，这已经超过了可接受的小型数据的限度，并使它们成为达到可伸缩性潜在的壁垒。（参见第</span><span
lang=EN-US>7</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US>push_back</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的魔法很简单：它是指数级地扩大容量，而不是按固定的大小递增。因此随着大小的增长，重新分配和复制的次数显著减少。对只用</span><span
lang=EN-US>push_back</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>来添加元素的容器来说，无论容器最终的大小是多少，平均每个元素只被复制一次。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>当然，</span><span lang=EN-US>resize</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>和</span><span lang=EN-US>insert</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>也可以采用相同的策略，但那取决于具体的实现；只有</span><span lang=EN-US>push_back</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>提供了这样的保证。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>算法不能直接使用</span><span lang=EN-US>push_back</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，因为它们不能访问容器。通过</span><span lang=EN-US>back_inserter</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，可以要求算法使用</span><span lang=EN-US>push_back</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><b style='mso-bidi-font-weight:normal'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>例外</span></b></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>如果知道要添加一个范围，那么即使是位于容器的末尾，也最好是使用范围插入函数（</span><span
lang=EN-US>range insertion function</span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）（参见第</span><span
lang=EN-US>81</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>指数级增长会过多地消耗内存。要对内存的增长进行微调，可以显式地调用</span><span
lang=EN-US>reserve</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>——如果有足够的空间，那么无论是</span><span
lang=EN-US>push_back</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，</span><span
lang=EN-US>resize</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，还是类似的函数都绝对不会重新分配内存。要给</span><span
lang=EN-US>vector</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>设定一个“最佳的大小”，可以使用</span><span
lang=EN-US>shrink-to-fit</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>惯用法（参见第</span><span
lang=EN-US>82</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。</span></p>
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