📄 item_082.htm
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<h3><span lang=EN-US>82. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>使用公认的惯用法来真正地缩小容量以及真正地删除元素。</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"'>使用一种有效的减肥方法：要真正地去除一个容器的额外容量，请使用“</span><span
lang=EN-US>swap trick</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>”。要真正地删除容器中的元素，请使用</span><span
lang=EN-US>erase-remove</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>惯用法。</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"'>一些容器（例如：</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"'>，</span><span lang=EN-US>string</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，</span><span class=SpellE><span lang=EN-US>deque</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）最终可能会具有不再需要的额外容量。虽然</span><span lang=EN-US>C++</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>标准库的容器没有提供一种切实可行的方法来去除额外的容量，但是下面的“</span><span lang=EN-US>swap
trick</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>”惯用法实际上可以用来去除下面类型为</span><span
lang=EN-US>container</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的</span><span
lang=EN-US>c</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>容器的额外容量。</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>container<T>(
c ).swap( c );<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// </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"'>用来去除额外容量的</span><span lang=EN-US>
shrink-to-fit</span></i><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"'>惯用法</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"'>或者要把</span><span lang=EN-US>c</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>完全清空，即清除所有被包含的元素并去除所有可能占用的容量，相应的惯用法为：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:36.0pt'><span lang=EN-US>container<T>().swap(
c );<span style='mso-tab-count:1'> </span><i
style='mso-bidi-font-style:normal'>// </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"'>用来去除全部内容和容量的惯用法</span><span
lang=EN-US><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"'>在与此有关的消息中，通常令新</span><span lang=EN-US>STL</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>程序员大吃一惊的是</span><span lang=EN-US>remove</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>算法并没有真正地从容器中移除元素。当然，它也不能；算法使用的只是迭代器的范围，如果不调用容器的成员函数，通常来说是</span><span
lang=EN-US>erase</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，那么是无法真正地从容器中移除什么东西的。</span><span
lang=EN-US>remove</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>所做的一切就是移动它们的位置，把不需要被“移除”的元素移到范围的前面，然后返回一个迭代器，该迭代器正好指向最后一个不要被移除的元素的下一个。要真正地去除它们，需要在调用</span><span
lang=EN-US>remove</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>之后再调用</span><span lang=EN-US>erase</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>——即“</span><span lang=EN-US>erase-remove</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>”惯用法的由来。例如，要从容器</span><span lang=EN-US>c</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中删除所有值为</span><span lang=EN-US>value</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的元素，可以用下面的代码：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span class=SpellE><span
class=GramE><span lang=EN-US>c.erase</span></span></span><span class=GramE><span
lang=EN-US>(</span></span><span lang=EN-US> remove( <span class=SpellE>c.begin</span>(),
<span class=SpellE>c.end</span>() ,value ) );</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"'>如果容器有</span><span lang=EN-US>remove</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>或</span><span class=SpellE><span lang=EN-US>remove_if</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>成员函数，那么应该尽量使用成员函数版本。</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"'>常用的</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"'>惯用法无法用于通过写时复制（</span><span lang=EN-US>copy-on-write</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>）来实现的</span><span class=SpellE><span lang=EN-US>std::string</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>。一般来说可行的方法是调用</span><span class=SpellE><span lang=EN-US>s.reserve</span></span><span
lang=EN-US>(0)</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>，或是用范围构造函数</span><span lang=EN-US>string(<span
class=SpellE>s.begin</span>(), <span class=SpellE>s.end</span>()).swap(s);</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>来伪去除</span><span lang=EN-US>string</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的额外容量。在实践中，这些方法都可以去除额外的容量。（另外，</span><span class=SpellE><span
lang=EN-US>std::string</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的实现正在弃用写时复制，这已经是一个过时的优化了；参见</span><span
lang=EN-US>[Sutter02]</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>。）</span></p>
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