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<h3><span lang=EN-US>28. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>最好是保持标准形式的</span><span lang=EN-US>++</span><span
style='font-family:宋体;mso-ascii-font-family:Arial;mso-hansi-font-family:Arial'>和</span><span
lang=EN-US>--</span><span style='font-family:宋体;mso-ascii-font-family:Arial;
mso-hansi-font-family:Arial'>。最好是调用前缀的形式。</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><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"'>如果允许</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"'>，那么</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"'>应该也允许：递增和递减操作符需要特殊对待，因为每一种都有前缀和后缀的形式，语义也略有不同。在定义</span><span
lang=EN-US>operator++</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>operator--</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>时应该模仿它们对应的内建操作符的行为。如果不需要原来的值，那么最好是调用前缀版本。</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"'>讨论</span><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"'>有一个关于</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"'>的很老的笑话，说该语言被称为</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"'>而不是</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"'>的原因是虽然语言已经改进了（递增了），但是许多人仍然把它当作</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"'>（原来的值）来使用。幸运的是，这个笑话已经过时了，不过它是个很形象的例子，有助于理解运算符的这两种形式之间的区别。</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"'>对</span><span lang=EN-US>++</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>--</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>来说，后缀的形式返回的是原来的值，而前缀的形式返回的则是新的值。最好是用前缀的形式实现后缀的形式。标准的形式为：</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>T&
T::operator++() {<span
style='mso-spacerun:yes'> </span>T&
T::operator--() {<span style='mso-spacerun:yes'>
</span>// the prefix form:</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>// perform increment<span
style='mso-spacerun:yes'> </span>//
perform decrement<span style='mso-spacerun:yes'> </span>//<span
style='mso-spacerun:yes'> </span>- do the work</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>return *this;<span
style='mso-spacerun:yes'>
</span>return *this;<span
style='mso-spacerun:yes'>
</span>//<span style='mso-spacerun:yes'> </span>- always return
*this;</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}<span
style='mso-spacerun:yes'>
</span>}</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>T
T::operator++(int) {<span
style='mso-spacerun:yes'> </span>T
T::operator--(int) {<span style='mso-spacerun:yes'> </span>// the
postfix form:</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>T old( *this );<span
style='mso-spacerun:yes'>
</span>T old( *this );<span
style='mso-spacerun:yes'>
</span>//<span style='mso-spacerun:yes'> </span>- remember old
value</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>++*this;<span
style='mso-spacerun:yes'>
</span>--*this;<span style='mso-spacerun:yes'>
</span>//<span style='mso-spacerun:yes'> </span>- call the prefix
version</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US><span
style='mso-spacerun:yes'> </span>return old;<span
style='mso-spacerun:yes'>
</span>return old;<span style='mso-spacerun:yes'>
</span>//<span style='mso-spacerun:yes'> </span>- return the old
value</span></p>
<p class=MsoNormal style='margin-left:21.0pt'><span lang=EN-US>}<span
style='mso-spacerun:yes'>
</span>}</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"'>在调用的时候，除非真的需要后缀版本返回的原值，否则最好还是使用前缀的形式。前缀的形式在语义上与后缀的形式是等价的，键盘敲得一样多，效率通常会因为少创建一个对象而略高。这不是过早的优化；而是避免过早的退而求次（参见第</span><span
lang=EN-US>9</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><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"'>表达式模板的框架（</span><span lang=EN-US>expression
template framework</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）会通过不同的方法来保持语义。</span></p>
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