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<h3><span lang=EN-US>26. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>在重载操作符时，要保留被重载操作符的自然语义。</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"'>程序员讨厌吃惊：仅在有正当的理由时才重载操作符，并保留它们的自然语义；如果这样做很困难，那么你可能正在误用操作符重载。</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"'>虽然任何人都会同意（我们希望）不应该在</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"'>中去实现减法，但是其它的情况可能并不那么明显。例如，你的</span><span
lang=EN-US>Tensor</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>类的</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"'>是表示标量乘呢还是向量乘呢？</span><span lang=EN-US>operator+=(
Tensor& t, unsigned u )</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>是把</span><span
lang=EN-US>u</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>加到</span><span lang=EN-US>t</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的每个元素中去呢，还是会调整</span><span lang=EN-US>t</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的大小？在此类模棱两可或与直觉不符的情况下，最好是使用具名函数，而不要编写隐晦的代码。</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"'>对值类型而言（但并非所有类型；参见第</span><span
lang=EN-US>32</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）：“如果有疑问，那么按照</span><span
class=SpellE><span lang=EN-US>int</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的做法做。”</span><span
lang=EN-US>[Meyers96] </span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>模仿内建类型的操作符的行为及它们之间的关系可以确保不会让任何人吃惊。如果你选择的语义可能会让人皱起眉头，那么操作符重载可能不是个好主意。</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"'>程序员认为操作符应该捆绑提供。如果对你定义的某个操作符</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（可能在转换之后），表达式</span><span
lang=EN-US>a @ b</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的形式是正确的，那么问一下自己：调用方是不是也可以编写</span><span
lang=EN-US>b @ a</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>而不会吃惊呢？调用方是不是可以编写</span><span
lang=EN-US>a @= b</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>呢？（参见第</span><span lang=EN-US>27</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>条。）如果操作符有逆操作符（例如：</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"'>，或</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"'>），那么是不是两者都支持呢？</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"'>对具名函数来说，它们有此类被认定的关系的可能性较小，所以，如果可能对语义产生任何疑问，那么最好是使用具名函数，这样代码会更清晰。</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"'>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"'>有许多高度专用的程序库（例如：</span><span
lang=EN-US> parser generators</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>和正则表达式引擎），它们为操作符定义了领域相关的约定，与</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"'>的含义大不相同（例如：正则表达式引擎可能会用</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"'>来表示“零个或更多”）。最好是找一个其它的方法，避免使用不符合惯例的操作符重载（例如：</span><span
lang=EN-US>[C++TR104]</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的正则表达使用字符串，这样就可以很自然地使用</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"'>而不用重载操作符）。如果经过深思熟虑你还是选择使用操作符，那么不仅要确保为你的约定定义一个一致的框架，而且要确保不会踩到内建操作符的脚趾。</span></p>
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