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<h3><span lang=EN-US>90. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>避免类型选择（</span><span lang=EN-US>type
switching</span><span style='font-family:宋体;mso-ascii-font-family:Arial;
mso-hansi-font-family:Arial'>）；尽量使用多态。</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 lang=EN-US>Switch off</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>（不要用</span><span
lang=EN-US>switch</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>）：避免通过对象类型选择来定制行为。用模板或虚函数，这样就可以让类型（而不是调用方的代码）来决定自己的行为。</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"'>通过类型选择来定制行为是脆弱的，容易出错，同时是一个清晰的迹象，表明正在</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"'>中写</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"'>或</span><span lang=EN-US>Fortran</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>代码。这是一种生硬的技术，迫使你在添加新特性时回头对既有的代码做外科手术。这也不安全，因为在添加新的类型时，编译器不会告诉你是否忘了修改所有的</span><span
lang=EN-US>switch</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><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>最理想的情况是，给程序添加新特性就等于添加新代码（参见第</span><span
lang=EN-US>37</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。实际上，我们知道事实并不总是这样——除了添加新代码，我们经常还需要回头修改一些既有代码。但是，我们不希望修改已经正常工作的代码，并要尽量避免这样做，这有两个原因：第一，这可能会破坏已有的功能；第二，它无法很好地适应变化，因为随着系统的演化，更多的特性被加入，你需要回头修改的“维护绳结”也随之增长。开放</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"'>封闭原则正是基于以上观察结果而产生的：一个实体（例如：类或模块）应该对扩充是开放的，但对修改则是封闭的（参见</span><span
lang=EN-US>[Martin96c]</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>[Meyer00]</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><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>那么我们怎么才能编写出既易于扩充又无需修改的代码呢？使用多态来描述抽象（参见第</span><span
lang=EN-US>36</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条），然后在添加功能时给这些抽象添加不同的实现。模板和虚函数调用隔离了使用抽象的代码和实现抽象的代码间存在的依赖性。</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，以这种方式管理依赖性取决于是否能找到正确的抽象。如果抽象不够完美，那么添加新功能时会需要修改接口（而不仅仅是添加接口的新的实现），而这通常都需要修改既有代码。但是抽象之所以被称为“抽象”是因为它们应该比“细节”要稳定得多，而“细节”则是该抽象的可能实现。</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"'>对比一下，如果代码很少使用抽象或不使用抽象，那么它会直接与具体的类型以及它们特有的操作打交道。这样的代码已经够“细节”了——事实上，它漂浮在细节的海洋中，并注定很快就会被淹没。</span></p>
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