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<div class=Section1>
<h3><span lang=EN-US>100. </span><span style='font-family:宋体;mso-ascii-font-family:
Arial;mso-hansi-font-family:Arial'>不要以多态方式处理数组</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"'>数组的排列方式并不如人愿：以多态方式处理数组是一个严重的类型错误，而编译器可能对此保持沉默。不要落入这个陷阱。</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"'>指针同时适用于两种情况：一种情况是作为别名（对象的小型标识），另一种情况则是作为数组迭代器（对指针进行运算来遍历数组）。作为别名，把指向</span><span
lang=EN-US>Derived</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的指针作为指向</span><span
lang=EN-US>Base</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的指针处理是完全正确的。但是，一旦把指针用作数组的迭代器，这样的替换就不成立了，因为</span><span
lang=EN-US>Derived</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的数组与</span><span
lang=EN-US>Base</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的数组并不相同。举个例子：老鼠和大象都是哺乳动物，但是这并不意味着护送一千只大象的时间就和护送一千只老鼠的时间一样长。</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"'>大小很重要。在把一个指向</span><span lang=EN-US>Derived</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的指针替换为指向</span><span lang=EN-US>Base</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的指针时，编译器很清楚地知道如何调整指针（如果需要的话），因为它有这两个类的足够的信息。但是，在对指向</span><span
lang=EN-US>Base</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的指针进行运算时，编译器对</span><span lang=EN-US>p[n]</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>执行的运算是</span><span lang=EN-US>*(p + n * <span class=SpellE>sizeof</span>(Base))</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，这就等于假定内存中的都是</span><span lang=EN-US>Base</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对象，而不是一些大小可能与之不同的派生类的对象。现在试想，如果把指向</span><span lang=EN-US>Derived</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>数组的指针转型为</span><span lang=EN-US>Base*</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>（编译器默许这样做），然后对该指针进行指针运算（编译器对此连眼皮都不会眨一下），那么要把数组完全破坏是多么轻而易举的事啊！</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"'>此类意外是由不当的相互替换造成的，因为</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"'>语言规定指向派生类的指针可以用作指向基类的指针，而</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"'>遗留的指针运算则假定指针不是多态的并完全根据静态信息来计算数组元素的跨度。</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"'>要在数组中存储多态对象，需要用一个基类指针（例如：原始指针，或</span><span
class=SpellE><span lang=EN-US>shared_ptr</span></span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>也不错，参见第</span><span
lang=EN-US>79</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）的数组（或用一个真正的容器也不错，参见第</span><span
lang=EN-US>77</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>条）。然后数组中的每个指针指向一个多态对象，对象很可能是从自由存储区中分配的。或者，如果想要给一个包含多态对象的容器提供接口，那么需要把整个数组进行封装，并为迭代操作提供一个多态的接口。</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"'>顺便说一下，在接口中最好使用引用而不要用指针，原因在于这样能够清楚地表明正在讨论的是一个对象，而不是一个对象数组。</span></p>
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