📄 csdn_文档中心_oo三步曲之浅析oo的基石.htm
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lang=EN-US> <o:p></o:p></SPAN></B></P>
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style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">多态，替换原则，对象切割</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><SPAN
style="mso-tab-count: 1">
</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">多态作为</SPAN><SPAN
lang=EN-US>OO</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中的核心机制之一拥有着丰富的内涵。顾名思义，多态就是一种名称多种形态的意思。其主要有三种形式：函数多态，对象变量多态，泛型多态。函数多态主要包括函数重载（</SPAN><SPAN
lang=EN-US>overload</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）和改写</SPAN><SPAN
lang=EN-US>(overriding)</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。泛型多态（</SPAN><SPAN
lang=EN-US>genericity</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）主要是提供了一种创建通用工具的方法，可以在特定的场合将其特化。在这里，我们重点要考量的是对象变量多态。在理解对象变量多态之前，我们首先了解一下</SPAN><SPAN
lang=EN-US>OO</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">核心机制之一的替换原则。静态类型的</SPAN><SPAN
lang=EN-US>OOPL</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的一个特征就是一个变量所包含的值（</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'">）的类型可能并不等于这个变量所声明的类型，在传统的编程语言中并不具备这样的特征，因为我们不可能把声明为整型的变量赋上字符串的变量值。而替换原则发生作用的情况就隐含的描叙了两种不同类型所具有的关联</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'">类型继承。</SPAN><SPAN
lang=EN-US>Barbara Liskov</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">曾经这样描叙替换原则以及起作用的类型之间的关联：对于类型为</SPAN><SPAN
lang=EN-US>S</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的每个对象</SPAN><SPAN
lang=EN-US>s</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，存在一个类型为</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'">的对象</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'">，对于根据类型</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'">所定义的所有程序</SPAN><SPAN
lang=EN-US>P</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，如果用对象</SPAN><SPAN
lang=EN-US>s</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">替换对象</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'">，程序</SPAN><SPAN
lang=EN-US>P</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的行为保持不变，那么类型</SPAN><SPAN
lang=EN-US>S</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">就是类型</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'">的子类型</SPAN><SPAN
lang=EN-US>[Liskov 1988]</SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US> <o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><SPAN
style="mso-tab-count: 1">
</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在理解了多态以及替换原则后，我们可以继续深入理解继承与替换原则相结合所带来的新的观点。可以说继承与替换原则的引入影响了几乎所有的</SPAN><SPAN
lang=EN-US>OOPL</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，包括类型系统，值语义</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'">引用语义，对象内存空间分配等等。下面，我将试图逐步的拨开其中的各种因果。</SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US> <o:p></o:p></SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><SPAN
style="mso-tab-count: 1">
</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">首先考虑</SPAN><SPAN
lang=EN-US>,people a; </SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">这样的代码在编译器中将如何实现？可以肯定是首先将把类型</SPAN><SPAN
lang=EN-US>people</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">绑定到对象</SPAN><SPAN
lang=EN-US>a</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">上，然后必须为对象</SPAN><SPAN
lang=EN-US>a</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">分配空间。同时，我们创建</SPAN><SPAN
lang=EN-US>people</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的子类</SPAN><SPAN
lang=EN-US>man</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，由于</SPAN><SPAN
lang=EN-US>man IS A people</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。根据多态以及替换原则，我们当然可以让对象</SPAN><SPAN
lang=EN-US>a</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">保存一个</SPAN><SPAN
lang=EN-US>man</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">类型的值（这就是替换原则的表现）。这是一种直观的描叙，但在编程语言的实现过程中就出现一些困难。我们知道继承是一种扩展接口与实现的方式，那么我们就很难保证</SPAN><SPAN
lang=EN-US>man</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">类型不对</SPAN><SPAN
lang=EN-US>people</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">类型做扩展，而一旦做出扩展，我们如何能用存储</SPAN><SPAN
lang=EN-US>people</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">对象的空间去存储</SPAN><SPAN
lang=EN-US>man</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">类型的对象值呢？</SPAN></P>
<P class=MsoNormal><SPAN lang=EN-US><SPAN
style="mso-tab-count: 1">
</SPAN>people a</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 style="TEXT-INDENT: 21.75pt"><SPAN lang=EN-US>man
b=new man();</SPAN></P>
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN
lang=EN-US>a=b;</SPAN></P>
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style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">这样的代码将首先把</SPAN><SPAN
lang=EN-US>b</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">对象进行切割，然后再存储到</SPAN><SPAN
lang=EN-US>a</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">对象空间去。然而这并不是我们所期望的。那么，为了支持</SPAN><SPAN
lang=EN-US>OOP</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的继承，多态，替换原则，但却需要避免对象切割的发生，面对对象</SPAN><SPAN
lang=EN-US>a</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">我们将采用何种分配空间模型呢？常用的有下面三种方式：</SPAN></P>
<P class=MsoNormal
style="MARGIN-LEFT: 39pt; TEXT-INDENT: -18pt; mso-list: l0 level1 lfo2; tab-stops: list 39.0pt"><SPAN
lang=EN-US>1，<SPAN style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">只为</SPAN><SPAN
lang=EN-US>a</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">分配基类</SPAN><SPAN
lang=EN-US>people</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的存储空间，不支持对象多态以及替换原则。这样的模型内存分配紧凑，存储效率很高。</SPAN></P>
<P class=MsoNormal
style="MARGIN-LEFT: 39pt; TEXT-INDENT: -18pt; mso-list: l0 level1 lfo2; tab-stops: list 39.0pt"><B><SPAN
lang=EN-US>2，<SPAN style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN></B><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">分配继承树中的最大类对象所需要空间（在这里是</SPAN><SPAN
lang=EN-US>man</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">类的对象值空间），这样的模型简单，同时可以实现多态和替换原则而避免对象切割问题，但是十分浪费内存空间十分明显。</SPAN><B><SPAN
lang=EN-US><o:p></o:p></SPAN></B></P>
<P class=MsoNormal
style="MARGIN-LEFT: 39pt; TEXT-INDENT: -18pt; mso-list: l0 level1 lfo2; tab-stops: list 39.0pt"><SPAN
lang=EN-US>3，<SPAN style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">只分配用于保存一个指针所需要的存储空间，在运行时通过堆来分配对象实际类型所需要的空间大小，这样也可以实现多态和替换原则而避免对象切割问题。（也就是说</SPAN><SPAN
lang=EN-US>a</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">只是一个对象的引用，而不是真实的对象，真实对象的生成必须靠程序员显式的声明）。</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'">对于上面提到的三种内存模型，</SPAN><SPAN
lang=EN-US>1</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>3</SPAN><SPAN
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