c++的多态性实现机制剖析.htm

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"Times New Roman"'>，接着定义了一个指向</span><span lang=EN-US>animal</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类的指针变量</span><span class=SpellE><span lang=EN-US>pAn</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，将</span><span class=SpellE><span lang=EN-US>fh</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的地址赋给了指针变量</span><span class=SpellE><span lang=EN-US>pAn</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，然后利用该变量调用</span><span class=SpellE><span lang=EN-US>pAn</span></span><span
lang=EN-US>-&gt;breathe()</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>。许多学员往往将这种情况和</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"'>的多态性搞混淆，认为</span><span class=SpellE><span
lang=EN-US>fh</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>实际上是</span><span
lang=EN-US>fish</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，应该是调用</span><span lang=EN-US>fish</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>breathe()</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，输出“</span><span lang=EN-US>fish bubble</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>”，然后结果却不是这样。下面我们从两个方面来讲述原因。</span></p>

<p class=MsoNormal style='margin-left:18.0pt;text-indent:-18.0pt;line-height:
150%;mso-list:l2 level1 lfo2;tab-stops:list 18.0pt'><![if !supportLists]><span
lang=EN-US style='mso-fareast-font-family:"Times New Roman"'><span
style='mso-list:Ignore'>1、<span style='font:7.0pt "Times New Roman"'>&nbsp; </span></span></span><![endif]><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>编译的角度</span></p>

<p class=MsoNormal style='margin-left:17.85pt;line-height:150%'><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"'>编译器在编译的时候，要确定每个对象调用的函数的地址，这称为早期绑定（</span><span
lang=EN-US>early binding</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），当我们将</span><span
lang=EN-US>fish</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</span><span class=SpellE><span
lang=EN-US>fh</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的地址赋给</span><span
class=SpellE><span lang=EN-US>pAn</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>时，</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"'>编译器进行了类型转换，此时</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"'>编译器认为变量</span><span class=SpellE><span lang=EN-US>pAn</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>保存的就是</span><span lang=EN-US>animal</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对象的地址。当在</span><span lang=EN-US>main()</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>函数中执行</span><span class=SpellE><span lang=EN-US>pAn</span></span><span
lang=EN-US>-&gt;breathe()</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>时，调用的当然就是</span><span
lang=EN-US>animal</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>对象的</span><span lang=EN-US>breathe</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>函数。</span></p>

<p class=MsoNormal style='margin-left:18.0pt;text-indent:-18.0pt;line-height:
150%;mso-list:l2 level1 lfo2;tab-stops:list 18.0pt'><![if !supportLists]><span
lang=EN-US style='mso-fareast-font-family:"Times New Roman"'><span
style='mso-list:Ignore'>2、<span style='font:7.0pt "Times New Roman"'>&nbsp; </span></span></span><![endif]><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>内存模型的角度</span></p>

<p class=MsoNormal style='text-indent:17.85pt;line-height:150%'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>我们给出了</span><span lang=EN-US>fish</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对象内存模型，如下图所示：</span></p>

<p class=MsoNormal style='line-height:150%'><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

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    <p class=MsoNormal><span lang=EN-US>animal</span><span style='font-family:
    宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的对象所占内存</span></p>
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</span><![endif]><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoCaption><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoCaption><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<br style='mso-ignore:vglayout' clear=ALL>

<p class=MsoCaption align=center style='text-align:center'><span
style='font-family:黑体;mso-ascii-font-family:Arial'>图</span><span lang=EN-US>1- </span><!--[if supportFields]><span
lang=EN-US><span style='mso-element:field-begin'></span><span
style='mso-spacerun:yes'>&nbsp;</span>SEQ </span><span style='font-family:黑体;
mso-ascii-font-family:Arial'>图</span><span lang=EN-US>1- \* ARABIC <span
style='mso-element:field-separator'></span></span><![endif]--><span lang=EN-US><span
style='mso-no-proof:yes'>1</span></span><!--[if supportFields]><span
lang=EN-US><span style='mso-element:field-end'></span></span><![endif]--><span
lang=EN-US> fish</span><span class=GramE><span style='font-family:黑体;
mso-ascii-font-family:Arial'>类对象</span></span><span style='font-family:黑体;
mso-ascii-font-family:Arial'>的内存模型</span></p>

<p class=MsoNormal><span lang=EN-US><o:p>&nbsp;</o:p></span></p>

<p class=MsoNormal style='text-indent:21.15pt;line-height:150%'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>我们构造</span><span lang=EN-US>fish</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类的对象时，首先要调用</span><span lang=EN-US>animal</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类的构造函数去构造</span><span lang=EN-US>animal</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，然后才调用</span><span lang=EN-US>fish</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类的构造函数完成自身部分的构造，从而拼接出一个完整的</span><span lang=EN-US>fish</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>对象。当我们将</span><span lang=EN-US>fish</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类的对象转换为</span><span lang=EN-US>animal</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类型时，该对象就被认为是原对象整个内存模型的上半部分，也就是图</span><span lang=EN-US>1-1</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>中的“</span><span lang=EN-US>animal</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的对象所占内存”。那么当我们利用类型转换后的对象指针去调用它的方法时，当然也就是调用它所在的内存中的方法。因此，输出</span><span
lang=EN-US>animal breathe</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，也就顺理成章了。</span></p>

<p class=MsoNormal style='text-indent:21.0pt;line-height:150%'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>正如很多学员所想，<span class=GramE>在例</span></span><span lang=EN-US>1-1</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>的程序中，我们知道</span><span class=SpellE><span lang=EN-US>pAn</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>实际指向的是</span><span lang=EN-US>fish</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，我们希望输出的结果是鱼的呼吸方法，即调用</span><span lang=EN-US>fish</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family: