📄 c++的多态性实现机制剖析.htm
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"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"'>方法。这个时候，就该轮到虚函数登场了。</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"'>前面输出的结果是因为编译器在编译的时候，就已经确定了对象调用的函数的地址，要解决这个问题就要使用迟绑定（</span><span
lang=EN-US>late binding</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>）技术。当编译器使用迟绑定时，就会在运行时再去确定对象的类型以及正确的调用函数。而要让编译器采用迟绑定，就要在基类中声明函数时使用</span><span
lang=EN-US>virtual</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>关键字（注意，这是必须的，很多学员就是因为没有使用虚函数而写出很多错误的例子），这样的函数我们称为虚函数。一旦某个函数在基类中声明为</span><span
lang=EN-US>virtual</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，那么在所有的派生类中该函数都是</span><span
lang=EN-US>virtual</span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，而不需要再显式地声明为</span><span
lang=EN-US>virtual</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:21.0pt;line-height:150%'><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>下面修改例</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"'>的代码，将</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"'>类中的</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"'>函数声明为</span><span lang=EN-US>virtual</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>，如下：</span></p>
<div style='mso-element:para-border-div;border:none;border-bottom:solid windowtext 1.5pt;
padding:0cm 0cm 1.0pt 0cm'>
<p class=MsoCaption align=center style='text-align:center;border:none;
mso-border-bottom-alt:solid windowtext 1.5pt;padding:0cm;mso-padding-alt:0cm 0cm 1.0pt 0cm'><span
style='font-family:黑体;mso-ascii-font-family:Arial'>例</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'> </span>SEQ </span><span style='font-family:黑体;
mso-ascii-font-family:Arial'>例</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'>2</span></span><!--[if supportFields]><span
lang=EN-US><span style='mso-element:field-end'></span></span><![endif]--></p>
</div>
<p class=MsoNormal><span lang=EN-US>#include <<span class=SpellE>iostream.h</span>></span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>class</span></span><span
lang=EN-US> animal</span></p>
<p class=MsoNormal><span lang=EN-US>{</span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>public</span></span><span
lang=EN-US>:</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=GramE>void</span> sleep()</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span>{</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:2'> </span><span
class=SpellE><span class=GramE>cout</span></span><<"animal
sleep"<<<span class=SpellE>endl</span>;</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span>}</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=GramE><b style='mso-bidi-font-weight:normal'>virtual</b></span><b
style='mso-bidi-font-weight:normal'> </b>void breathe()</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span>{</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:2'> </span><span
class=SpellE><span class=GramE>cout</span></span><<"animal
breathe"<<<span class=SpellE>endl</span>;</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span>}</span></p>
<p class=MsoNormal><span lang=EN-US>};</span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>class</span></span><span
lang=EN-US> <span class=SpellE>fish:public</span> animal</span></p>
<p class=MsoNormal><span lang=EN-US>{</span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>public</span></span><span
lang=EN-US>:</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=GramE>void</span> breathe()</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span>{</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:2'> </span><span
class=SpellE><span class=GramE>cout</span></span><<"fish
bubble"<<<span class=SpellE>endl</span>;</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span>}</span></p>
<p class=MsoNormal><span lang=EN-US>};</span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>void</span></span><span
lang=EN-US> main()</span></p>
<p class=MsoNormal><span lang=EN-US>{</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=GramE>fish</span> <span class=SpellE>fh</span>;</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=GramE>animal</span> *<span class=SpellE>pAn</span>=&<span
class=SpellE>fh</span>;</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:1'> </span><span
class=SpellE><span class=GramE>pAn</span></span>->breathe();</span></p>
<div style='mso-element:para-border-div;border:none;border-bottom:solid windowtext 1.5pt;
padding:0cm 0cm 1.0pt 0cm'>
<p class=MsoNormal style='border:none;mso-border-bottom-alt:solid windowtext 1.5pt;
padding:0cm;mso-padding-alt:0cm 0cm 1.0pt 0cm'><span lang=EN-US>}</span></p>
</div>
<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"'>大家可以再次运行这个程序，你会发现结果是“</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"'>”，也就是根据对象的类型调用了正确的函数。</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"'>那么当我们将</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"'>声明为</span><span lang=EN-US>virtual</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>时，在背后发生了什么呢？</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"'>编译器在编译的时候，发现</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"'>类中有虚函数，此时编译器会为每个包含虚函数的类创建一个虚表（即</span><span class=SpellE><span
lang=EN-US>vtable</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），该表是一个一维数组，在这个数组中存放每个虚函数的地址。<span
class=GramE>对于例</span></span><span lang=EN-US>1-2</span><span style='font-family:
宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>的程序，</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"'>和</span><span lang=EN-US>fish</span><span
class=GramE><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>类都包含</span></span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>了一个虚函数</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"'>，因此编译器会为这<span class=GramE>两个类都建立</span>一个虚表，如下图所示：</span></p>
<p class=MsoNormal style='text-indent:21.0pt;line-height:150%'><span
lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='text-indent:21.0pt;line-height:150%'><!--[if gte vml 1]><v:rect
id="_x0000_s1081" style='position:absolute;left:0;text-align:left;
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z-index:7'>
<v:textbox style='mso-next-textbox:#_x0000_s1081'>
<![if !mso]>
<table cellpadding=0 cellspacing=0 width="100%">
<tr>
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<div>
<p class=MsoNormal><span lang=EN-US>&<span class=SpellE>animal:<span
class=GramE>:breathe</span></span>()</span></p>
</div>
<![if !mso]></td>
</tr>
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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"'>类的</span><span
class=SpellE><span lang=EN-US>vtable</span></span></p>
</div>
<![if !mso]></td>
</tr>
</table>
<![endif]></v:textbox>
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<div>
<p class=MsoNormal><span class=SpellE><span lang=EN-US>animal::<span
class=GramE>breathe</span></span></span><span class=GramE><span lang=EN-US>()</span></span></p>
</div>
<![if !mso]></td>
</tr>
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<p class=MsoNormal><span lang=EN-US>&<span class=SpellE>fish:<span
class=GramE>:breathe</span></span>()</span></p>
</div>
<![if !mso]></td>
</tr>
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<p class=MsoNormal><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
class=SpellE><span lang=EN-US>vtable</span></span></p>
</div>
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