📄 c++的多态性实现机制剖析.htm
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<p class=MsoNormal><span class=SpellE><span lang=EN-US>fish::<span
class=GramE>breathe</span></span></span><span class=GramE><span lang=EN-US>()</span></span></p>
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</span><![endif]><span lang=EN-US><o:p> </o:p></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%'><span
lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal style='line-height:150%'><span lang=EN-US><o:p> </o:p></span></p>
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<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'> </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'>2</span></span><!--[if supportFields]><span
lang=EN-US><span style='mso-element:field-end'></span></span><![endif]--><span
lang=EN-US> animal</span><span style='font-family:黑体;mso-ascii-font-family:
Arial'>类和</span><span lang=EN-US>fish</span><span style='font-family:黑体;
mso-ascii-font-family:Arial'>类的虚表</span></p>
<p class=MsoNormal style='line-height:150%'><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"'>那么如何定位虚表呢？编译器另外还为每个类的对象提供了一个虚表指针（即</span><span class=SpellE><span
lang=EN-US>vptr</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>），这个指针指向了对象所属类的虚表。在程序运行时，根据对象的类型去初始化</span><span
class=SpellE><span lang=EN-US>vptr</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>，从而让</span><span
class=SpellE><span lang=EN-US>vptr</span></span><span style='font-family:宋体;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>正确的指向所属类的虚表，从而在调用虚函数时，就能够找到正确的函数。<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，由于</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"'>实际指向的对象类型是</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"'>，因此</span><span class=SpellE><span
lang=EN-US>vptr</span></span><span style='font-family:宋体;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"'>指向的</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"'>类的</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"'>，当调用</span><span
class=SpellE><span lang=EN-US>pAn</span></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"'>时，根据虚表中的函数地址找到的就是</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</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='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"'>正是由于每个对象调用的虚函数都是通过虚表指针来索引的，也就决定了虚表指针的正确初始化是非常重要的。换句话说，在虚表指针没有正确初始化之前，我们不能够去调用虚函数。那么虚表指针在什么时候，或者说在什么地方初始化呢？</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"'>答案是在构造函数中进行虚表的创建和虚表指针的初始化。还记得构造函数的调用顺序吗，在构造子<span
class=GramE>类对象</span>时，要先<span class=GramE>调用父类的</span>构造函数，此时编译器只“看到了”父类，并不知道后面是否后还有继承者，它初始化<span
class=GramE>父类对象</span>的虚表指针，该虚表指针<span class=GramE>指向父类的</span>虚表。当执行子类的构造函数时，子<span
class=GramE>类对象</span>的虚表指针被初始化，指向自身的虚表。<span class=GramE>对于例</span></span><span
lang=EN-US>2-2</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>的程序来说，当</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 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"'>对象构造完毕后，其内部的虚表指针也就被初始化为指向</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"'>类的虚表。在类型转换后，调用</span><span class=SpellE><span lang=EN-US>pAn</span></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"'>，由于</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，该对象内部的虚表指针指向的是</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"'>类的虚表，因此最终调用的是</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</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='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"'>要注意：对于虚函数调用来说，每一个对象内部都有一个虚表指针，该虚表指针被初始化为本类的虚表。所以在程序中，不管你的对象类型如何转换，但该对象内部的虚表指针是固定的，所以呢，才能实现动态的对象函数调用，这就是</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"'>多态性实现的原理。</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"'>总结（<span class=GramE>基类有</span>虚函数）：</span></p>
<p class=MsoNormal style='margin-left:39.0pt;text-indent:-18.0pt;line-height:
150%;mso-list:l1 level1 lfo3;tab-stops:list 39.0pt'><![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"'> </span></span></span><![endif]><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>每一个类都有虚表。</span></p>
<p class=MsoNormal style='margin-left:39.0pt;text-indent:-18.0pt;line-height:
150%;mso-list:l1 level1 lfo3;tab-stops:list 39.0pt'><![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"'> </span></span></span><![endif]><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>虚表可以继承，如果子<span class=GramE>类没有</span>重写虚函数，那么子类虚表中仍然会有该函数的地址，只不过这个地址指向的<span
class=GramE>是基类的</span>虚函数实现。<span class=GramE>如果基类<span lang=EN-US
style='font-family:"Times New Roman"'>3</span>个</span>虚函数，<span class=GramE>那么基类的</span>虚表中就有三项（虚函数地址），派生类也会有虚表，至少有三项，如果重写了相应的虚函数，那么虚表中的地址就会改变，指向自身的虚函数实现。如果派生类有自己的虚函数，那么虚表中就会添加该项。</span></p>
<p class=MsoNormal style='margin-left:39.0pt;text-indent:-18.0pt;line-height:
150%;mso-list:l1 level1 lfo3;tab-stops:list 39.0pt'><![if !supportLists]><span
lang=EN-US style='mso-fareast-font-family:"Times New Roman"'><span
style='mso-list:Ignore'>3、<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>派生类的虚表中虚函数地址的排列顺序和<span class=GramE>基类</span>的虚表中虚函数地址排列顺序相同。</span></p>
<h1 style='margin-left:21.25pt;text-indent:-21.25pt;mso-list:l0 level1 lfo1;
tab-stops:list 21.25pt'><![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"'>
</span></span></span><![endif]><span lang=EN-US>VC</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>视频第三课</span><span lang=EN-US>this</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>指针说明</span></h1>
<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"'>我在论坛的</span><span lang=EN-US>VC</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>教学视频版面发了帖子，是模拟</span><span lang=EN-US>MFC</span><span
style='font-family:宋体;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"'>类库的例子写的，主要是说明<span class=GramE>在基类的</span>构造函数中保存的</span><span
lang=EN-US>this</span><span style='font-family:宋体;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"'>指针是指向子类的，我们在看一下这个例子：</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'>3</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 lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>class</span></span><span
lang=EN-US> base;</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>base</span></span><span
lang=EN-US> * <span class=SpellE>pbase</span>;</span></p>
<p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p>
<p class=MsoNormal><span class=GramE><span lang=EN-US>class</span></span><span
lang=EN-US> base</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>base()</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:2'> </span><span
class=SpellE><span class=GramE>pbase</span></span><span class=GramE>=</span>this;</span></p>
<p class=MsoNormal><span lang=EN-US><span style='mso-tab-count:2'> </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>virtual</span> void fn()</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'> &nbs⌨️ 快捷键说明
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