subject_45701.htm

一些关于vc的问答

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序号：45701 发表者：罗兹维尔 发表日期：2003-07-02 23:13:05
<br>主题：虚析构的疑惑
<br>内容：#include "iostream.h"<BR><BR>class Base<BR>{<BR>public:<BR>&nbsp;&nbsp;&nbsp;&nbsp;~Base(){ cout&lt;&lt;"~Base"&lt;&lt;endl; }<BR>};<BR><BR>class Derived:public Base<BR>{<BR>public:<BR>&nbsp;&nbsp;&nbsp;&nbsp;~Derived(){ cout&lt;&lt;"~Derived"&lt;&lt;endl; }<BR>};<BR><BR>void main()<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;Derived D;<BR>}<BR><BR>输出为<BR>~Derived<BR>~Base<BR><BR>这样与析构声明为virtual有什么区别呢？//修饰为virtual之后结果也事，但什么情况下virtual的虚函数有价值？<BR>为什么会有这样的输出
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回复者：_DAWN_ 回复日期：2003-07-03 07:00:19
<br>内容：在定义接口时会很有价值，就是说定义纯虚的基类.
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回复者：Neil Gan 回复日期：2003-07-03 10:00:51
<br>内容：Base *pCBase=new Derived;<BR><BR>delete pCBase; //when virtual destructor, call Derived::~Derived<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; //when not virtual destructor, call Base::~Base
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<font color=red>答案被接受</font><br>回复者：非常可乐(用VC的老头) 回复日期：2003-07-03 11:29:20
<br>内容：补充楼上的<BR>虚析构函数<BR><BR>在析构函数前面加上关键字virtual进行说明，称该析构函数为虚析构函数。例如：<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;class B<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;virtual ~B();<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;…<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;};<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;该类中的析构函数就是一个虚析构函数。<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;如果一个基类的析构函数被说明为虚析构函数，则它的派生类中的析构函数也是虚析构函数，不管它是否使用了关键字virtual进行说明。<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;说明虚析构函数的目的在于在使用delete运算符删除一个对象时，能保析构函数被正确地执行。因为设置虚析构函数后，可以采用动态联编方式选择析构函数。<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;下面举一个用虚析构函数的例子。<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;#include &lt;iostream.h&gt;<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;class A<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;public:<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;virtual ~A() { cout&lt;&lt;"A::~A() Called.\n"; }<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;};<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;class B : public A<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;public:<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;B(int i) { buf = new char[i]; }<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;virtual ~B()<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;delete [] buf;<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cout&lt;&lt;"B::~B() Called.\n";<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;private:<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;char * buf;<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;};<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;void fun(A *a)<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;delete a;<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;void main()<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;A *a = new B(15);<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;fun(a);<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;执行该程序输出如下结果：<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;B::~B() Called.<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;A::~A() Called.<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;如果类A中的析构函数不用虚函数，则输出结果如下：<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;A::~A() Called.<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;当说明基类的析构函数是虚函数时，调用fun(a)函数，执行下述语句：<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;delete a;<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;由于执行delete语句时自动调用析构函数，采用动态联编，调用它基类的析构函数，所以输出上述结果。<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;当不说明基类的析构函数为虚函数时，delete隐含着对析构函数的调用，故产生<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;A::~A() Called.<BR><BR> <BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;的结果。<BR><BR>
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