虚基类的作用

当一个基类被声明为虚基类后，即使它成为了多继承链路上的公共基类，最后的派生类中也只有它的一个备份。例如：
class CBase { }；
class CDerive1：virtual public CBase{ }；
class CDerive2：virtual public CBase{ }；
class CDerive12：public CDerive1，CDerive2{ }；
则在类CDerive12的对象中，仅有类CBase的一个对象数据

虚基类的特点：

       虚基类构造函数的参数必须由最新派生出来的类负责初始化（即使不是直接继承）；
               虚基类的构造函数先于非虚基类的构造函数执行。

/**//************************************************************************

* 混合继承：多基类继承与多重继承

************************************************************************/

#include <IOSTREAM.H>

//基类

class CBase

...{

protected:

    int a;

public:

    CBase(int na)

    ...{

        a=na;

        cout<<"CBase constructor! ";

    }



    ~CBase()...{cout<<"CBase deconstructor! ";}

};



//派生类1(声明CBase为虚基类)

class CDerive1:virtual public CBase

...{

public:

    CDerive1(int na):CBase(na)

    ...{

        cout<<"CDerive1 constructor! ";

    }

    

    ~CDerive1()...{cout<<"CDerive1 deconstructor! ";}



    int GetA()...{return a;}

};



//派生类2(声明CBase为虚基类)

class CDerive2:virtual public CBase

...{

public:

    CDerive2(int na):CBase(na)

    ...{

        cout<<"CDerive2 constructor! ";

    }

    ~CDerive2()...{cout<<"CDerive2 deconstructor! ";}

    int GetA()...{return a;}

};



//子派生类

class CDerive12:public CDerive1,public CDerive2

...{

public:

    CDerive12(int na1,int na2,int na3):CDerive1(na1),CDerive2(na2),CBase(na3)

    ...{

        cout<<"CDerive12 constructor! ";

    }

    ~CDerive12()...{cout<<"CDerive12 deconstructor! ";}

};

void main()

...{

    CDerive12 obj(100,200,300);

    //得到从CDerive1继承的值

    cout<<" from CDerive1 : a = "<<obj.CDerive1::GetA();

    //得到从CDerive2继承的值

    cout<<" from CDerive2 : a = "<<obj.CDerive2::GetA()<<endl<<endl;

}

运行结果：