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：