虚拟继承和虚基类

#include <iostream>using namespace std;class A{public:    A()    {        cout<< "this is A constructor"<<endl;    }    //needly    virtual ~A()    {        cout<<"this is A destructor"<<endl;    }    void Speak()    {        cout<<"this is A speak"<<endl;    }};class B1 : public virtual A{public:    B1()    {        cout<<"this is B1 constructor"<<endl;    }    ~B1()    {        cout<<"this is B1 destructor"<<endl;    }};class B2 : public virtual A{public:    B2()    {        cout<<"this is B2 constructor"<<endl;    }    ~B2()    {        cout<<"this is B2 destructor"<<endl;    }};class C: public B1,public B2{public:    C()    {        cout<<"this is C constructor"<<endl;    }    ~C()    {        cout<<"this is C destructor"<<endl;    }};int main(){    C *p = new C();    p->Speak();    delete p;    p = NULL;    return 0;}

执行结果：
this is A constructor
this is B1 constructor
this is B2 constructor
this is C constructor
this is A speak
this is C destructor
this is B2 destructor
this is B1 destructor
this is A destructor

虚拟继承即在继承时再基类前面加一个virtual关键字，而虚基类便是A。
通过虚继承我们可以解决三个问题：
存储问题：因为当非虚继承时，类C的内存中，将会产生两份类A的拷贝。
效率问题：非虚继承时，基类的构造（析构）函数被调用两次。
编译问题：调用基类的成员函数时会产生二义性，如这里的Speak（），当非虚继承时，B1跟B2均会继承A的Speak（）成员函数，那么当子类C调用时将产生二义性，虽然可用：：指定，但这不符合设计原则跟开发原则。