[C++ rudiment]多重继承中二义性错误的解决：虚基类

C++中允许多重继承，看以下例子：

#include <iostream>
using namespace std;
class A
class B:public A
class C:public A
class D:public B,public C
int main()
#include <iostream>
using namespace std;
class A
class B:public virtual A //A是B的虚基类
class C:public virtual A //A是C的虚基类
class D:public B,public C
int main()
{
D d;
d.a = 100;
return 0;
}