虚继承

class person
{
public:
int a;
};

class son1: virtual public person
{

};

class son2: virtual public person
{

};

class son: public son1,public son2
{

};

sizeof(虚基类) = 4

sizeof(son1) = 4+4（vbptr指针）

sizeof(son2) = 4+4（vbptr指针）

sizeof(son) = 4+4（son1vbptr指针）+4（son2vbptr指针）