关于虚继承和析构函数的一个奇怪的问题

class A{
public :
int _a1;
int _b;
void virtual f(){};
};

class B:virtual public A{
public:

int _b;

~B(){}; //line1

void virtual f(){}; //line2

void virtual f2(){};
void virtual f3(){};
void virtual f4(){};

};
int main(){

printf("A: %d\n", sizeof(A));
printf("B: %d\n", sizeof(B));
return 0;
}