如何停止隐式转换为虚函数

struct A{
    virtual void fun(){cout<<"A";}
};
struct B:public A{
    void fun(){cout<<"B";}
};
struct C:public B{
    void fun(){cout<<"C";}
};
int main() 
{
    C c;B b1;   
    A *a=&b1;
    a->fun(); //1
    B *b=&c;
    b->fun(); //2
    return 0;
}
struct A{
    virtual void fun(){cout<<"A";}
};