static_cast疑问

#include <iostream>
class A
{
public:
    A()
    {
        std::cout << "n A_Constructor t" << this <<std::endl;
    }
    void A_Method()
    {
        std::cout <<"n A_Method t" << this <<std::endl;
    }
};
class B:public A
{
public:
    B()
    {
        std::cout <<"n B_Constructor n";
    }
    void B_Method()
    {
        std::cout <<"n B_Method t" << this <<std::endl;
    }
};

int main()
{
    A *a_obj = new A;
    B *b_obj = static_cast<B*> (a_obj);  // This isn't safe.
    b_obj->B_Method();      
    getchar();
    return 0;
}

static_cast

B::B()

b_obj

B::B()

class B