Can anyone help me for 64bit os i am expecting answer for this below code is 28 but showing result 32?

//size is showing 32 bytes
class B1 { public: virtual ~B1() {} void f0() {} virtual void f1() {} int int_in_b1; };

        class B2 {
        public:
          virtual ~B2() {}
          virtual void f2() {}
          int int_in_b2;
        };
        class D : public B1, public B2 {
        public:
          void d() {}
          void f2() {}  // override B2::f2()
          int int_in_d;
        };

        int main()
        {
                B2 *b2 = new B2();
                D  *d  = new D();
                D d1;

                cout<<"sizeof :"<<sizeof(d1);
        }