下列程序的输出结果是 【15】 。#include ＜iostream＞using namespace std;class A {int a;public:A():a(9){}virtual void print() const { cout＜＜a; };};class B: public A {char b;public:B(){b=’S’;}void print() const {cout ＜＜b;}};void show(A &x){ x. print();}int main(){A d1,*p;B d2;p=&d2;dl1.print();d2.print();p-＞print();show(d1);show(d2);return 0;}