运算符重载

#include<iostream.h>

class RMB{           //人民币类
public:
  RMB(double d){ yuan=d; jf=(d-yuan)/100; }
  RMB interest(double rate);          //计算利息
  RMB add(RMB d);                     //人民币加
  void display(){ cout <<(yuan + jf / 100.0) << endl; }
  RMB operator+(RMB d){ return RMB(yuan+d.yuan+(jf+d.jf)/100); }
                                      //人民币加的运算符重载
  RMB operator*(double rate){ return RMB((yuan+jf/100)*rate);}
private:
  unsigned int yuan;                  //元
  unsigned int jf;                    //角分
};

RMB RMB::interest(double rate)
{
  return RMB((yuan + jf / 100.0) * rate);
}
RMB RMB::add(RMB d)
{
  return RMB(yuan + d.yuan + jf / 100.0 + d.jf / 100.0);
}

//以下是计算应付人民币的两个版本
RMB expense1(RMB principle, double rate)
{
  RMB interest = principle.interest(rate);
  return principle.add(interest);
}

RMB expense2(RMB principle, double rate)
{
  RMB interest = principle * rate;    //本金乘利息
  return principle + interest;        //连本带利
}

void main()
{
  RMB x = 10000.0;
  double yrate = 0.035;
  expense1(x,yrate).display();
  expense2(x,yrate).display();
}
运行结果：
10350
10350
在上述程序中，输出结果为何不是double型的？