驀然回首，那人卻在燈火闌珊處

#include <iostream>
using namespace std;
class A
{
 friend double count(A&);
public:
 A(double t, double r):total(t),rate(r){}
 private:
 double total;
 double rate;
};
double count(A&a)
{
 a.total+=a.rate*a.total;
 return a.total;
}
int main(void )
{
 A x(100,0.5),y(50,0.1);

 cout<<count(x)<<"  "<<count(y)<<endl;
 cout<<count(x) <<"\n";
    return 0;
}

這段代碼是什么意思？？