面向对象c++友元使用

#include<iostream.h>
class bbank;
class gbank;
class Cbank
{
private:
double balance1;
public:
Cbank(double m)
{
balance1=m;
}
friend double total(Cbank &p1,bbank &p2,gbank &p3);
};
class bbank
{
private:
double balance2;
public:
   bbank(double p)
{
balance2=p;
}
friend double total(Cbank &p1,bbank &p2,gbank &p3);
};
class gbank
{
private:
double balance3;
public: