1.友元可以偽裝成成員函數...為了訪問數據成員.
#include "stdafx.h"

class Fish
{
public:	
    void Setfloat1( Fish &fish1);
	friend void Setfloat3(Fish &fish3);
	  void Setfloat4(int CCDeath);
private:
	int m_s;
};

void Fish::Setfloat1(Fish &fish1)
{
	fish1.m_s=100;
	cout<<fish1.m_s<<endl;
}
void Setfloat2(Fish &fish2)//肯定失敗...
{
//	fish2.m_s=100;
//	cout<<fish2.m_s<<endl;
}
void Setfloat3(Fish &fish3)//Setfoat只要是Fish成員函數或友元函數就對數據成員進行操控...
{
	fish3.m_s=100;
	cout<<fish3.m_s<<endl;
}
void Fish::Setfloat4(int CCDeath)
{
	m_s=CCDeath;
	cout<<m_s<<endl;
}
void main()
{
	Fish fish;	
	fish.Setfloat1(fish);
//	Setfloat2(fish);
	Setfloat3(fish);	
	fish.Setfloat4(5);
}
2.具有相同友元的了兩個類....
#include "stdafx.h"
//Show(): 顯示日期--class Day  ;  顯示時間  class Time

class Time;//記得
class Day
{
public:
	Day(int year,int month,int day);
	friend void  ShowDayTime(const Day &day,const Time &time);
private:
	int m_year,m_month,m_day;
};
Day::Day(int year,int month,int day)
{
	m_year=year;
	m_month=month;
	m_day=day;
}
class Time
{
public:
	Time(int hour,int min,int second);
	friend void  ShowDayTime(const Day &day,const Time &time);
private:
	int m_hour,m_min,m_second;
};
Time::Time(int hour,int min,int second)
{
	m_hour=hour;
	m_min=min;
	m_second=second;
}
void  ShowDayTime(const Day &day,const Time &time)
{
	cout<<"日期:"<<day.m_year<<"-"<<day.m_month<<"-"<<day.m_day<<endl;
	cout<<"時間:"<<time.m_hour<<"-"<<time.m_min<<"-"<<time.m_second<<endl;
}
void main()
{
	Time wo(15,44,56);
	Day  ni(2007,11,9);
	ShowDayTime(ni,wo);
}
3.類的成員函數可以做另外一個類友元......
#include "stdafx.h"
//Show(): 顯示日期--class Day  ;  顯示時間  class Time

class Time;//記得
class Day
{
public:
	Day(int year,int month,int day);
	void  ShowDayTime(const Day &day,const Time &time);
private:
	int m_year,m_month,m_day;
};
Day::Day(int year,int month,int day)
{
	m_year=year;
	m_month=month;
	m_day=day;
}
class Time
{
public:
	Time(int hour,int min,int second);
	friend void  Day::ShowDayTime(const Day &day,const Time &time);
private:
	int m_hour,m_min,m_second;
};
Time::Time(int hour,int min,int second)
{
	m_hour=hour;
	m_min=min;
	m_second=second;
}
void  Day::ShowDayTime(const Day &day,const Time &time)
{
	cout<<"日期:"<<day.m_year<<"-"<<day.m_month<<"-"<<day.m_day<<endl;
	cout<<"時間:"<<time.m_hour<<"-"<<time.m_min<<"-"<<time.m_second<<endl;
}
void main()
{
	Time wo(15,44,56);
	Day  ni(2007,11,9);
	ni.ShowDayTime(ni,wo);
}