#ifndef TV_H_
#define TV_H_

/*一个类 电视 */
class Tv
{
public:
  friend class Remote; //Remote类可以访问Tv Privite 的私有部分
  enum {
    off,on  //开关 
  };
  enum 
  {
    MinVal,MaxVal=20  //音量
  };
  enum {
    Antena,Cable //使用的天线、还是电缆
  };
  enum 
  {
    TV ,DVD  //工作模式
  };

  Tv(int s = off, int mc = 125) :state(s), volume(5), maxchannel(mc),
    channel(5), mode(Cable), input(TV) {}
  void onoff() { state = (state == on) ? off : on; }
  bool ison()const { return state == on; }
  bool volup();  //增大声音
  bool voldown(); //减小声音
  void chanup(); //频道 +
  void chandown();//频道 -
  void set_mode() { mode = (mode == Antena) ? Cable : Antena; }
  void set_input() { input = (input == TV) ? DVD : TV; }
  void settings()const; //显示所有设置


private:
  int state;  // 开或者 关
  int volume; // 音量
  int maxchannel; //最大
  int channel;  //当前频道
  int mode;  // 广播还是 电缆
  int input; //Tv 或者 DVD
};

/*Remote 的定义 （遥控器） */
class Remote {
private :
  int mode; // 控制 TV 或 DVD
public:
  Remote(int m = Tv::TV) :mode(m) {}
  bool volup(Tv & t) { return t.volup(); }
  bool voldown(Tv & t) { return t.voldown(); }
  void onoff(Tv & t) { return t.onoff(); }
  void chanup(Tv & t) { return t.chanup(); }
  void chandown(Tv & t) { return t.chandown(); }
  void set_chan(Tv &t, int c) { t.channel = c; } //访问了Tv的私有成员
  void set_mode(Tv &t) { t.set_mode(); }
  void set_input(Tv &t) { t.set_input(); }
};
#endif // TV_H_

#include "stdafx.h"
#include "Tv.h"
#include <iostream>

bool Tv::volup() {
  if (volume < MaxVal) {
    volume++;
    return true;
  }
  else {
    return false;
  }
}

bool Tv::voldown() {
  if (volume > MinVal) {
    volume--;
    return true;
  }
  else {
    return false;
  }
}

void Tv::chanup() {
  if (channel < maxchannel) channel++;
  else channel = 1;
}

void Tv::chandown() {
  if (channel > 1) channel--;
  else channel = maxchannel;
}

void Tv::settings() const {
  using std::cout;
  using std::endl;
  cout << "TV is " << (state == off ? "off" : "on") << endl;
  if (state == on) {
    cout << "Volume setting =" << volume << endl;
    cout << "Channel setting = " << channel << endl;
    cout << "Mode = " << (mode == Antena ? "antenna" : "cable") << endl;
    cout << "Input = " << (input == TV ? "TV" : "DVD") << endl;
  }
}

#include "stdafx.h"
#include "tv.h"
#include <iostream>

int main()
{
  using std::cout;
  Tv s42;
  cout << "Initial settings for 42 \" Tv: \n";
  s42.settings();
  s42.onoff();
  s42.chanup();

  cout << " \n Adjusted settings for 42 \" Tv: \n";
  s42.chanup();
  cout << "\n Adjusted settings for 42 \" Tv: \n";
  s42.settings();

  Remote grey;
  grey.set_chan(s42, 10);
  grey.volup(s42);
  grey.volup(s42);
  cout << " \n s42 \" settings after using remote: \n";
  s42.settings();

  Tv s58(Tv::on);
  s58.set_mode();
  grey.set_chan(s58, 58);
  cout << " \n s58 \" setting: \n";
  s58.settings();

  system("pause");
  return 0;
}

Initial settings for 42 " Tv:
TV is off

 Adjusted settings for 42 " Tv:

 Adjusted settings for 42 " Tv:
TV is on
Volume setting =5
Channel setting = 7
Mode = cable
Input = TV

 s42 " settings after using remote:
TV is on
Volume setting =7
Channel setting = 10
Mode = cable
Input = TV

 s58 " setting:
TV is on
Volume setting =5
Channel setting = 58
Mode = antenna
Input = TV
请按任意键继续. . .

clas Tv
{
  friend void Remote::set_chan(Tv & t,int c ) ;
}

class Tv ; //前向声明
class Remote{...}
class Tv {...}

class Tv; //前向声明
class Remote {...} //如要用到Tv 只能是方法声明
class Tv{...}
//接着写Remote的定义

#ifndef TV_H_
#define TV_H_
class Tv; //前向声明
class Remote {

public:
  enum {
    off, on  //开关 
  };
  enum
  {
    MinVal, MaxVal = 20  //音量
  };
  enum {
    Antena, Cable //使用的天线、还是电缆
  };
  enum
  {
    TV, DVD  //工作模式
  };

private:
  int mode; // 控制 TV 或 DVD
public:
  Remote(int m = TV) :mode(m) {}
  //用到了Tv 只能是声明
  bool volup(Tv & t);
  bool voldown(Tv & t);
  void onoff(Tv & t);
  void chanup(Tv & t);
  void chandown(Tv & t);
  void set_chan(Tv &t, int c);
  void set_mode(Tv &t);
  void set_input(Tv &t);
};

class Tv
{
public:
  friend void Remote::set_chan(Tv & t,int c); //友元成员函数
  enum {
    off, on  //开关 
  };
  enum
  {
    MinVal, MaxVal = 20  //音量
  };
  enum {
    Antena, Cable //使用的天线、还是电缆
  };
  enum
  {
    TV, DVD  //工作模式
  };
  Tv(int s = off, int mc = 125) :state(s), volume(5), maxchannel(mc),
    channel(5), mode(Cable), input(TV) {}
  void onoff() { state = (state == on) ? off : on; }
  bool ison()const { return state == on; }
  bool volup();  //增大声音
  bool voldown(); //减小声音
  void chanup(); //频道 +
  void chandown();//频道 -
  void set_mode() { mode = (mode == Antena) ? Cable : Antena; }
  void set_input() { input = (input == TV) ? DVD : TV; }
  void settings()const; //显示所有设置
private:
  int state;  // 开或者 关
  int volume; // 音量
  int maxchannel; //最大
  int channel;  //当前频道
  int mode;  // 广播还是 电缆
  int input; //Tv 或者 DVD
};

inline bool Remote::volup(Tv & t) { return t.volup(); }
inline bool Remote::voldown(Tv & t) { return t.voldown(); }
inline void Remote::onoff(Tv & t) { return t.onoff(); }
inline void Remote::chanup(Tv & t) { return t.chanup(); }
inline void Remote::chandown(Tv & t) { return t.chandown(); }
inline void Remote::set_chan(Tv &t, int c) { t.channel = c; }
inline void Remote::set_mode(Tv &t) { return t.set_mode(); }
inline void Remote::set_input(Tv &t) { return t.set_input(); }
#endif // TV_H_

class Remote
class Tv
{
friend clas Remote
public:
  void buzz(Remote & r) ;
  ...
}

class Remote
{
friend class Tv;
public:
  void Bool volup(Tv & t){t.volup();}
  ...
}
inline void Tv::buzz(Remote & r)
{
...
}

class Analyzer;
class Probe
{
  friend void sync (Analyzer & a,const Probe & p) ;
  friend void sync (Probe & p,const Analyzer & a);
  ...
};
class Analyzer
{
  friend void sync (Analyzer & a,const Probe & p) ;
  friend void sync (Probe & p,const Analyzer & a);
}
inline void sync (Analyzer & a,const Probe & p)
{
  ...
}
inline void sync (Probe & p,const Analyzer & a)
{
  ...
}