c++——友元类

代码摘自c++primer plus
【注】：友元类的声明的声明可以调用另一个类的私有成员
tv.h头文件初始化类的声明

#ifndef _TV_H_
#define _TV_H_

class Tv
{
     
private:
	int state;       //开关机状态
	int volume;     //音量
	int maxchannel;//最大频道数
	int channel;  //频道数
	int mode;     //广播或有线电视
	int input;    //
public:
	friend class Remote; //Remote类可以访问Tv类的私有部分
	enum {
     OFF,ON};
	enum {
     MinVal,MaxVal = 20};
	enum {
     Antenna,Cable};
	enum {
     TV,DVD};

	Tv(int s = OFF,int mc = 125) : state(s),volume(5),maxchannel(mc),channel(2),mode(Cable),input(TV)
	{
     }
	void onoff() {
     state = (state == ON) ? OFF : ON;}
	bool ison() const {
     return state == ON;}
	bool volup();
	bool voldown();
	void chanup();
	void chandown();
	void set_mode() {
     mode = (mode == Antenna) ? Cable : Antenna;}
	void set_input() {
     input = (input == TV) ? DVD : TV;}
	void settings() const;//显示所有设置
};

class Remote
{
     
private:
	int mode;//控制TV或者DVD模式
public:
	Remote(int m = Tv::TV) : mode(m) {
     }
	bool volup(Tv & t) {
     return t.volup();}
	bool voldown(Tv & t) {
     return t.voldown();}
	bool onoff(Tv & t) {
      t.onoff();}
	bool chanup(Tv & t) {
      t.chanup();}
	bool chandown(Tv & t) {
      t.chandown();}
	void set_chan(Tv & t,int c) {
      t.channel = c;}
	void set_mode(Tv & t) {
     t.set_mode();}
	void set_input(Tv & t) {
     t.set_input();}
};

#endif

tv.cpp实现类的函数

#include 
#include "TV.h"

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 = 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 == Antenna ? "antenna" : "cable") << endl;
		cout << "Input = " << (input == TV ? "TV" : "DVD")<< endl;
	}
}

user_tv.cpp对类的使用

#include
#include"TV.h"

int main()
{
     
	using std::cout;
	Tv s42;
	cout << "Inint settings for 42 号电视\" TV:\n";
	s42.settings();
	s42.onoff();
	s42.chanup();
	cout << "\nAdjusted settings for 42号电视机\" 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 << "\n42\" settings after using remote:\n";
	s42.settings();

	Tv s58(Tv::ON);
	s58.set_mode();
	grey.set_chan(s58,28);
	cout << "\n58\" settings:\n";
	s58.settings();
	return 0;
}

Makefile
makefile基础编写讲解

CC = g++
objects = tv.o use_tv.o
tv:$(objects)
	$(CC) -o tv $(objects)
tv.o: TV.h
use_tv.o:TV.h
clean:
	rm -rf  tv $(objects)

运行结果：