C++学习笔记－－友元类

最新推荐文章于 2024-11-07 14:07:13 发布

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最新推荐文章于 2024-11-07 14:07:13 发布

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分类专栏：学习笔记文章标签：友元类

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本文通过电视和遥控器的关系，介绍了C++中的友元类概念。示例中，遥控器作为友元类可以操作电视的开关状态和频道增减，设定的最大频道数为125。

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生活中简单的电视和遥控器，遥控器既不能继承电视，也不属于组合关系，此时使用友元类，让遥控器可以控制电视。

对例子进行简化，只有电视的开关状态和频道的增加/减少，设置最大频道数125

// tv.h -- Tv and Remote classes
#ifndef TV_H_
#define TV_H_
#include <iostream>
using namespace std;
class Tv
{
public:
    friend class Remote;    // Remote can access Tv private parts
    enum {Off, On};
    Tv(int s = Off, int mc = 125) : state(s), maxchannel(mc),
        channel(2) {}
    void onoff() {state = (state == On) ? Off : On;}
    bool ison() const {return state == On;}
    void chanup();
    void chandown();
private:
    int state;
    int maxchannel;
    int channel;
};
class Remote
{
public:
    Remote() {}
    void onoff(Tv & t) {t.onoff(); cout << "Tv is on/off\n";}
    void chanup(Tv & t) {t.chanup();}
    void chandown(Tv & t) {t.chandown();}
};
#endif

// tv.cpp -- methods for the Tv class (Remote methods are inline
#include <iostream>
#include "tv.h"
using namespace std;
void Tv::chanup()
{
    if (channel < maxchannel)
        channel++;
    else
        channel = 1;
    cout << "Now channel is " << channel << endl;
}
void Tv::chandown()
{
    if (channel > 1)
        channel--;
    else
        channel = maxchannel;
    cout << "Now channel is " << channel << endl;
}

// use_tv.cpp -- using the Tv and Remote classes
#include <iostream>
#include "tv.h"
using namespace std;
int main()
{
    Tv s42;
    s42.onoff();
    Remote grey;
    grey.onoff(s42);
    grey.chanup(s42);
    grey.chanup(s42);
    grey.chandown(s42);
}

输出：

wang@wang:~/c++$ ./a.out
Tv is on/off
Now channel is 3
Now channel is 4
Now channel is 3