C++友元函数和友元类

原创已于 2022-02-14 21:52:43 修改 · 223 阅读

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于 2022-02-14 12:26:26 首次发布

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本文介绍了C++中的友元函数和友元类的概念及用法。通过示例展示了如何使用友元函数访问类的私有和保护成员，以及友元类如何允许一个类访问另一个类的私有和保护成员。代码示例包括了友元函数的声明和实现，以及友元类的使用。

C++友元函数和友元类

C++友元函数1
友元函数2
友元类

C++友元函数1

#include<iostream>
using namespace std;

class book
{
public:
    book(){}
    book(char* a, double p);
    //void display(book &b);
    friend void display(book &b);
private:
    double price;
    char * title;
};

book::book(char* a, double p)
{
    title = a;
    price = p;
}

void display(book &b)
{
    cout<<"The price of "<<b.title<<" is $"<<b.price<<endl;
}

int main()
{
    book Alice("Alice in Wonderland",29.9);
    display(Alice);
    book Harry("Harry potter", 49.9);
    display(Harry);
    return 0;
}

在这里插入图片描述

friend void display(book &b); //error
//friend void display(book &b);

友元函数2

#include<iostream>
using namespace std;
class time;

class date
{
public:
    date(int y,int m,int d);
    void display(time &t);
private:
    int year;
    int month;
    int day;
};

class time
{
public:
    time(int s,int m,int h);
    friend void date::display(time & t);
private:
    int second;
    int minute;
    int hour;
};

time::time(int s,int m,int h)
{
    second = s;
    minute = m;
    hour = h;
}

date::date(int y,int m,int d)
{
    year = y;
    month = m;
    day = d;
}

void date::display(time &t)
{
    cout<<"The time is year month:"<<endl;
    cout<<year<<"/"<<month<<"/"<<day<<" " << endl;

    cout<<"The time is t.hour t.minute:"<<endl;
    cout<<t.hour<<":"<<t.minute<<":"<<t.second<<endl;
}

int main()
{
    date d(2015,1,16);
    time t(20,2,30);
    d.display(t);
    return 0;
}

在这里插入图片描述

友元类

#include<iostream>
using namespace std;

class time;

class date
{
public:
    date(int y,int m,int d);
    void display(time &t);
private:
    int year;
    int month;
    int day;
};

class time
{
public:
    time(int s,int m,int h);
    friend void date::display(time & t);
private:
    int second;
    int minute;
    int hour;
};

time::time(int s,int m,int h)
{
    second = s;
    minute = m;
    hour = h;
}

date::date(int y,int m,int d)
{
    year = y;
    month = m;
    day = d;
}

void date::display(time &t)
{
    cout<<"The time is:"<<endl;
    cout<<year<<"/"<<month<<"/"<<day<<" ";
    cout<<t.hour<<":"<<t.minute<<":"<<t.second<<endl;
}

int main()
{
    date d(2015,1,16);
    time t(20,2,30);
    d.display(t);
    return 0;
}