c++-友元函数和友元类

友元函数

• 友元函数和友元类（破坏类的封装性）
• 面向对象编程思想

#define _crt_secure_no_warnings
#include <iostream>
#include <cmath>


using namespace std;

class point;

class pointmanager {
public:
    double pointdistance(point &p1, point &p2);
};

class point
{

public:
    //声明全局函数 pointdistance 是我类point类的一个友元函数。
    //friend double pointdistance(point &p1, point &p2);
    friend double pointmanager::pointdistance(point &p1, point &p2);

    point(int x, int y) {
        this->x = x;
        this->y = y;
    }


    int getx()
    {
        return this->x;
    }

    int gety()
    {
        return this->y;
    }
private:
    int x;
    int y;

};

double pointmanager::pointdistance(point &p1, point &p2)
{
    double dis;
    int dd_x = p1.x - p2.x;
    int dd_y = p1.y - p2.y;

    dis = sqrt(dd_x*dd_x + dd_y *dd_y);

    return dis;
}

# if 0
double pointdistance(point &p1, point &p2)
{
    double dis;
    int dd_x = p1.x - p2.x;
    int dd_y = p1.y - p2.y;

    dis = sqrt(dd_x*dd_x + dd_y *dd_y);

    return dis;
}
#endif



int main(void)
{
    
    point p1(1, 2);
    point p2(2, 2);

    //cout << pointdistance(p1, p2) << endl;

    pointmanager pm;
    cout << pm.pointdistance(p1, p2) << endl;

    return 0;
}

友元类

#define _crt_secure_no_warnings
#include <iostream>

using namespace std;

class a 
{
public:

    a(int a)
    {
        this->a = a;
    }

    void printa() {
        cout << "a = " << this->a << endl;
        // b objb(3000);
        // cout << objb.b << endl;
    }

    //声明一个友元类b
    friend class b;
private:
    int a;
};


class b
{
public:
    b(int b)
    {
        this->b = b;
    }
    void printb() {
        a obja(100);
        cout << obja.a << endl;
        cout << "b = " << this->b << endl;
    }
    friend class a;
private:
    int b;
};

int main(void)
{
    b bobj(200);

    bobj.printb();
    
    return 0;
}