c++友元

友元函数

#include <iostream>
#include <math.h>

using namespace std;

class Point{
    private:
        int m_x;
        int m_y;
    public:
        Point(int x, int y) {
            m_x = x;
            m_x = y;
        }
        int getX() {
            return m_x;
        }
        int getY() {
            return m_y;
        }
        // 声明getDistance2是类Point的友元函数，因此在getDistance2里可以直接通过对象访问其私有成员变量
        friend void getDistance2(Point& p1, Point& p2);
};

void getDistance(Point& p1, Point& p2) {
    double dis;
    int x_dis = p1.getX() - p2.getX();
    int y_dis = p2.getY() - p2.getY();
    dis = sqrt(x_dis * x_dis + y_dis * y_dis);
    cout<<"getDistance dis = "<<dis<<endl;
}

void getDistance2(Point& p1, Point& p2) {
    double dis;
    int x_dis = p1.m_x - p2.m_x;
    int y_dis = p2.m_y - p2.m_y;
    dis = sqrt(x_dis * x_dis + y_dis * y_dis);
    cout<<"getDistance2 dis = "<<dis<<endl;
}

int main() {
    Point p1(2,3);
    Point p2(3,4);
    getDistance(p1, p2);
    getDistance2(p1, p2);
    return 0;
}

#include <iostream>
#include <math.h>

using namespace std;

class Point;

class PointManager {
    public:
        void getDistance(Point& p1, Point& p2);
};

class Point{
    private:
        int m_x;
        int m_y;
    public:
        Point(int x, int y) {
            m_x = x;
            m_x = y;
        }
        int getX() {
            return m_x;
        }
        int getY() {
            return m_y;
        }
        friend void PointManager::getDistance(Point& p1, Point& p2);
};

void PointManager::getDistance(Point& p1, Point& p2) {
    double dis;
    int x_dis = p1.m_x - p2.m_x;
    int y_dis = p2.m_y - p2.m_y;
    dis = sqrt(x_dis * x_dis + y_dis * y_dis);
    cout<<"getDistance dis = "<<dis<<endl;
}

int main() {
    Point p1(2,3);
    Point p2(3,4);
    PointManager pm;
    pm.getDistance(p1, p2);
    return 0;
}

友元类

#include <iostream>

using namespace std;

class ClassA {
    private:
        int m_a;
    public:
        ClassA(int a) {
            m_a = a;
        }
        friend class ClassB; //声明ClassB为当前类的友元类后，在ClassB中就可以通过ClassA的对象，直接访问其私有成员
};

class ClassB {
    private:
        int m_b;
    public:
        ClassB(int b) {
            m_b = b;
        }
        void printB() {
            ClassA clazzA(22);
            cout << "m_b = " << m_b << "  clazzA.m_a = "<<clazzA.m_a<<endl;
        }
};

int main() {
    ClassB b(44);
    b.printB();
    return 0;
}