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;
}