友元函数的用法

外部的函数，访问类中的私有变量

 

#include <iostream>
using namespace std;
class Message
{
	public:
		Message(int a,int b):x(a),y(b){}
		friend void showMessage(Message &msg);
	
	private:
		int x;
		int y;
};
void showMessage(Message &msg)
{
	cout<<"a:"<<msg.x<<endl;
	cout<<"b:"<<msg.y<<endl;
}

int main()
{
	Message msg(1,2);
	showMessage(msg);
	return 0;
}

 

A的成员函数，作为B的友元函数，访问B的私有成员

 

#include <iostream>
using namespace std;
class Message_B;
class Message_A{
public:
	void showMessage_B(Message_B &msg_b);
};

class Message_B{
public:
	Message_B(int x,int y):a(x),b(y){}
	friend void Message_A::showMessage_B(Message_B &msg_b);
private:
	int a;
	int b;

};
void Message_A::showMessage_B(Message_B &msg_b)
{
	cout<<"a:"<<msg_b.a<<endl;
	cout<<"b:"<<msg_b.b<<endl;
}

int main()
{
	Message_A msg_a;
	Message_B msg_b(1,2);
	msg_a.showMessage_B(msg_b);

	return 0;
}