c++友元函数、友元类、友成员函数

class Point{
public:
    friend void fun(Point t);//友元函数
private:
    int a;
protected:
    int b;
};

void fun(Point t)
{
    t.a = 100;
    t.b = 2000;
    cout << t.a << "   " << t.b << endl;
}

int main()
{
    Point test;
    fun(test);
    system("pause");
    return 0;
}

class Point{
    friend class Line;
public:
    void fun()
    {
        this->x = 100;
        this->y = 120;
    }
private:
    int x;
protected:
    int y;
};

class Line{
public:
    void printXY()
    {
        t.fun();
        cout << t.x << "  " << t.y << endl;
    }

private:
    Point t;
};

int main()
{
    Line test;
    test.printXY();
    system("pause");
    return 0;
}

 1 class Point;//在此必须对Point进行声明，如不声明将会导致第5行（void fun(Point t);）“Point”报错（无法识别的标识符）
 2 
 3 class Line{
 4 public:
 5     void fun(Point t);
 6     //void Line::fun(Point t)//在此编译器无法获知class Point中到底有哪些成员，所以应在Point后面定义
7     //{
 8     //    t.x = 120;
 9     //    t.y = 130;
10     //    t.print();
11     //}
12 
13 };
14 
15 class Point{
16 public:
17     friend void Line::fun(Point t);
18     Point(){}
19     void print()
20     {
21         cout << this->x << endl;
22         cout << this->y << endl;
23     }
24 private:
25     int x;
26 protected:
27     int y;
28 };
29 
30 void Line::fun(Point t)//应在此定义fun函数
31 {
32     t.x = 120;
33     t.y = 130;
34     t.print();
35 }
36 
37 int main()
38 {
39     Point test;
40     Line LL;
41     LL.fun(test);
42     system("pause");
43     return 0;
44 }