c friend -- 友元

友元用于突破protected 或者 private 保护的限制，首先要做的是在被访问者的类中声明是友元函数或者友元类。代码如下

#include <iostream>
using namespace std;

class Square{
        private:
                int side;
        public:
                Square(int a):side(a){}
        friend class Rectangle ; //declare the Class is friend


};

class Rectangle {
        private:
                int width, height;
        public:
                Rectangle(int a, int b):width(a),height(b){}
                void set_values (int a, int b){
                        width = a;
                        height = b;
                }
                int girth();
                friend int area (Rectangle &);  //declare the friend function here
                int get_width(){return width;}
                int get_height(){return height;}
                void conver_from_square(Square &s){
                        width = height = s.side; //access so easily
                }
};

//implement the function here , access easily ,too
int area (Rectangle &r){ return ( r.width * r.height); }
int r_area(Rectangle &r){ return r.get_width() * r.get_height();}
int Rectangle::girth(){return width + width + height + height; }

int main () {
test_sizeof:
        cout << "sizeof: Square " << sizeof(Square)
             << ",\tRectangle " << sizeof(Rectangle) << "\n\n" ;
test_access:
        Rectangle r(2,3);
        cout << "area:" << area(r) << "\tgirth:" << r.girth()  << endl;
        cout << "onather way:area " << r_area(r) << endl;

        Rectangle r1(2,3);
        Square s(5);
        r1.conver_from_square(s);
        cout << "rectangle convering from square , girth is " << r1.girth() << endl;
        return 0;
}

结果

sizeof: Square 4,       Rectangle 8

area:6  girth:10
onather way:area 6
rectangle convering from square , girth is 20

看函数

• area
• r_area
• girth

如果不是友元函数或类，访问情况如 r_area()函数，友元函数就可以直接访问成员。但是和成员函数比起来还是要有区别的，看函数girth()

看看size，友元类或者友元函数并不增加类的大小，只是声明一下。

参考：

1. Friendship and inheritance %20%20/inheritance/