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

1. #include <iostream>
2. using namespace std;
3. class Square{
4. private:
5. int side;
6. public:
7. Square(int a):side(a){}
8. friend class Rectangle ; //declare the Class is friend
9. };
10. class Rectangle {
11. private:
12. int width, height;
13. public:
14. Rectangle(int a, int b):width(a),height(b){}
15. void set_values (int a, int b){
16. width = a;
17. height = b;
18. }
19. int girth();
20. friend int area (Rectangle &); //declare the friend function here
21. int get_width(){return width;}
22. int get_height(){return height;}
23. void conver_from_square(Square &s){
24. width = height = s.side; //access so easily
25. }
26. };
27. //implement the function here , access easily ,too
28. int area (Rectangle &r){ return ( r.width * r.height); }
29. int r_area(Rectangle &r){ return r.get_width() * r.get_height();}
30. int Rectangle::girth(){return width + width + height + height; }
31. int main () {
32. test_sizeof:
33. cout << "sizeof: Square " << sizeof(Square)
34. << ",\tRectangle " << sizeof(Rectangle) << "\n\n" ;
35. test_access:
36. Rectangle r(2,3);
37. cout << "area:" << area(r) << "\tgirth:" << r.girth() << endl;
38. cout << "onather way:area " << r_area(r) << endl;
39. Rectangle r1(2,3);
40. Square s(5);
41. r1.conver_from_square(s);
42. cout << "rectangle convering from square , girth is " << r1.girth() << endl;
43. return 0;
44. }

结果

1. sizeof: Square 4, Rectangle 8
2. area:6 girth:10
3. onather way:area 6
4. 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。