c friend -- 友元

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c friend -- 友元

友元用於突破protected 或者 private 保護的限制，首先要做的是在被訪問者的類中聲明是友元函數或者友元類。代碼如下

1. #include <iostream>
   
2. using namespace std;
   
4. class Square{
   
5.         private:
   
6.                 int side;
   
7.         public:
   
8.                 Square(int a):side(a){}
   
9.         friend class Rectangle ; //declare the Class is friend
   
12. };
    
14. class Rectangle {
    
15.         private:
    
16.                 int width, height;
    
17.         public:
    
18.                 Rectangle(int a, int b):width(a),height(b){}
    
19.                 void set_values (int a, int b){
    
20.                         width = a;
    
21.                         height = b;
    
22.                 }
    
23.                 int girth();
    
24.                 friend int area (Rectangle &); //declare the friend function here
    
25.                 int get_width(){return width;}
    
26.                 int get_height(){return height;}
    
27.                 void conver_from_square(Square &s){
    
28.                         width = height = s.side; //access so easily
    
29.                 }
    
30. };
    
32. //implement the function here , access easily ,too
    
33. int area (Rectangle &r){ return ( r.width * r.height); }
    
34. int r_area(Rectangle &r){ return r.get_width() * r.get_height();}
    
35. int Rectangle::girth(){return width + width + height + height; }
    
37. int main () {
    
38. test_sizeof:
    
39.         cout << "sizeof: Square " << sizeof(Square)
    
40.              << ",\tRectangle " << sizeof(Rectangle) << "\n\n" ;
    
41. test_access:
    
42.         Rectangle r(2,3);
    
43.         cout << "area:" << area(r) << "\tgirth:" << r.girth() << endl;
    
44.         cout << "onather way:area " << r_area(r) << endl;
    
46.         Rectangle r1(2,3);
    
47.         Square s(5);
    
48.         r1.conver_from_square(s);
    
49.         cout << "rectangle convering from square , girth is " << r1.girth() << endl;
    
50.         return 0;
    
51. }

結果

1. sizeof: Square 4, Rectangle 8
3. area:6 girth:10
4. onather way:area 6
5. rectangle convering from square , girth is 20

看函數

• area
• r_area
• girth

如果不是友元函數或類，訪問情況如 r_area()函數，友元函數就可以直接存取成員。但是和成員函數比起來還是要有區別的，看函數girth() 

看看size，友元類或者友元函數並不增加類的大小，只是聲明一下。

c friend -- 友元