Mutual Friendship

What if you want a function of one class to be a friend of a second class, and a function of the second class to be a friend of the first class? How do you do it?

Make the bark() function of the dog class a friend of the cat class and the meow() function a friend of the dog class.

Example 5-8 - Mutual friends

1      // File: ex5-8.cpp
2     
3      #include <iostream>
4      #include <cstring>
5      using namespace std;
6     
7      class cat;                       // forward declare the cat class
8     
9      class dog {
10          char name[10];
11        public:
12          void bark(const cat&) const;
13          dog(const char* n) { strcpy(name,n); }
14        friend class cat;      // name the entire cat class as a friend
15      };
16     
17      class cat {
18          char name[10];
19        public:
20          void meow(const dog&) const;
21          cat(const char* n) { strcpy(name,n); }
22        friend void dog::bark(const cat&) const;
23      };
24     
25      // a dog barks at a cat exactly the number of times as
26      // the length of the cat's name
27      void dog::bark(const cat& c) const
28      {
29        for (unsigned i = 0; i < strlen(c.name); i++)
30           cout << " woof "; cout << endl;
31      }
32     
33      // a cat meows at a dog exactly the number of times as 
34      // the length of the dog's name
35      void cat::meow(const dog& d) const
36      {
37        for (unsigned i = 0; i < strlen(d.name); i++)
38           cout << " meow "; cout << endl;
39      }
40      main()
41      {
42        dog bart("Bart");
43        cat socks("Socks");
44     
45      // Bart barks at Socks
46        bart.bark(socks);
47     
48      // Socks meows at Bart
49        socks.meow(bart);
50     
51        return 0;
52      }

****** Output ******

woof  woof  woof  woof  woof
meow  meow  meow  meow