Friend Function:

Is a function that:
1. is not a member of a class.
2. has access to the class's private and protected members.
3. is considered as a normal external function that is given special access privileges.
4. is not in the class's scope.
5. is not called using the member-selection operators (. and –>) unless they are members of another class.
6. is declared by the class that is granting access.
We can define a friend function anywhere in the class declaration. It is not affected by the access control keywords.

For Example:

1. // Modified Count class
   
2. class
   
   Count
   
3. {
   
4. friend
   
   void
   
   setX(
   
   Count &
   
   , int
   
   )
   
   ;
   
   // friend declaration
   
5. public
   
   :
   
6. Count(
   
   )
   
   {
   
   x =
   
   0
   
   ;
   
   }
   
   // constructor
   
7. void
   
   print(
   
   )
   
   const
   
   {
   
   cout
   
   <<
   
   x <<
   
   endl;
   
   }
   
   // output
   
8. private
   
   :
   
9. int
   
   x;
   
   // data member
   
10. }
    
    ;
    

- setX() is a friend function of Count class, it has access to the class's private and protected members.
- We can implement setX() function outside Count class as a normal function, as follow:

1. // Can modify private data of Count because
   
2. // setX is declared as a friend function of Count
   
3. void
   
   setX(
   
   Count &
   
   c, int
   
   val )
   
4. {
   
5. c.x
   
   =
   
   val;
   
   // legal: setX is a friend of Count
   
6. }
   

We can test it in the main function as follow:

1. int
   
   main(
   
   )
   
2. {
   
3. Count counter;
   
4. 
   
5. cout
   
   <<
   
   "counter.x after instantiation: "
   
   ;
   
6. counter.print
   
   (
   
   )
   
   ;
   
7. cout
   
   <<
   
   "counter.x after call tosetX friend function: "
   
   ;
   
8. 
   
9. setX(
   
   counter, 8
   
   )
   
   ;
   
   // set x with a friend
   
10. counter.print
    
    (
    
    )
    
    ;
    
11. return
    
    0
    
    ;
    
12. }
    

The output:
counter.x after instantiation: 0
counter.x after call to setX friend function: 8

We will re-implement the previous example without using a friend function as follow:

1. // Modified Count class
   
2. class
   
   Count
   
3. {
   
4. public
   
   :
   
5. Count(
   
   )
   
   {
   
   x =
   
   0
   
   ;
   
   }
   
   // constructor
   
6. void
   
   print(
   
   )
   
   const
   
   {
   
   cout
   
   <<
   
   x <<
   
   endl;
   
   }
   
   // output
   
7. private
   
   :
   
8. int
   
   x;
   
   // data member
   
9. }
   
   ;
   
10. 
    
11. // Function tries to modifyprivate data of Count,
    
12. // but cannot because it isnot a friend of Count.
    
13. void
    
    cannotSetX(
    
    Count &
    
    c, int
    
    val )
    
14. {
    
15. c.x
    
    =
    
    val;
    
    // ERROR: 'Count::x'is not accessible
    
16. }
    
17. 
    
18. int
    
    main(
    
    )
    
19. {
    
20. Count counter;
    
21. 
    
22. cannotSetX(
    
    counter, 3
    
    )
    
    ;
    
    // cannotSetX is not a friend
    
23. return
    
    0
    
    ;
    
24. }
    

The output with errors:

Compiling...
Fig07_06.cpp
D:\books\2000\cpphtp3\examples\Ch07\Fig07_06\Fig07_06.cpp(22) :
error C2248: 'x' : cannot access private member declared in
class 'Count'
D:\books\2000\cpphtp3\examples\Ch07\Fig07_06\
Fig07_06.cpp(15) : see declaration of 'x'
Error executing cl.exe.
test.exe -1error(s), 0 warning(s)

Note: You can find the full source code of this example in code.zip file.

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