1. Time sunset(
   
   19
   
   ,0
   
   ,0
   
   )
   
   ;
   
   // object of type Time
   
2. Time &
   
   dinnerTime =
   
   sunset;
   
   // reference to a Time object
   

Defining a reference to an object is equal to defining an alias to this object. We can use the reference in the left side of an assignment, that means we can assign a new value to this reference and change the original object that is referenced.
We can write a public member function in a class that returns a reference to private data member, let us have the following class:

1. class
   
   Time {
   
2. public
   
   :
   
3. // Constructor function to initialize private data.
   
4. // Calls member function setTime to set variables.
   
5. // Default values are 0 (see class definition).
   
6. Time(
   
   int
   
   hr,int
   
   min, int
   
   sec )
   
7. {
   
8. setTime(
   
   hr, min, sec )
   
   ;
   
9. }
   
10. // Set the values of hour, minute, and second.
    
11. void
    
    setTime(
    
    int
    
    h,int
    
    m, int
    
    s )
    
12. {
    
13. hour =
    
    (
    
    h >=
    
    0
    
    &&
    
    h <
    
    24
    
    )
    
    ?
    
    h :
    
    0
    
    ;
    
14. minute =
    
    (
    
    m >=
    
    0
    
    &&
    
    m <
    
    60
    
    )
    
    ?
    
    m :
    
    0
    
    ;
    
15. second =
    
    (
    
    s >=
    
    0
    
    &&
    
    s <
    
    60
    
    )
    
    ?
    
    s :
    
    0
    
    ;
    
16. }
    
17. // Get the hour value
    
18. int
    
    getHour(
    
    )
    
19. {
    
20. return
    
    hour;
    
21. }
    
22. // POOR PROGRAMMING PRACTICE:
    
23. // Returning a reference toa private data member.
    
24. int
    
    &
    
    badSetHour(
    
    int
    
    hh)
    
    // DANGEROUS reference return
    
25. {
    
26. hour =
    
    (
    
    hh >=
    
    0
    
    &&
    
    hh <
    
    24
    
    )
    
    ?
    
    hh :
    
    0
    
    ;
    
27. return
    
    hour;
    
    // DANGEROUS reference return
    
28. }
    
29. private
    
    :
    
30. int
    
    hour;
    
31. int
    
    minute;
    
32. int
    
    second;
    
33. }
    
    ;
    

You can notice that badSetHour() member function returns a reference to 'hour' data member, so any change has been made on this reference will change the value of 'hour' data member like in the following example:

1. int
   
   main(
   
   )
   
2. {
   
3. Time t;
   
4. int
   
   &
   
   hourRef =
   
   t.badSetHour
   
   (
   
   20
   
   )
   
   ;
   
5. 
   
6. cout
   
   <<
   
   "Hour before modification: "
   
   <<
   
   hourRef;
   
7. hourRef=
   
   30
   
   ;
   
   // modification with invalid value
   
8. cout
   
   <<
   
   "\n
   
   Hourafter modification: "
   
   <<
   
   t.getHour
   
   (
   
   )
   
   ;
   
9. 
   
10. // Dangerous: Function call that returns
    
11. // a reference can be used as anlvalue!
    
12. t.badSetHour
    
    (
    
    12
    
    )
    
    =
    
    74
    
    ;
    
13. cout
    
    <<
    
    "\n
    
    \n
    
    *********************************\n
    
    "
    
14. <<
    
    "POOR PROGRAMMING PRACTICE!!!!!!!!\n
    
    "
    
15. <<
    
    "badSetHour as an lvalue, Hour: "
    
16. <<
    
    t.getHour
    
    (
    
    )
    
17. <<
    
    "\n
    
    *********************************"
    
    <<
    
    endl;
    
18. 
    
19. return
    
    0
    
    ;
    
20. }
    

The output:

Hour before modification: 20
Hour after modification: 30
*********************************
POOR PROGRAMMING PRACTICE!!!!!!!!
badSetHour as an lvalue, Hour: 74
*********************************

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

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