1. Classes: Implementation and Testing
Edited for CMPSC 122
Penn State University
Prepared by Doug Hogan

2. Preview of Today… Implementation Client end -- Test drivers
Tips for implementing each kind of function
More Examples
Client end -- Test drivers

3. Review
Questions on the blue worksheet?

4. Implementing Functions
Remember
Implementation outside the class interface
Name of the class and scope resolution operator (::) before the FUNCTION NAME

5. Implementing Constructors
Do whatever is necessary to set up the class
Initialize each private member
Default constructors:
default values
Initializer constructors:
some from input parameters
validate?
set others to default values

6. Exercise Implement the default constructor for tvShow.
tvShow::tvShow() {
name = “New show”;
channel = 2;
startHour = 0;
startMin = 0; }

7. Implementing initializer constructors
Most specify SOME, not ALL, initial values
Set the others as in the default constructor

8. Initializer constructor example
tvShow::tvShow(string initName, int initChannel) {
name = initName; // initialized from
channel = initChannel; // parameters
// we still need to handle two data members!
startHour = 0; // initialized to
startMin = 0; // defaults }
tvShow object constructed with this method
name
channel
startHour
startMin d
from init- Channel
value of initName

9. Implementing modifiers
Ultimately: must have assignment statements (or function calls) that change private data

10. Modifier Example
void tvShow::reschedule(int hoursLater, int minutesLater)
// PRE: hoursLater >= 0, 0 <= minutesLater <= 59
// POST: this tvShow now starts hoursLater hours and
// minutesLater minutes after it did before {
startHour = ((startMin + minutesLater)/60
+ startHour + hoursLater)%24; // add on hour from minutes rolling over // add on hours, correct for day rolling over
startMin = (startMin + minutesLater)%60;
// add on time, correct for new hour }

11. Implementing Accessors
“Get” accessors
return a private variable
ex: int tvShow::getChannel() const { return channel; }
Don’t forget the const if it’s in the prototype!

12. Accessors That Print vs. Accessors That Return
int tvShow::getChannel() const // POST: Method returns this show’s channel { return channel; }
void tvShow::printChannel() const // POST: Method displays this show’s channel { cout << "Channel: " << channel; }
Always provide the first type. The second type can be useful, but from a user interface design perspective, really ought to be avoided. A general principle is that you should separate logic from user interface at all times.

13. An Accessor Exercise
Write an accessor that prints the time of a tvShow in the format “8:30 a.m.”

14. An Accessor Exercise void tvShow::printTimeAMPM() const {
// POST: a string is displayed with this
// show's start time in "h:mm a.m." form {
int hr; // hour in format 1..12
hr = startHour%12; // get hour in range
if(hr == 0) // correct for hour 0 == 12 hr = 12;
if(min <= 9) // need leading zero
cout << hr << ":0" << min;
else // no leading zero
cout << hr << ":" << min;
if(startHour < 12) // check for a.m.
cout << " a.m.";
else // otherwise it's p.m.
cout << " p.m."; }
Question: Can any of the if
statements be optimized?

15. Test Drivers
Main program that checks whether the class is working properly
Create a few objects
Call each of the member functions
Check the results
Good practice: test drive classes before writing programs with them
Find the errors WITHIN THE CLASS vs. outside the class

16. Example Test Driver
int main() { bankAccount acct1; // def. constructor bankAccount acct2("Homer", 100); // one init constr bankAccount acct3("Lisa"); // another init con acct1.resetAcct("Marge", 75); // test resetAcct cout << acct1.getName(); // test getName // did resetAcct work? cout << acct1.getBalance(); // test getBalance acct1.deposit(50); // deposit cout << acct1.getBalance(); // did deposit work? acct1.withdraw(100); // withdraw cout << acct1.getBalance(); // did withdraw work? // additional calls, use accessors to see that other // constructors worked }
Note: Comments in this test driver are for you. I'll never require test driver comments.

17. Summary Implementation Test drive classes before using them
Scope resolution operator and class name
Constructors – initialize each private member
Modifiers – change private members
Accessors – remember const, printing vs. returning
Test drive classes before using them
Lab time now: Your turn to make your own class!