1. 1 CMSC 202 ADTs and C++ Classes

2. 2 Announcements Project 1 due Sunday February 25 th at midnight – don’t be late! Notes and clarifications for Project 1 HelpCenter (ECS 332) is open for business Quiz #2 this week Exam 1 – Next Wed/Thurs –Up to and including Monday/Tuesday Lecture –Chapters 2 – 8 and 14 of the text

3. 3 OOP and Classes OOP encapsulates data (characteristics) and functions (behaviors) into packages called classes. Classes support information hiding –Users of a class know how to communicate with the class using well defined interfaces, they do not know how the classes are implemented – implementation details are hidden within each class

4. 4 User Defined Types In C, software components are functions In OOP (C++), software components are user defined data types called classes. Each class contains data as well as the functions to manipulate the data An object is the instantiation of a class

5. 5 The TIME ADT in C typedef struct time { int hour; int minute; int second; } TIME; void InitTime (int h, int m, int s); void PrintMilitary (TIME *tp); void PrintStd (TIME *tp); void Increment (TIME *tp);

6. 6 The TIME ADT in C++ class Time { public: Time ( ); void InitTime (int h, int m, int s); void PrintMilitary ( void ); void PrintStd ( void ); void Increment ( void ); private: int hour; int minute; int second; }

7. 7 Terminology This is a class, not an object. private and public are “member access specifications”. (protected later) Time ( ); is a special function called a constructor. Same name as the class. Class functions also known as member functions or methods. In this course, ALL DATA MEMBERS OF ALL CLASSES WILL BE PRIVATE.

8. 8 Member Functions // should have error checking void Time::InitTime (int h, int m, int s) { hour = h; minute = m; second = s; }

9. 9 Another Member Function void Time::PrintMilitary ( void ) { cout << (hour < 10 ? “0” : “”) << hour; cout << “:” cout << (minute < 10 ? “0” : “”) << minute; }

10. 10 Accessing Private Members main ( ) { Time t2; t2.hour = 11;// compiler error }

11. 11 Manipulating Private Data Public “Accessor” functions –int GetMinute ( void ); –int GetHour ( void ); –int GetSecond ( void ); Public “Mutator” functions –void SetMinute (int m); –void SetHour (int h); –void SetSecond (int s);

12. 12 A Subtle Trap This is okay int Time::GetHour ( ) { return hour; } This is wrong int& Time::GetHour ( ) { return hour; }

13. 13 The constructor A special member function which is called automatically whenever an object is instantiated Time::Time ( ) { hour = minute = second = 0; } NO RETURN TYPE (not even “void”) There can be more than one Compiler provides a default constructor

14. 14 Default Arguments with Constructors class Time { public: Time (int h = 0, int m = 0, int s = 0);... } Any function can have default arguments

15. 15 Using the Time Constructor Time t1 (3, 5, 9); Time t2 (4, 7);// second = 0 Time t3 ( 5);// minute = second = 0 Time t4;// all are 0

16. 16 int main ( ) { Time t1;// instantiates a Time object – constructor cout << “Initial Military Time is :” ; t1.PrintMilitary ( ); cout << endl; t1.InitTime (13, 9, 10 ); cout << “New Military Time is:” t1.PrintMilitary ( ); cout << endl; t1.Increment ( ); cout << “New Military Time is:” t1.PrintMilitary ( ); cout << endl; }

17. 17 // main( ) with pointers int main ( ) { Time t1;// instantiates a Time object – constructor Time *tp = &t1;// pointer to Time object cout PrintMilitary ( ); cout << endl; tp->InitTime (13, 9, 10 ); tp->Increment ( ); cout PrintMilitary ( ); cout << endl; }

18. 18 Member Initialization List An alternative for constructors Time::Time (int h, int m, int s) : hour(h), minute(m), second(s) { // no code }

19. 19 Destructor A special member function that is automatically called when an object goes out of scope to “deallocate” the object Special Naming -- Time::~Time ( ); NO RETURN TYPE CAN BE ONLY ONE Compiler provides a default destructor –Not always sufficient

20. 20 Default Assignment The compiler provides a default assignment operator for every class Time t1, t2; t1 = t2;// assignment A member-by-member copy Not always sufficient

21. 21 friend functions A friend function of a class is a non- member function yet still has the right to access the right to access the private (and protected) members of the class.

22. 22 A friendly example Within the Time class, we can allow the PrintTime( ) function to be our friend friend void PrintTime (const Time &); Now PrintTime doesn’t need to use the accessors

23. 23 PrintTime with Accessors void PrintTime (Time& t) { cout << t.GetHour( ) << “:”; cout << t.GetMinute( ) << “:”; cout << t.GetSecond( ); }

24. 24 PrintTime as a friend void PrintTime (const Time& t) { cout << t.hour << “:”; cout << t.minute << “:”; cout << t.second; }

25. 25 Classes can be friends too We can allow all the functions of a class to be our friend. AlarmClock can let Time be it’s friend by declaring friend class Time; within its class definition

26. 26 A philosophical Question Does the concept of friends violate the principles of OOP (i.e. data hiding)? Advantages of friends Disadvantages of friends

27. 27 Separating Interface and Implementation Interface – the header file (Time.H) –Given to the user –Substantial documentation –NO CODE –public section first Implementation – member function code (Time.C) –Only for the programmer –Programmer-oriented documentation

28. 28 Good Programming Practices 1.Use private and public just once in a class 2.List public first 3.ALL DATA is private 4.Interface in.H and implementation in.C 5.Don’t return a (non-const) reference to private data