1. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-1 Learning Objectives  Classes  Constructors  Principles of OOP  Class type member data

2. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-2 Topic 1: Classes  Similar to structures  Adds member FUNCTIONS  Not just member data  Object-oriented programming  Class: Abstract data type that contains data and operations  Object: Instances of a class

3. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-3 Class Definitions  Similar to structures  Example: class DayOfYear { public: int month; int day; public : void output(); };

4. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-4 Declaring Objects  Declared same as all variables  Example: DayOfYear today, birthday;  Declares two objects of class type DayOfYear  Objects include:  Data (member data)  month, day  Operations (member functions)  output()

5. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-5 Class Member Access  Members accessed same as structures  Example: today.month today.day  And to access member function: today.output();  Invokes member function

6. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-6 Class Member Functions  Must define or "implement" class member functions  Like other function definitions  Must specify class: return_type class_name::function_name(parameters) Example: void DayOfYear::output() {…}  :: is scope resolution operator  Instructs compiler "what class" member is from  Item before :: called type qualifier

7. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-7 Dot and Scope Resolution Operator  Dot (. ) operator:  Specifies member of particular object  Scope resolution ( :: ) operator:  Specifies what class the function definition comes from

8. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-8 Class Member Functions Definition  Function used for all objects of the class  Will refer to "that object’s" data when invoked  Example: today.output();  Displays "today" object’s data

9. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-9 Topic 2: Constructors of Classes  Initialization of objects  Initialize some or all member variables  Other actions possible as well, but not typicall  A special kind of member function  Automatically called when object declared

10. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-10 Constructor Definitions  Constructors defined like any member function  Except: 1. Must have same name as class 2. Cannot return a value; not even void!

11. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-11 Constructor Definition Example  Class definition with constructor: class DayOfYear { public: int month; int day; public: DayOfYear(int m, int d); //Constructor initializes month & day void output(); };

12. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-12 Constructor Code  Constructor definition is like all other member functions: DayOfYear::DayOfYear(int m, int d) { month = m; day = d; }

13. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-13 Constructor Notes  Notice name of constructor: DayOfYear  Same name as class itself!  Constructor declaration has no return-type  Not even void!  Constructor in public section

14. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-14 Calling Constructors  Declare objects: DayOfYear date1(7, 4); DayOfYear date2(5, 5);  Objects are created here  Constructor is called  Values in parenthes passed as arguments to constructor

15. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-15 Constructor Equivalency  Consider:  DayOfYear date1, date2 date1.DayOfYear(7, 4);// ILLEGAL! date2.DayOfYear(5, 5);// ILLEGAL!  Seemingly OK…  CANNOT call constructors like other member functions!

16. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-16 Overloaded Constructors  Can overload constructors just like other functions  Recall: a signature consists of:  Name of function  Parameter list  Provide constructors for all possible argument-lists

17. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-17 Constructor with No Arguments  Can be confusing  Standard functions with no arguments:  Called with syntax: callMyFunction();  Including empty parentheses  Object declarations with no "initializers":  DayOfYear date1; // This way!  DayOfYear date(); // Not this way!

18. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-18 Topic 3: Principles of OOP  Information Hiding  Details of how operations work not known to "user" of class  Data Abstraction  Details of how data is handled within class not known to user  Encapsulation  Bring together data and operations, but keep "details" hidden

19. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-19 Public and Private Members  In C++, Principles of OOP are implemented through the qualifier public and private keywords  Private data in class allows manipulation only via member functions  Public items (usually member functions) can be accessed by other functions.

20. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-20 Public and Private Example  This class definition satisfies principles of OOP class DayOfYear { private: int month; int day; public: void input(); void output(); };  Data now private  Objects have no direct access to private data

21. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-21 Public and Private Example (continued) DayOfYear d; …….. d.month =1; // No longer allowed d.input(); // This is allowed  How data members are handled is hidden

22. Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 6-22 Topic 4: Class Type Member Variables  Class member variables can be any type  Including objects of other classes!