Classes

1. Classes EECE 351 Spring 2000 Lecture # 11 EECE 351 – Lecture 11

2. Introduction • Overview – What is a class? • The Details – How a class looks? • The Reasoning – Why use a class? • The Method – How do you use a class? EECE 351 – Lecture 11

3. OverviewWhat is a class? • Classes are what put the ++ in C. • A class is: • A user-defined type! • A way to group data with manipulating functions and/or operators (an object). • A type that defines the interface of a particular kind of object. • Also contains the implementation. EECE 351 – Lecture 11

4. OverviewWhat is a class? (cont.) • However, classes are not objects. • Objects are instantiated classes. • Why do SEs like classes? • Classes allow different access levels to internal data. • This means we can hide some of the information from the user. • Data Abstraction – GOOD THING!!! • Classes open up a whole new world. • OOP EECE 351 – Lecture 11

5. OOP Methodology Attributes • Data Abstraction • Inheritance • Polymorphism • Communication Through Messages Object Oriented Programming Data Abstraction Polymorphism Inheritance EECE 351 – Lecture 11

6. Classes (cont.) OOP vs. “Old School” • OOP? • Object Oriented Programming • Code and data that belong together can be combined into objects. • Break the overall task over multiple objects with different data variables and member functions. • “Old school?” • Functional • Code and data are separate. • Divide task into steps each function accomplishes. EECE 351 – Lecture 11

7. OverviewWhat is an object? • Think of an object like this: Chest of Drawers • Contains things you may or may not want others to see. • Sweaters, “tighty-whities” • Properties (Variables) • You can get things in & out of the chest by using a drawer (Method – function). • Good SEs always use the drawers!! EECE 351 – Lecture 11

8. OverviewWhat is an object? (cont.) • Understanding the drawer concept • 1st, Put things in the drawer (Input) • Then, close the drawer and wait (Function CALL) • Finally, wait for drawer to pop open (Answer – what function returns, Output) • Use what you get back any way you want • This drawer concept is DATA ABSTRACTION EECE 351 – Lecture 11

9. The DetailsHow a class looks? • A class has two parts. • Interface • Implementation • The (skeleton) interface looks similar to this: class CTeach //ClassName our “parents” go here too { //Begin interface list public: //Hold on, we’ll get to this in 1 min. CTeach(); //Constructor – 1st thing our class does ~CTeach(); //Destructor – Last thing our class does //Anything else we want };//End interface list ‘;’ is NEEDED HERE! EECE 351 – Lecture 11

10. The DetailsHow a class looks? (cont.) • The (skeleton) implementation looks similar to this: CTeach::CTeach() { //Initialization goes here! Initialization is important } CTeach::~CTeach() { //Kill me code goes here. Usually used for deletes } <return type> CTeach::FuncName(<input type> pName, …) { //How the others look } EECE 351 – Lecture 11

11. Constructors • NEVER have return types (not even void) • Can accept parameters • Begins the life of class members • Called automatically when class is instantiated CTeach::CTeach() //CTeach:: links func with class { //Initialize all members. SetSSN(262890329); //SetX is other class func SetName(“Adrian Michael Nida”); SetDOB(07111976); } EECE 351 – Lecture 11

12. Destructors • NEVER have return types (not even void) • NEVER accept parameters • Ends the life of class members • Called by user orautomatically when scope ends • Usually used to give memory back to OS CTeach::~CTeach() //CTeach:: links func with class { } EECE 351 – Lecture 11

13. The ReasoningWhy use a class? • The class language construct provides a way to link functions with data. • More importantly, the class construct enables specific types of access to the data. • public • Everyone can get to it • protected • Only this and derived classes can access • THERE IS NO NEED TO USE THIS ONE! • private • Only this class has the key EECE 351 – Lecture 11

14. The ReasoningWhy use a class? (cont.) • Access levels let us put things in the chest with us. • Keeps others from getting our stuff • Locked Drawers (private Functions) • Interface goes in the .h file. Implementation goes in the .cpp file. • Member variables should always be private (and have the m_ prefix)! • Create “get” and “set” functions to encapsulate the variables. EECE 351 – Lecture 11

15. The ReasoningWhy use a class? – Gets & Sets • If a class only has private data, how does the user access it? • Implementer “allows” access through certain public functions. • These functions all called: • <type> GetNAME() – Returns current value of NAME; • intSetNAME(<type> pNewVal) – Changes NAME to value of pNewVal. int is used for error checking. • Encapsulation!!! EECE 351 – Lecture 11

16. The ReasoningWhy use a class? – Gets & Sets (cont.) • Why this extra step? • IT WILL SAVE YOU FROM ETERNAL CODING DAMNNATION! • Imagine when something goes wrong… • Why change Millions of NAME = XX; ? • When you can just change 1 SetName? • What could go wrong? • Porting – NT to LINUX (98 to 2000) • Location – feet vs. meters (NASA) • User errors – Grade of -5682 EECE 351 – Lecture 11

17. The Method How do you use a class? • To use the member functions (or variables depending on access level) one must invoke the dot “.” operator. CHuman cMan; //Instantiate an object of type CHuman // Calls the CHuman function “Set” with the 3 parameters. cMan.Set(123456789, “Joe Doe”, 050178); // Attempts illegal access of private data. WILL NOT COMPILE!!! cMan.m_nDOB = 050178; EECE 351 – Lecture 11

18. The Method How do you use a class? (cont.) • However, when a pointer to a class is used, one must invoke the structure pointer deference “->” operator. CHuman * pcMan = new CHuman; //Pointer to new CHuman // Call the “Set” function of what pcMan points to. pcMan->Set(123456789, “Joe Doe”, 050178); // Attempts illegal access of private data of what pcMan points to. WILL NOT COMPILE!!! pcMan->m_nDOB = 050178; delete pcMan; EECE 351 – Lecture 11

19. What we know • Classes • How? • What? • Why? EECE 351 – Lecture 11

20. Where we are going • LAB!!! • How to build a class using MSDEV • Lecture • More classes • so much to know… so little time • Homework • Critical Numbers • Remember the quadratic equation? EECE 351 – Lecture 11