1. C++ OOP :: Class Structure 29/09/2009 1 Hadziq Fabroyir - Informatics ITS

2. C++ Structures: Classes In C++, the concept of structure has been generalized in an object-oriented sense: classes are types representing groups of similar instances each instance has certain fields that define it (instance variables) instances also have functions that can be applied to them (represented as function fields) -- called methods the programmer can limit access to parts of the class (to only those functions that need to know about the internals) 29/09/2009 2 Hadziq Fabroyir - Informatics ITS

3. A Motivating Example You declare a simple structure: struct Robot { float locX; float locY; float facing; }; Robot r1; What if you never want locX or locY to be negative ? Someone unaware of your restriction could set: r1.locX = -5; and you have a problem ! 29/09/2009 3 Hadziq Fabroyir - Informatics ITS

4. General Class Definition in C++ class ClassName { access_modifier1: field11_definition; field12_definition; … access_modifer2: field21_definition; field22_definition; … }; ClassName is a new type (just as if declared as struct with typedef in C) Possible access modifiers: public private Protected Fields can be variables (as in C structures) or functions 29/09/2009 4 Hadziq Fabroyir - Informatics ITS

5. A Simple Class class Robot { public: float getX() { return locX; } float getY() { return locY; } float getFacing() { return facing; } void setFacing(float f) { facing = f; } void setLocation(float x, float y); private: float locX; float locY; float facing; }; 29/09/2009 5 Hadziq Fabroyir - Informatics ITS

6. private Access Modifier Fields marked as private can only be accessed by functions that are part of that class In the Robot class, locX, locY, and facing are private float fields, these fields can only be accessed by functions that are in class Robot (getX, getY, getFacing, setFacing, setLocation) Example: void useRobot() { Robot r1; r1.locX = -5; // Error 29/09/2009 6 Hadziq Fabroyir - Informatics ITS

7. publicAccess Modifier Fields marked as public can be accessed by anyone In the Robot class, the methods getX, getY, etc. are public these functions can be called by anyone Example: void useRobot() { Robot r1; r1.setLocation(-5,-5); // Legal to call 29/09/2009 7 Hadziq Fabroyir - Informatics ITS

8. structVS class In C++ struct and class can be used interchangeably to create a class with one exception What if we forget to put an access modifier before the first field? struct Robot { OR class Robot { float locX; float locX; In a class, until an access modifer is supplied, the fields are assumed to beprivate In a struct, the fields are assumed to bepublic 29/09/2009 8 Hadziq Fabroyir - Informatics ITS

9. What about protected? C++ allows users to create classes based on other classes: a FordCar class based on a general Car class idea: the general Car class has fields (variables and functions that describe all cars) the FordCar class then uses the fields from the general Car class and adds fields specific to FordCars done with inheritance public fields are inherited (as public) private fields are not inherited protected fields are like private, but can be inherited 29/09/2009 9 Hadziq Fabroyir - Informatics ITS

10. Class Methods Functions associated with a class are declared in one of two ways: ReturnType FuncName(params) { code } function is both declared and defined (code provided) ReturnType FuncName(params); function is merely declared, we must still define the body of the function separately To call a method we use the . form: classinstance.FuncName(args); FuncName is a field just like any other field in the structured variable classinstance 29/09/2009 10 Hadziq Fabroyir - Informatics ITS

11. Defined Methods class Robot { public: float getX() { return locX; } }; Robot r1; The function getX is defined as part of class Robot To call this method: cout << r1.getX() << endl; // prints r1’s locX 29/09/2009 11 Hadziq Fabroyir - Informatics ITS

12. Defining Methods Separately For methods that are declared but not defined in the class we need to provide a separate definition To define the method, you define it as any other function, except that the name of the function is ClassName::FuncName :: is the scope resolution operator, it allows us to refer to parts of a class or structure 29/09/2009 12 Hadziq Fabroyir - Informatics ITS

13. A Simple Class class Robot { public: void setLocation(float x, float y); private: float locX; float locY; float facing; }; void Robot::setLocation(float x, float y) { if ((x < 0.0) || (y < 0.0)) cout << “Illegal location!!” << endl; else { locX = x; locY = y; } } 29/09/2009 13 Hadziq Fabroyir - Informatics ITS

14. Types of Class Methods Generally we group class methods into three broad categories: accessors - allow us to access the fields of a class instance (examples: getX, getY, getFacing), accessors do not change the fields of a class instance mutators - allow us to change the fields of a class instance (examples: setLocation, setFacing), mutators do change the fields of a class instance manager functions- specific functions (constructors, destructors) that deal with initializing and destroying class instances (more in a bit) 29/09/2009 14 Hadziq Fabroyir - Informatics ITS

15. Accessors & Mutators Why do we bother with accessors and mutators? Why provide getX()?? Why not just make the locX field publicly available? By restricting access using accessors and mutators we make it possible to change the underlying details regarding a class without changing how people who use that class interact with the class Example: what if I changed the robot representation so that we no longer store locX and locY, instead we use polar coordinates (distance and angle to location)? If locX is a publicly available field that people have been accessing it, removing it will affect their code If users interact using only accessors and mutators then we can change things without affecting their code 29/09/2009 15 Hadziq Fabroyir - Informatics ITS

16. Rewritten Robot Class class Robot { public: float getX() { return dist * cos(ang); } float getY() { return dist * sin(ang); } void setLocation(float x, float y); private: float dist; float ang; }; void Robot::setLocation(float x, float y) { if ((x < 0.0) || (y < 0.0)) cout << “Illegal location!!” << endl; else { dist = sqrt(x * x + y * y); ang = atan2(y,x); } } 29/09/2009 16 Hadziq Fabroyir - Informatics ITS

17. Object-Oriented Idea Declare implementation-specific fields of class to be private Provide public accessor and mutator methods that allow other users to connect to the fields of the class often provide an accessor and mutator for each instance variable that lets you get the current value of that variable and set the current value of that variable to change implementation, make sure accessors and mutators still provide users what they expect 29/09/2009 17 Hadziq Fabroyir - Informatics ITS

18. Static Instance Variables C++ classes may also contain, static instance fields -- a single field shared by all members of a class Often used when declaring class constants (since you generally only need one copy of a constant) To make a field static, add the static keyword in front of the field can refer to the field like any other field (but remember, everybody shares one copy) static variables are also considered to be global, you can refer to them without an instance static fields can be initialized (unlike other fields) 29/09/2009 18 Hadziq Fabroyir - Informatics ITS

19. Static Fields Example class Robot { public: static int numRobots = 0; static const float minX = 0.0; static const float maxX = 100.0; void initializeRobot() { numRobots++; } void setLocation(float x, float y); } void Robot::setLocation(float x, float y) { if ((x < minX) || (x > maxX)) … } 29/09/2009 19 Hadziq Fabroyir - Informatics ITS

20. Static Fields as Global Variables One can examine public static fields of a class outside of that class using the form: ClassName::StaticFieldName Example: void main() { Robot r1, r2; r1.initializeRobot(); r2.initializeRobot(); cout << Robot::numRobots << “ robots.”; 29/09/2009 20 Hadziq Fabroyir - Informatics ITS

21. The Need for Manager Functions To make the previous program work we have to explicitly call the routine initializeRobot, wouldn’t it be nice to have a routine that is automatically used to initialize an instance -- C++ does C++ provides for the definition of manager functions to deal with certain situations: constructors (ctor) - used to set up new instances default - called for a basic instance copy - called when a copy of an instance is made (assignment) other - for other construction situations destructors (dtor) - used when an instance is removed 29/09/2009 21 Hadziq Fabroyir - Informatics ITS

22. When Managers Called void main() { Robot r1; // default ctor (on r1) Robot r2; // default ctor (on r2) Robot* r3ptr; r3ptr = new Robot; // default ctor on // Robot object r3ptr points to r2 = r1; // copy ctor delete r3ptr; // dtor } // dtor (on r1 and r2) when main ends 29/09/2009 22 Hadziq Fabroyir - Informatics ITS

23. The Default Constructor (ctor) A default constructor has no arguments, it is called when a new instance is created C++ provides a default ctor that initializes all fields to 0 To write your own, add following to your class: class MyClass { public: … MyClass() { // repeat class name, no code here // return type } } 29/09/2009 23 Hadziq Fabroyir - Informatics ITS

24. Example Default ctor class Robot { public: static int numRobots = 0; Robot() { numRobots++; locX = 0.0; locY = 0.0; facing = 3.1415 / 2; } private: float locX; float locY; float facing; } 29/09/2009 24 Hadziq Fabroyir - Informatics ITS

25. Destructor (dtor) A destructor is normally not critical, but if your class allocates space on the heap, it is useful to deallocate that space before the object is destroyed C++ provides a default dtor that does nothing can only have one dtor To write your own, add following to your class: class MyClass { public: … ~MyClass() { code here } } 29/09/2009 25 Hadziq Fabroyir - Informatics ITS

26. Example dtor class Robot { public: char *robotName; Robot() { robotName = 0; } void setRobotName(char *name) { robotName = new char[strlen(name)+1]; strcpy(robotName,name); } ~Robot() { delete [] robotName; } } 29/09/2009 26 Hadziq Fabroyir - Informatics ITS

27. The Copy ctor A copy constructor is used when we need a special method for making a copy of an instance example, if one instance has a pointer to heap-allocated space, important to allocate its own copy (otherwise, both point to the same thing) To write your own, add following to your class: class MyClass { public: … MyClass(const MyClass& obj) { code here } } 29/09/2009 27 Hadziq Fabroyir - Informatics ITS

28. Example Copy ctor class Robot { public: char *robotName; void setRobotName(char *name) { robotName = new char[strlen(name)+1]; strcpy(robotName,name); } Robot(const Robot& obj) { robotName = new char[strlen(obj.robotName)+1]; strcpy(robotName,obj.robotName); } } 29/09/2009 28 Hadziq Fabroyir - Informatics ITS

29. Other ctors It is often useful to provide constructors that allow the user to provide arguments in order to initialize arguments Form is similar to the copy ctor, except parameters are chosen by programmer: class MyClass { public; … MyClass(parameters) { code here } } 29/09/2009 29 Hadziq Fabroyir - Informatics ITS

30. Example ctor class Robot { public: Robot(float x, float y, float face) { locX = x; locY = y; facing = face; } } calling: Robot r1(5.0,5.0,1.5); Robot r2(5.0,10.0,0.0); Robot* rptr; rptr = new Robot(10.0,5.0,-1.5); 29/09/2009 30 Hadziq Fabroyir - Informatics ITS

31. A Combination ctor Can combine a ctor that requires arguments with the default ctor using default values: class Robot { public: Robot(float x = 0.0, float y = 0.0, float face = 1.57075) { locX = x; locY = y; facing = face; } } calling: Robot r1; // ctor called with default args Robot r2(); // ctor called with default args Robot r3(5.0); // ctor called with x = 5.0 Robot r4(5.0,5.0); // ctor called with x,y = 5.0 … 29/09/2009 31 Hadziq Fabroyir - Informatics ITS

32. Hiding the Default ctor Sometimes we want to make sure that the user gives initial values for some fields, and we don’t want them to use the default ctor To accomplish this we declare an appropriate ctor to be used in creating instances of the class in the public area, then we put the default ctor in the private area (where it cannot be called) 29/09/2009 32 Hadziq Fabroyir - Informatics ITS

33. Example ctor class Robot { public: Robot(float x, float y, float face) { locX = x; locY = y; facing = face; } private: Robot() {} } calling: Robot r1(5.0,5.0,1.5); Robot r2; // ERROR, attempts to call default ctor 29/09/2009 33 Hadziq Fabroyir - Informatics ITS

34. For your practice … Exercise 9.5 Complex Class Properties: float realPart, float imaginaryPart Method (Mutators): Complex addBy(Complex), ComplexSubstractBy(Complex) Lab Session for upper order (Senin, 15.00-17.00) Lab Session for lower order (Senin, 19.00-21.00) Please provide: the softcopy(send it by email – deadline Sunday 23:59) the hardcopy(bring it when attending the lab session) 29/09/2009 Hadziq Fabroyir - Informatics ITS 34

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