1 Chapter 13 Introduction to Classes

2 Topics 12.1 Procedural and Object-Oriented Programming 12.2 Introduction to Classes 12.3 Defining an Instance of a Class 12.4 Why Have Private Members? 12.5 Software Design Considerations 12.6 Using Private Member Functions 12.7 Inline Member Functions 12.8 Constructors

3 Topics 12.9 Destructors Constructors That Accept Arguments Input Validation Objects Overloading Constructors Only One Default Constructor and One Destructor Arrays of Objects An Object-Oriented System Development Primer

Procedural and Object- Oriented Programming Procedural programming: focuses on the processes/actions that occur in a program Object-Oriented programming: is based on the data and the functions that operate on it. Objects are instances of ADTs that represent the data and its functions

5 Problems with Procedural Programming global variables versus complex function hierarchies Use of global data may allow data corruption Programs based on complex function hierarchies are: difficult to understand and maintain difficult to modify and extend easy to break

6 Object-Oriented Programming Terminology class: like a struct (allows bundling of related variables), but variables and functions in the class can have different properties than in a struct object: an instance of a class, in the same way that a variable can be an instance of a struct

7 Object-Oriented Programming Terminology attributes: member data of a class behaviors: member functions of a class

8 More on Objects data hiding: restricting access to certain data members of an object public interface: members of an object that are available outside of the object. This allows the object to provide access to some data and functions without sharing its internal details and design, and provides some protection from data corruption objects can be general-purpose or application-specific

9 Member Variables float width; float length; float area; Member Functions void setData(float w, float l) { … function code … } void calcArea(void) { … function code … } float getWidth(void) { … function code … } float getLength(void) { … function code … } float getArea(void) { … function code … } Encapsulation

10 Figure 12-2

11 General Purpose Objects Creating data types that are improvements on C++’s built-in data types. For example, an array object could be created that works like a regular array, but additionally provides bounds-checking. ( Vector ) Creating data types that are missing from C++. For instance, an object could be designed to process currencies or dates as if they were built-in data types.The string class provides an alternative to using C-string. Creating objects that perform commonly needed tasks, such as input validation and screen output in a graphical user interface.

Introduction to Classes Objects are created from a class (instances of a class) Format: class { member data declarations member function declarations };

13 Class Example class Square { private: int side; public: void setSide(int s) { side = s; } int getSide() { return side; } };

14 Access Specifiers Used to control access to members of the class public: can be accessed by functions outside of the class private: can only be called by or accessed by functions that are members of the class

15 More on Access Specifiers Can be listed in any order in a class Can appear multiple times in a class If not specified, the default is private

16 // This program demonstrates a simple class. #include using namespace std; // Rectangle class declaration. class Rectangle // class declaration { private:// private member data float width; float length; public:// public member functions void setWidth(float); void setLength(float); float getWidth(); float getLength(); float getArea(); }; Function Prototypes private data public functions Example

17 :: (ParameterList) // setData copies the argument w to private member // width and len to private member length. void Rectangle :: setWidth(float w) { width = w; } void Rectangle :: setLength(float len) { length = len; } Example (Cont.) Scope resolution operator

18 // getWidth returns the value in the private member width. float Rectangle::getWidth(void) { return width; } // getLength returns the value in the private member length. float Rectangle::getLength( ) { return length; } // getArea returns the value in the private member area. float Rectangle::getArea( ) { return length * width; } Example (Cont.)

19 int main( ) { Rectangle box; float rectWidth, rectLength; cout << "This program will calculate the area of a\n"; cout << "rectangle. What is the width? "; cin >> rectWidth; cout << "What is the length? "; cin >> rectLength; box.setWidth(rectWidth); box.setLength(rectLength); cout << "Here is the rectangle's data:\n"; cout << "width: " << box.getWidth() << endl; cout << "length: " << box.getLength() << endl; cout << "area: " << box.getArea() << endl; return 0; } Example (Cont.)

20 Program Output This program will calculate the area of a rectangle. What is the width? 10 [Enter] What is the length? 5 [Enter] Here is the rectangle's data: width: 10 length: 5 area: 50

Defining an Instance of a Class Class objects must to be defined after the class is declared An object is an instance of a class Defined like structure variables: Square sq1, sq2; Access public members using dot operator: sq1.setSide(5); cout << sq2.getSide(); A compiler error is generated if you attempt to access a private member using a dot operator Example: Prog 12-1Prog 12-1

22 Pointer to an Object Can define a pointer to an object: Square *sqPtr; Can access public members via pointer: sqPtr = &sq1; sqPtr->setSide(12); sqPtr = &sq2; sqPtr->setSide(sq1.getSide()); cout getSide();

Why Have Private Members? Making data members private provides data protection Data can be accessed only through public functions Public functions define the class’s public interface

24 Set versus Get Functions Set function: function that stores a value in a private member variable Get function: function that retrieves a value from a private member variable Common class design practice: make all member variables private, provide public set and get functions

Software Design Considerations Place class declaration in a header file that serves as the class specification file. Name the file “classname.h”, for example, square.h Place member function definitions in “ classname.cpp”, for example, square.cpp File should #include the class specification file Programs that use the class must #include the class specification file, and be compiled and linked with the member function definitions

26 Software Design Considerations Usually: class declarations are stored in their own header files. (.h ) Member function definitions are stored in their own source files. (.cpp ) The #ifndef directive allows statements to be conditionally compiled. This prevents a header file from accidentally being included more than once.

27 Contents of Rectangle.h #ifndef RECTANGLE_H #define RECTANGLE_H // Rectangle class declaration. class Rectangle { private: float width; float length; float area; public: void setData(float, float); void calcArea( ); float getWidth( ); float getLength( ); float getArea( ); }; #endif Example

28 Contents of Rectangle.cpp #include “Rectangle.h" // Rectangle Class Declaration // Rectangle Class member function definitions follow // setData copies the argument w to private member width and // l to private member length. void Rectangle::setData(float w, float l) { width = w; length = l; }... Example Cont.

29 Defining a Member Function When defining a member function: Put the function prototype in the class declaration Define the function outside of the class declaration using class name and scope resolution operator (::) int Square :: getSide() { return side; } Example:.h file,.cpp file, main file.h file.cpp filemain file

30 Input/Output with Objects Class should be designed to provide functions to store and retrieve data Functions that use objects of a class should perform input/output operations, not class member functions There can be exceptions to these rules: a class can be designed to display a menu, or specifically to perform I/O

Using Private Member Functions A private member function can only be called by another member function It is used for internal processing by the class, not for use outside of the class A private member function is also referred to as a utility function Example:.h file,.cpp file, main file.h file.cpp filemain file

Inline Member Functions Member functions can be defined in the class declaration ( inline ) after the class declaration Inline member functions are appropriate for short function bodies: int getSide() { return side; }

33 Tradeoffs: Inline vs. Regular Member Functions Regular functions: when called, compiler stores return address of call, allocates memory for local variables, etc. Inline functions: Code for an inline function is copied into program in place of call – larger executable program, but no function call overhead, hence faster execution

Constructors Member function that is called when an object is created Called automatically Constructor function name is class name Has no return type ( not even void )

35 Constructor Example class Square { private: int side; public: Square(); //constructor prototype... }; Square::Square() //function header { side = 1; } Example: Progs 12-4,

Destructors Member function automatically called when an object is destroyed Destructor name is ~ classname, e.g., ~Square ~ character is called the tilde Has no return type; takes no arguments Only 1 destructor per class, i.e., it cannot be overloaded If constructor allocates dynamic memory, destructor should release it Example: Prog

Constructors That Accept Arguments Default constructor: constructor that takes no arguments To create an object using the default constructor, use no argument list and no () : Square square1; To create a constructor that takes arguments: indicate parameters in prototype( if used ), definition provide arguments when object is created: Square square2(12);

38 Constructors That Accept Arguments Constructor may have default arguments: Square(int = 1); // prototype Square::Square(int s) // heading { side = s; } Example:.h file, main file,.h filemain file.h file, main file.h filemain file

Input Validation Objects Objects can be designed to validate user input: Acceptable menu choice Test score in range of valid scores etc.

Overloading Constructors A class can have more than 1 constructor Overloaded constructors ( like any overloaded function ) in a class must have different parameter lists: Square(); Square(int);

41 Member Function Overloading Non-constructor member functions can also be overloaded: void setSide(); void setSide(int); Must have unique parameter lists as for constructors

Only One Default Constructor and One Destructor Do not provide > 1 default constructor for a class: one that takes no arguments and one that has default arguments for all parameters Square(); Square(int = 0); // will not compile Since a destructor takes no arguments, there can only be one destructor for a class

Arrays of Objects Objects can be the elements of an array: Square lottaSquares[10]; Default constructor for object is used when array is defined Must use initializer list to invoke constructor that takes arguments: Square triSqu[3] = {5,7,11}; More complex initialization if constructor takes > 1 argument

44 Accessing Objects in an Array Objects in an array are referenced using subscripts Member functions are referenced using dot notation: lottaSquares[3].setSide(6); cout << triSqu[i].getSide();

An Object-Oriented System Development Primer Procedural Programming: program is made up of procedures: sets of programming statements that perform tasks Object-Oriented Programming: program is made up of objects: entities that contain data (attributes) and actions (methods)

46 Benefits of Object-Oriented Programming Simplification of software development for graphical (GUI) applications visual components (menus, text boxes, etc.) modeled as objects visual components (e.g., windows) may be made up of multiple objects some objects (e.g., buttons) may have methods associated with them

47 Benefits of Object-Oriented Programming Simplification of software development for non-GUI applications - the problem: procedural programming enforces separation between code and data changes in data format require extensive analysis, testing to ensure program functionality

48 Benefits of Object-Oriented Programming Simplification of software development for non-GUI applications - a solution: OO programming addresses the problem through encapsulation: combination of data and actions in an object data hiding: protection of an object’s data by its public member functions

49 Component Reusability Component: a software object that performs a well-defined task or that provides a service Component Reusability: the ability to use a component in multiple programs without (or with little) modification

50 Relationships Between Objects A program may contain objects of different classes Objects may be related by one of the following: Access (‘knows’ relationship) Ownership (‘has a’ relationship) Inheritance (‘is a’ relationship)

51 Messages and Polymorphism Message: request to an object to perform a task. Implemented as a member function call Polymorphism: ability to take many forms. Sending the same message to different objects may produce different behaviors

52 Object Oriented Analysis Used to create the logical design of a system, what the system is to do Usually includes examine the problem domain; model the system from within that perspective identify the objects that exist within the boundaries of that system identify the relationships between the objects realize that there may be many ‘right’ solutions

53 Object Oriented Design Used to determine how requirements from OO Analysis will be implemented Usually includes determine hierarchical relation between objects determine object ownership of attributes implement and test; refine as required