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Chapter 12 Introduction to Classes

Chapter 12 Introduction to Classes. 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

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Chapter 12 Introduction to Classes

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  1. Chapter 12 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 12.10 Constructors That Accept Arguments 12.11 Input Validation Objects 12.12 Overloading Constructors 12.13 Only One Default Constructor and One Destructor 12.14 Arrays of Objects 12.17 An Object-Oriented System Development Primer

  4. 12.1 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. Encapsulation 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 … }

  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 datesas 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.

  12. 12.2 Introduction to Classes • Objects are created from a class (instances of a class) • Format: class <class name> { 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. Example // This program demonstrates a simple class. #include <iostream> using namespace std; // Rectangle class declaration. classRectangle // 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(); }; private data Function Prototypes public functions

  17. Example (Cont.) Scope resolution operator <ReturnType> <ClassName>::<functionName>(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; }

  18. Example (Cont.) // 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; }

  19. Example (Cont.) int main() { Rectanglebox; 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; }

  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

  21. 12.3 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 12-1

  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 << sqPtr->getSide();

  23. 12.4 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 publicset and get functions

  25. 12.5 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. Example Contents of Rectangle.h #ifndef RECTANGLE_H #define RECTANGLE_H // Rectangle class declaration. classRectangle { private: float width; float length; float area; public: void setData(float, float); void calcArea(); float getWidth(); float getLength(); float getArea(); }; #endif

  28. Example Cont. 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; } ...

  29. Defining a Member Function • When defining a member function: • Put the function prototypein 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

  30. Input/Output with Objects • Class should be designed to provide functions to store andretrieve 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

  31. 12.6 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

  32. 12.7 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

  34. 12.8 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 classSquare { private: int side; public: Square(); //constructor prototype ... }; Square::Square() //function header { side = 1; } • Example: Progs 12-4, 12-5

  36. 12.9 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 12-7

  37. 12.10 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 (): Squaresquare1; • To create a constructor that takes arguments: • indicate parameters in prototype( if used ) , definition • provide arguments when object is created: Squaresquare2(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 file, main file

  39. 12.11 Input Validation Objects • Objects can be designed to validate user input: • Acceptable menu choice • Test score in range of valid scores • etc.

  40. 12.12 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

  42. 12.13 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

  43. 12.14 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;

  45. 12.17 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)

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