Object Oriented Programming Examples with explanation

1 • A pointer variable is a variable whose value is the address of a location in memory int x; x = 5; int* ptr1; ptr1 = &x; int* ptr2; ptr2 = ptr1; *ptr1 = 6; cout << ptr1 << endl; cout << *ptr2 << endl; Review: Pointers & Dynamic Data int* ptr3; ptr3 = new int; *ptr3 = 5; delete ptr3; ptr3 = NULL; int *ptr4; ptr4 = new int[5]; ptr4[0] = 100; ptr4[4] = 123; delete [] ptr4; ptr4 = NULL;

2 void increment(int b1, int &b2, int *b3) { b1 += 2; b2 += 2 *b3 += 2; } Review: Reference Types • Reference Types – Alias for another variable – Must be initialized when declared – Are primarily used as function parameters int main (void){ int a1 = 5, a2 = 10; int *a3 = new int; *a3 = 15; int &a4 = a3; cout << a1 << a2 << a3 << endl; increment(a1, a2, a3); cout << a1 << a2 << a3 << endl; delete a3; a3 = NULL; return 0; }

3 Object-Oriented Programming Introduction to Classes • Class Definition • Class Examples • Objects • Constructors • Destructors

4 Class • The class is the cornerstone of C++ – It makes possible encapsulation, data hiding and inheritance • Type – Concrete representation of a concept • Eg. float with operations like -, *, + (math real numbers) • Class – A user defined type – Consists of both data and methods – Defines properties and behavior of that type • Advantages – Types matching program concepts • Game Program (Explosion type) – Concise program – Code analysis easy – Compiler can detect illegal uses of types • Data Abstraction – Separate the implementation details from its essential properties

5 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); }; Classes & Objects Rectangle r1; Rectangle r2; Rectangle r3; …… int a; Objects: Instance of a class

6 Define a Class Type class class_name { permission_label: member; permission_label: member; ... }; class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); }; Body Header

7 Class Definition Data Members • Can be of any type, built-in or user-defined • non-static data member – Each class object has its own copy • static data member – Acts as a global variable – One copy per class type, e.g. counter

8 class Rectangle { private: int width; int length; static int count; public: void set(int w, int l); int area(); } Static Data Member Rectangle r1; Rectangle r2; Rectangle r3; width length width length width length r1 r3 r2 count

9 Class Definition Member Functions • Used to – access the values of the data members (accessor) – perform operations on the data members (implementor) • Are declared inside the class body • Their definition can be placed inside the class body, or outside the class body • Can access both public and private members of the class • Can be referred to using dot or arrow member access operator

10 Define a Member Function class Rectangle { private: int width, length; public: void set (int w, int l); int area() {return width*length; } }; void Rectangle :: set (int w, int l) { width = w; length = l; } inline class name member function name scope operator r1.set(5,8); rp->set(8,10);

11 • const member function – declaration • return_type func_name (para_list) const; – definition • return_type func_name (para_list) const { … } • return_type class_name :: func_name (para_list) const { … } – Makes no modification about the data members (safe function) – It is illegal for a const member function to modify a class data member Class Definition Member Functions

12 Const Member Function class Time { private : int hrs, mins, secs ; public : void Write ( ) const ; } ; void Time :: Write( ) const { cout <<hrs << “:” << mins << “:” << secs << endl; } function declaration function definition

13 • Information hiding – To prevent the internal representation from direct access from outside the class • Access Specifiers – public • may be accessible from anywhere within a program – private • may be accessed only by the member functions, and friends of this class – protected • acts as public for derived classes • behaves as private for the rest of the program Class Definition - Access Control

14 class Time Specification class Time { public : void Set ( int hours , int minutes , int seconds ) ; void Increment ( ) ; void Write ( ) const ; Time ( int initHrs, int initMins, int initSecs ) ; // constructor Time ( ) ; // default constructor private : int hrs ; int mins ; int secs ; } ; 14

15 Class Interface Diagram Private data: hrs mins secs Set Increment Write Time Time Time class

16 • The default access specifier is private • The data members are usually private or protected • A private member function is a helper, may only be accessed by another member function of the same class (exception friend function) • The public member functions are part of the class interface • Each access control section is optional, repeatable, and sections may occur in any order Class Definition Access Control

17 What is an object? OBJECT Operations Data set of methods (member functions) internal state (values of private data members)

18 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); }; Declaration of an Object main() { Rectangle r1; Rectangle r2; r1.set(5, 8); cout<<r1.area()<<endl; r2.set(8,10); cout<<r2.area()<<endl; }

19 Another Example #include <iostream.h> class circle { private: double radius; public: void store(double); double area(void); void display(void); }; // member function definitions void circle::store(double r) { radius = r; } double circle::area(void) { return 3.14*radius*radius; } void circle::display(void) { cout << “r = “ << radius << endl; } int main(void) { circle c; // an object of circle class c.store(5.0); cout << "The area of circle c is " << c.area() << endl; c.display(); }

20 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); }; Declaration of an Object main() { Rectangle r1; r1.set(5, 8); } r1 is statically allocated width length r1 width = 5 length = 8

21 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); }; Declaration of an Object main() { Rectangle r1; r1.set(5, 8); Rectangle *r2; r2 = &r1; r2->set(8,10); } r2 is a pointer to a Rectangle object width length r1 width = 5 length = 8 5000 ??? r2 6000 5000 width = 8 length = 10 //dot notation //arrow notation

22 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); }; Declaration of an Object main() { Rectangle *r3; r3 = new Rectangle(); r3->set(80,100); delete r3; r3 = NULL; } r3 is dynamically allocated ??? r3 6000 width length 5000 5000 width = 80 length = 100 ??? NULL //arrow notation

23 #include <iostream.h> class circle { public: double radius; }; Object Initialization int main() { circle c1; // Declare an instance of the class circle c1.radius = 5; // Initialize by assignment } 1. By Assignment • Only work for public data members • No control over the operations on data members

24 #include <iostream.h> class circle { private: double radius; public: void set (double r) {radius = r;} double get_r () {return radius;} }; int main(void) { circle c; // an object of circle class c.set(5.0); // initialize an object with a public member function cout << "The radius of circle c is " << c.get_r() << endl; // access a private data member with an accessor } Object Initialization 2. By Public Member Functions

25 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); } Declaration of an Object main() { Rectangle r1; r1.set(5, 8); Rectangle *r2; r2 = &r1; r2->set(8,10); } r2 is a pointer to a Rectangle object //dot notation //arrow notation r1 and r2 are both initialized by public member function set

26 class Rectangle { private: int width; int length; public: Rectangle(); Rectangle(const Rectangle &r); Rectangle(int w, int l); void set(int w, int l); int area(); } Object Initialization 3. By Constructor • Default constructor • Copy constructor • Constructor with parameters There is no return type Are used to initialize class data members Have the same name as the class They are publicly accessible They have different signatures

27 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); }; Object Initialization • Default constructor When a class is declared with no constructors, the compiler automatically assumes default constructor and copy constructor for it. Rectangle :: Rectangle() { }; • Copy constructor Rectangle :: Rectangle (const Rectangle & r) { width = r.width; length = r.length; };

28 class Rectangle { private: int width; int length; public: void set(int w, int l); int area(); } Object Initialization • Initialize with default constructor Rectangle r1; Rectangle *r3 = new Rectangle(); • Initialize with copy constructor Rectangle r4; r4.set(60,80); Rectangle r5 = r4; Rectangle r6(r4); Rectangle *r7 = new Rectangle(r4);

29 class Rectangle { private: int width; int length; public: Rectangle(int w, int l) {width =w; length=l;} void set(int w, int l); int area(); } Object Initialization If any constructor with any number of parameters is declared, no default constructor will exist, unless you define it. Rectangle r4; // error • Initialize with constructor Rectangle r5(60,80); Rectangle *r6 = new Rectangle(60,80);

30 class Rectangle { private: int width; int length; public: Rectangle(); Rectangle(int w, int l); void set(int w, int l); int area(); } Object Initialization Write your own constructors Rectangle :: Rectangle() { width = 20; length = 50; }; Rectangle *r7 = new Rectangle(); width length width = 20 length = 50 5000 ??? r7 6000 5000

31 class Account { private: char *name; double balance; unsigned int id; public: Account(); Account(const Account &a); Account(const char *person); } Object Initialization With constructors, we have more control over the data members Account :: Account() { name = NULL; balance = 0.0; id = 0; }; Account :: Account(const Account &a) { name = new char[strlen(a.name)+1]; strcpy (name, a.name); balance = a.balance; id = a.id; }; Account :: Account(const char *person) { name = new char[strlen(person)+1]; strcpy (name, person); balance = 0.0; id = 0; };

32 So far, … • An object can be initialized by a class constructor – default constructor – copy constructor – constructor with parameters • Resources are allocated when an object is initialized • Resources should be revoked when an object is about to end its lifetime

33 Cleanup of An Object class Account { private: char *name; double balance; unsigned int id; //unique public: Account(); Account(const Account &a); Account(const char *person); ~Account(); } Destructor Account :: ~Account() { delete[] name; } • Its name is the class name preceded by a ~ (tilde) • It has no argument • It is used to release dynamically allocated memory and to perform other "cleanup" activities • It is executed automatically when the object goes out of scope

34 Putting Them Together class Str { char *pData; int nLength; public: //constructors Str(); Str(char *s); Str(const Str &str); //accessors char* get_Data(); int get_Len(); //destructor ~Str(); }; Str :: Str() { pData = new char[1]; *pData = ‘0’; nLength = 0; }; Str :: Str(const Str &str) { int n = str.nLength; pData = new char[n+1]; nLength = n; strcpy(pData,str.pData); }; Str :: Str(char *s) { pData = new char[strlen(s)+1]; strcpy(pData, s); nLength = strlen(s); };

35 Putting Them Together class Str { char *pData; int nLength; public: //constructors Str(); Str(char *s); Str(const Str &str); //accessors char* get_Data(); int get_Len(); //destructor ~Str(); }; char* Str :: get_Data() { return pData; }; Str :: ~Str() { delete[] pData; }; int Str :: get_Len() { return nLength; };

36 Putting Them Together class Str { char *pData; int nLength; public: //constructors Str(); Str(char *s); Str(const Str &str); //accessors char* get_Data(); int get_Len(); //destructor ~Str(); }; int main() { int x=3; Str *pStr1 = new Str(“Joe”); Str *pStr2 = new Str(); }

37 Interacting Objects Class A Class B Private: data members Private: data members Member methods Member methods Constructor Other public methods Public: Destructor Constructor Other public methods Public: Destructor Message passing Private: methods Private: methods

38 Working with Multiple Files • To improve the readability, maintainability and reusability, codes are organized into modules. • When working with complicated codes, – A set of .cpp and .h files for each class groups • .h file contains the prototype of the class • .cpp contains the definition/implementation of the class – A .cpp file containing main() function, should include all the corresponding .h files where the functions used in .cpp file are defined

39 Example : time.h // SPECIFICATION FILE ( time .h ) // Specifies the data members and // member functions prototypes. #ifndef _TIME_H #define _TIME_H class Time { public: . . . private: . . . } ; #endif

40 // IMPLEMENTATION FILE ( time.cpp ) // Implements the member functions of class Time #include <iostream.h> #include “ time.h” // also must appear in client code … … bool Time :: Equal ( Time otherTime ) const // Function value == true, if this time equals otherTime // == false , otherwise { return ( (hrs == otherTime.hrs) && (mins == otherTime.mins) && (secs == otherTime.secs) ) ; } . . . Example : time.cpp

41 Example : main.cpp // Client Code ( main.cpp ) #include “ time.h” // other functions, if any int main() { … … } Compile and Run g++ -o mainExec main.cpp time.cpp

42 Separate Compilation and Linking of Files time.h main.cpp time.cpp main.o mainExec time.o Compiler Compiler Linker #include “time.h” implementation file specification file main program

Object Oriented Programming Examples with explanation

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