OBJECTS. An object is an entity around us, perceivable through our senses. Types of Object: Objects that operate independently.

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1 OBJECTS An object is an entity around us, perceivable through our senses. Types of Object: Objects that operate independently. Objects that work in associations with each others. Objects that frequently interact with each other. Objects that are contained in other objects.

2 CLASSES AND OBJECTS IN C++

3 Characteristics of Object 1. Identity: It is the name associated with an object. 2. State: An object can be in many states. 3. Behavior: What the object does or what is it capable of doing?

4 CLASSES Human has a very strong ability to categorize objects. In Object Oriented Programming, a category is called a class. The objects belonging to a class may have different states but their behaviors remain the same.

5 A class defines common states and behaviours of a group of objects. Pictorially a class is represented as: Class Name States Behavior Pitcher Upright /Upside down Full /Empty Broken Material clay Colour brown Can be filled Can be emptied Can be transported Can be broken

6 For a given problem following steps are followed: Identify the objects that comprise the problem. Categorize the objects into classes. Establish the relationship between the classes. Create instances of objects as per the requirement Manage the objects to solve the problem

7 DECLARATION OF CLASSES IN class <name > {private : variable declarations function declarations public : variable declarations function declarations }; C++ where class : is a keyword <name >: is the user defined name of the class. It is a C++ identifier. private : is a keyword public : is a keyword

8 Class declaration is similar to a structure declaration. Therefore, everything related to structure applies to classes. Structure is open and accessible to all whereas the access to members of the class is rather restricted. The class has clearly two sections: Private Public Third special section available in a class called protected.

9 Private members The members declared in private section are not accessible outside the class. The states (i.e. variables) are always made private. The reason being, that the internal state of an object should only be changed by an internal behavior or function and not by outside stimuli. This feature of hiding states and data structures is called data or information hiding.

10 Public members The functions that need to interact with outside world are made public. Functions interact with the outside world for the following purposes: Taking input from users or other objects. Providing information about states of the object to users. Providing services offered by the object to users or other objects.

11 Data and implementation hiding behind public interface

12 Abstraction and Encapsulation Abstraction: The activity of separating essential members of a class in the form of public interface is known as abstraction. It provides a simple view of the object so that the user can easily use the object. Encapsulation: The activity of hiding data and implementation within a class as a unit is called as encapsulation. It creates a separation between the interface and the implementation thereby reducing the dependency between the components of the system.

13 Member Function Definition The member functions can be defined inside as well as outside the class declaration. //EXAMPLE OF MEMBER FUNCTION DEFINITION INSIDE CLASS DECLARATION class test { private: int val; public : void readvar() { cin >>val; } void dispvar() { cout <<val; } };

14 //EXAMPLE OF MEMBER FUNCTION DEFINITION OUTSIDE A CLASS DECLARATION class test { private: int val; public : void readvar(); void dispvar(); }; void test :: readvar() { cin >>val; } void test :: dispvar() { cout <<val; } The operator `::' is known as scope resolution operator. This is used to associate member functions to their corresponding class.

15 The format of a member function declaration is : type class_name :: function_name (parameter list) { } function body

16 CREATING OBJECTS The syntax for defining an object is: <class_name> <object_name>; where <class_name> <object_name> : is the name of the class : is the name of the object being defined. For example, we can create an object studobj of class student by the following declaration : student studobj; The above declaration says that studobj is an object of class student.

17 C++ allows assignment of objects of same type. class xyz { // CLASS DECLARATION : }; xyz obj1, obj2; : obj2 = obj1; // OBJECT CREATION // OBJECT ASSIGNMENTS

18 Calling Member Functions Once an object has been created, its member functions (public) can be called in the program with the help of dot operator. studob.read_data();

19 ARRAY OF OBJECTS // This program reads a list of students and prints their grade # include <iostream.h> # include <conio.h> class student { private: char name[20]; int roll; int marks; char grade; void compute_grade(); public: void read_data(); void display_grade(); };

20 // Function to read data of a student void student:: read_data() { cout <<"\n Enter Name:"; cin >> name; cout << "\n Enter Roll:"; cin >> roll; cout << "\n Enter Marks:"; cin >> marks; compute_grade(); // compute the grade } void student:: display_grade() { cout<< "\n" << name; cout << "\t" << roll; cout << "\t" <<grade; }

21 void student::compute_grade() { if (marks >= 80) grade = 'A'; else if (marks >=70 ) grade ='B'; else if (marks >=60) grade ='C'; else if (marks >=50) grade ='D'; else grade ='E'; }

22 void main() { int n; student stud_list[50]; cout <<"\nenter THE NUMBER OF STUDENT"; cin >>n; for(int i=0;i <n;i++) stud_list[i].read_data(); clrscr(); cout <<"\n LIST OF STUDENTS: \n"; cout <<"\nname\t ROLLNO\t GRADE"; for( i=0;i <n;i++) stud_list[i].display_grade(); }

23 OBJECTS AS FUNCTION ARGUMENTS C++ considers objects as built-in data type and therefore, an object can be passed to a function as an argument. 1. Pass by value: In pass by value method of argument passing, the objects are passed to functions by value. A copy of the object is received by the called function, not the actual object.

24 The object being passed by pass by value method

25 2. Pass by Reference: In this method the address of an object is passed to the function as an argument. The address of the object is represented by preceding the object name with the character `&'.

26 Pass by reference sob & studob refer to the same object

27 Pass by value void test (student sob) { sob.read_data(); cout <<"\n Display in test()"; sob.display_grade(); } void main() { student studob; clrscr(); test (studob); // call test() by pass by value cout <<"\n \n Display in main()"; studob.display_grade(); }

28 Pass by reference void test (student &sob) { sob.read_data(); cout <<"\n Display in test()"; sob.display_grade(); } // pass by reference void main() { student studob; clrscr(); test (studob); // call test() by pass by value cout <<"\n \n Display in main()"; studob.display_grade(); }

29 SCOPE RESOLUTION OPERATOR The scope resolution operator is a double colon (::) used to connect the definition of functions to their classes. The general form of usage of this operator is shown below: <returntype><class_name>::<function _name > (parameter list)

30 We can use the scope resolution operator (::) on the hidden global variable to make it accessible inside the class as shown below: int val; // global variable int count; class xyz { int val; // local variable public: void in_val() { :: val = ++ val; cout < < \n THE LOCAL VAL = < < val; cout < <}\n THE GLOBAL VAL =" < < :: val; } }

31 The major difference between scope resolution operator and the dot operator in context of classes and objects is : The dot operator is used to specify a member of an object whereas the scope resolution operator is used to specify the class name while defining a member function of the class. The scope resolution operator is also used to make hidden global variable visible inside a class or a block.

32 Static data members A static variable is shared among all instantiated objects of the class. A static variable, declared within a function of a class has following characteristics: The static variable is initialized to zero at the time of creation of first object of the class. It is shared by all instances of the class. The scope of the static variable is within the function of the class Its life time starts with creation of first object and ends with destruction of last object of the class.

33 PROPERTIES OF CLASSES And Properties of classes are: OBJECTS Classes consist of both data (variables) and functions. Since a class is only processed at compiler level, no variable can be initialized inside a class declaration. Classes allow private, protected, and public members. A member function defined inside the class is by default an inline function. A member of a class is by default a private member unless otherwise specified. A private member is not available outside the class. However, a member function can always access such a member. Objects can be passed as parameters to functions. Objects can be returned from a function.

34 The general properties of objects are: Objects are alive: because an object comes into existence. It gets the resources like memory. It has a lifetime. Objects are active: because an object provides services through its interface. Objects are intelligent: because every object has the coded logic to provide a service or to solve the problem at hand.

Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat

Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat Classes Chapter 4 Classes and Objects Data Hiding and Encapsulation Function in a Class Using Objects Static Class members Classes Class represents a group of Similar objects A class is a way to bind the