C++ Objects and Classes

In object-oriented programming languages like C++, the data and functions (procedures to manipulate the data) are packed together as a self-contained unit called a class. This tutorial explains how to use C++ classes.

Class is an extended concept similar to the struct in the C programming language. In C++, a class describes objects' properties (data) and behaviors (functions). Classes are not objects, but they are used to instantiate objects.

What is a Class in C++?

• Class is an abstract data type similar to the C structure.
• A class is a representation of objects and some set of operations that can apply to such objects.
• The class consists of Data members and methods.
• The primary purpose of a class is to hold data/information. This is achieved with attributes which are also known as data members.
• The member functions determine the class's behavior, i.e., provide a definition for supporting various operations on data held in the form of an object.

Class Definition

Syntax:

Class class_name
{
    Data Members;
    Methods;
}

Example:

class A
{
    public:
    double length; // Length of a box
    double breadth; // Breadth of a box
    double height; // Height of a box 
}

• private, protected, and public is called visibility labels.
• The members that are declared private can be accessed only from within the class.
• Public members can be accessed from outside the class also.
• In C++, data can be hidden by making it private.

Class Members

Data and functions are members. Data Members and methods must be declared within the class definition.

Example:

Class A
{
    int i; // i is a data member of class A
    int j; // j is a data member of class A
    int i; // Error redefinition of i 
}

• A member cannot be redeclared within a class.
• No member can be added elsewhere other than in the class definition.

Example:

Class A
{
    int i;
    int j;
    void f (int, int);
    int g(); 
}

f and g are a member function of class A. They determine the behavior of the objects of class A.

Accessing Data Members

The public data members of objects of a class can be accessed using the direct member access operator (.). Let us try the following example to make things clear:

Example:

#include <iostream> 
using namespace std;

class Box
{
public:
    double length; // Length of a box
    double breadth; // Breadth of a box
    double height; // Height of a box
};

int main( )
{
    Box Box1; // Declare Box1 of type Box
    Box Box2; // Declare Box2 of type Box
    double volume = 0.0; // Store the volume of a box here

    // box 1 specification
    Box1.height = 4.0; 
    Box1.length = 6.0; 
    Box1.breadth = 3.0;

    // box 2 specification
    Box2.height = 10.0;
    Box2.length = 12.0;
    Box2.breadth = 12.0;

    // volume of box 1
    volume = Box1.height * Box1.length * Box1.breadth;
    cout << "Volume of Box1 : " << volume <<endl; 

    // volume of box 2
    volume = Box2.height * Box2.length * Box2.breadth;
    cout << "Volume of Box2 : " << volume <<endl;
    return 0;
}

Program Output:

It is important to note that private and protected members cannot be accessed directly using a direct member access operator (.). We will learn how private and protected members can be accessed.

Program to Enter Student’s Details and Display It

Example:

#include <iostream>
using namespace std;

class stud 
{
    public:
    char name[30],clas[10];
    int rol,age;

    void enter() 
    { 
        cout<<"Enter Student Name: "; cin>>name;    
        cout<<"Enter Student Age: "; cin>>age;        
        cout<<"Enter Student Roll number: "; cin>>rol;        
        cout<<"Enter Student Class: "; cin>>clas;
    }
    
    void display() 
    {
        cout<<"\n Age\tName\tR.No.\tClass";
        cout<<"\n"<<age<<"\t"<<name<<"\t"<<rol<<"\t"<<clas; 
    }
};

int main()
{
    class stud s;
    s.enter();
    s.display();
    cin.get();  //use this to wait for a keypress 
} 

Program Output: