C++ Classes and Objects

In the C++ programming language, classes and objects are the base building block that leads to object-oriented programming in C++. They provide the code in a more organized, reusable, and maintainable way. In C++, classes act like a blueprint that defines how objects are structured and behave in the program. On the other hand, objects are instances of classes that are used to hold data and methods to create and manipulate several entities.

C++ Class

In C++, a class is a user-defined data type that acts as a blueprint to create an object. It is used to collect the data members and member functions into a single unit. A class keyword is used to create a class in C++. A class has several features, including fields, methods, constructors, etc.

For Example:

Consider a class of Cars. There can be several cars with different names and brands, but these cars will share many common properties, such as wheels, mileage range, Speed limit, and many others. It also contains turns, brakes, gears, accelerator, etc. Now, the Car is the class, and its data members are wheels, mileage range, and Speed limit, and its member functions are turn, brakes, gears, and accelerator.

Defining a Class

A class should be defined and declared before its use in C++. It is defined via using the class keyword in C++. The body of the class is defined inside the curly brackets and terminated by a semicolon at the end.

Syntax

It has the following syntax:

In this syntax,

• Data Members:Data members are the variables that are defined inside the class of the program.
• Member Functions: The functions that are declared inside a class are referred to as member method..
• Access Specifiers: These are the keywords (such as public, private, and protected) that are used to control the accessibility of data members and methods.

Example

Let's take an example to demonstrate the class in C++.

Output:

Enter the Name: John
Enter the Age: 25

Explanation:

In this example, we define a class named Employee with two public data members, name and age, and a member function displayInfo() that displays the employee's details.

In the main() function, an object e1 of the Employee class is created. The values "John" and 25 are assigned to the object's name and age. Finally, the displayInfo() function is called to display these values.

Class Access Specifiers

In C++ classes, the access specifiers can be used to control access to the members of the class. The access specifiers are also known as access modifiers in C++. These specifier keywords are defined in the class, and all the class members will have particular access levels under access specifiers.

There are mainly 3 types of access specifiers in C++.

1. Public: In C++, public data members may be accessed from outside the class using the dot (.) operator.
2. Private: Private data members may only be accessed within the class in C++. They cannot be accessed directly from outside the class.
3. Protected: A protected member variable of a class acts as a private member. It can be accessed inside the class and by derived classes.

C++ Object

An object represents a real-world entity, such as a chair, car, pen, mobile, or laptop. It consists of state (data) and behavior (function). Objects allow us to access the members of a class, and we can create more than one object from a single class.

In C++, an object is an instance of a class. When a class is declared in the program, no memory is allocated until an object of that class is created.

Creating Objects in C++

Once the class is defined in the program, we can easily create its object in the same manner that we define the variable of any other built-in data types.

Where,

• class_Name: It represents the name of the Class.
• object_Name: It represents the name of the class objects.

For Instance:

Here, Employee is the class name and emp1 and emp2 are the object names.

Accessing Data Members

In C++, class members may be accessed within the class directly by using their allocated name. If we need to access the class members outside the class, we can access its members using the dot(.) operator.

For Instance:

In this example, obj refers to the object name of the class, member1 represents the data member, and member2 represents the member function.

Example

Let's take an example to illustrate the objects in C++.

Output:

Name of the Course: C++
Time Duration of Course: 1 Year

Explanation:

In this example, we have taken a class, Tpoint, that has two data members and one function. Tpoint1 is an object of the Tpoint class. The object Tpoint1 is used to access and manipulate class members using the dot (.) operator.

Characteristics of an Object

Several characteristics of an Object in C++ are as follows:

• State: It refers to the properties or data stored within the object. It defines the current condition or values stored in the object.
• Behavior: It refers to the function that is related to an object. It defines what operations the object can perform. For example, a method to study (such as study()) for a Student object, or a method to display the object's information (such as displayInfo())
• Identity: Every object has a unique identity, and its properties are also identical to another object.

Example of Classes and Objects

Here, we are going to discuss several examples that demonstrate the working of Class and Object.

Example 1: Creating and Accessing Class Members

Let's see an instance of a class that contains two fields: id and name. It creates an instance of the class, initializes the object, and prints the object value.

Output:

101
John Miller

Explanation:

In this example, we have taken a class named Employee that is defined with two public data members: emp_id and emp_name. In the main() function, we create an obj emp1 of the type Employee. After that, the data members of the object are assigned values directly. The values of the object's members are accessed using the dot(.) operator.

Example 2: Initializing and Displaying data through Method

Let's take another example of a C++ class where we are initializing and displaying objects through a method.

Output:

101  John
102  Alice

Explanation:

In this example, we have taken a class named Employee with members that give the employee's name and the ID number as type string and integer. In the main() function, we define the value for emp1.id and emp2.id to 101 and 102, and the name to John and Alice.

Example 3: Store and Display Employee Information

Let's take another example of a C++ class where we are storing and displaying employee information using the method.

Output:

101  John  995000
102  Michael  29700

Objects as Function Arguments

If we need to pass an object to a function in C++, we may pass the object in the same manner as we pass any other primitive data. We only need to define the object in the function's argument and call the function.

Syntax

It has the following syntax:

In this syntax, function_name refers to the name of the function, and object_name refers to the name of the object.

Example of Objects as Function Arguments

Let's take an example to illustrate how to use objects as function arguments in C++.

Output:

Employee Name: John, Employee ID: 101
Inside Function: 
Employee Name: John
Employee ID: 101

Explanation:

In this example, we define a class Employee that contains two data members: emp_name and emp_id. We also take a member function setDetails() to assign values.

In the main() function, an object emp1 is created and initialized with the employee name and id. After that, the object is passed to an external function showEmployee() that takes an employee object as a parameter and prints its details.

Differences between Classes and Objects in C++

Several main differences between classes and objects are as follows: