C++ Classes & Objects

C++ adds object-oriented programming to the C language, supporting object-oriented design. Classes are the core feature of C++ and are often referred to as user-defined types.

Classes specify the form of an object, including its data representation and methods for processing the data. Data and methods within a class are called members. Functions within a class are known as member functions.

C++ Class Definition

Defining a class is essentially defining a blueprint for a data type. This does not define any data, but it defines what the class name means, i.e., what an object of the class will include and what operations can be performed on such an object.

A class definition starts with the keyword class followed by the class name. The body of the class is enclosed in curly braces. The class definition must be followed by a semicolon or a declaration list. For example, we define the Box data type using the keyword class as follows:

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

The keyword public determines the access attributes of the class members. Public members are accessible outside the class within the scope of the class object. You can also specify class members as private or protected, which we will discuss later.

Defining C++ Objects

A class provides the blueprint for objects, so objects are created based on the class. Declaring an object of a class is similar to declaring a variable of a basic type. The following statements declare two objects of class Box:

Box Box1;          // Declare Box1, of type Box
Box Box2;          // Declare Box2, of type Box

Objects Box1 and Box2 have their own data members.

Accessing Data Members

Public data members of an object of a class can be accessed using the direct member access operator ..

To better understand these concepts, let's try the following example:

Example

#include <iostream>

using namespace std;

class Box
{
   public:
      double length;   // Length
      double breadth;  // Breadth
      double height;   // Height
      // Member function declarations
      double get(void);
      void set(double len, double bre, double hei);
};
// Member function definitions
double Box::get(void)
{
    return length * breadth * height;
}

void Box::set(double len, double bre, double hei)
{
    length = len;
    breadth = bre;
    height = hei;
}
int main()
{
   Box Box1;        // Declare Box1, of type Box
   Box Box2;        // Declare Box2, of type Box
   Box Box3;        // Declare Box3, of type Box
   double volume = 0.0;     // To store the volume

   // Box 1 specification
   Box1.height = 5.0; 
   Box1.length = 6.0; 
   Box1.breadth = 7.0;

   // Box 2 specification
   Box2.height = 10.0;
   Box2.length = 12.0;
   Box2.breadth = 13.0;

   // 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;

   // Box 3 specification
   Box3.set(16.0, 8.0, 12.0); 
   volume = Box3.get(); 
   cout << "Volume of Box3: " << volume << endl;
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Volume of Box1: 210
Volume of Box2: 1560
Volume of Box3: 1536

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

Detailed Explanation of Classes & Objects

So far, we have gained a basic understanding of classes and objects in C++. The following list also includes other concepts related to C++ classes and objects, which can be learned by clicking on the respective links.