Classes & Objects In Cpp

C++ Class:

We can think of class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows etc. Based on these descriptions we build the house. House is the object.

A class is defined in C++ using keyword class followed by the name of class.

The body of class is defined inside the curly brackets and terminated by a semicolon at the end.

class className
   {
   // some data
   // some functions
   };

class Test
{
    private:
        int data1;
        float data2;  

    public:  
        void function1()
        {
            data1 = 2; 
        } 

        float function2()
        { 
            data2 = 3.5;
            return data2;
        }
   };

Here, we defined a class named Test.

This class has two data members: data1 and data2 and two member functions: function1() and function2().

Keywords: private and public

You may have noticed two keywords: private and public in the above example.

The private keyword makes data and functions private. Private data and functions can be accessed only from inside the same class.

The public keyword makes data and functions public. Public data and functions can be accessed out of the class.

Here, data1 and data2 are private members where as function1() and function2() are public members.

If you try to access private data from outside of the class, compiler throws error. This feature in OOP is known as data hiding.

C++ Objects:

When class is defined, only the specification for the object is defined; no memory or storage is allocated.

To use the data and access functions defined in the class, you need to create objects.

className objectVariableName;

You can create objects of Test class (defined in above example) as follows:

class Test
{
    private:
        int data1;
        float data2;  

    public:  
        void function1()
        {
           data1 = 2;  
        } 

        float function2()
        { 
            data2 = 3.5;
            return data2;
        }
   };

int main()
{
    Test o1, o2;
}

Here, two objects o1 and o2 of Test class are created.

In the above class Test, data1 and data2 are data members and function1() and function2() are member functions.

You can access the data members and member functions by using a . (dot) operator. For example,

o2.function1();

This will call the function1() function inside the Test class for objects o2.

Similarly, the data member can be accessed as:

o1.data2 = 5.5;

It is important to note that, the private members can be accessed only from inside the class.

So, you can use o2.function1(); from any function or class in the above example. However,

the code o1.data2 = 5.5;

should always be inside the class Test.

// Program to illustrate the working of objects and class in C++ Programming
#include <iostream>
using namespace std;

class Test
{
    private:
        int data1;
        float data2;

    public:
       
       void insertIntegerData(int d)
       {
          data1 = d;
          cout << "Number: " << data1;
        }

       float insertFloatData()
       {
           cout << "\nEnter data: ";
           cin >> data2;
           return data2;
        }
};

 int main()
 {
      Test o1, o2;
      float secondDataOfObject2;

      o1.insertIntegerData(12);
      secondDataOfObject2 = o2.insertFloatData();

      cout << "You entered " << secondDataOfObject2;
      return 0;
 }

Number: 12
Enter data: 23.3
You entered 23.3

In this program, two data members data1 and data2 and two member functions insertIntegerData() and insertFloatData() are defined under Test class.

Two objects o1 and o2 of the same class are declared.

The insertIntegerData() function is called for the o1 object using:

o1.insertIntegerData(12);

This sets the value of data1 for object o1 to 12. Then, the insertFloatData() function for object o2 is called and the return value from the function is stored in variable secondDataOfObject2 using:

In this program, data2 of o1 and data1 of o2 are not used and contains garbage value.