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C++ Class:

  • Before you create an object in C++, you need to define a class.
  • A class is a blueprint for the object.
  • learn c++ tutorials - class object model in c++

    learn c++ tutorials - class object model in c++ Example

  • 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.
  • As, many houses can be made from the same description, we can create many objects from a class.
  • learn c++ tutorials - class and object in c++

    learn c++ tutorials - class and object in c++ Example

How to define a class in C++?

  • 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
   };

Example: Class in C++

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().
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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.
  • learn c++ tutorials - class and object in c++

    learn c++ tutorials - class and object definition in c++ Example

  • 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.
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Syntax to Define Object in C++

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() andfunction2() are member functions.

How to access data member and member function in C++?

  • 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.

Example: Object and Class in C++ Programming

// 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;
 }
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Output

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:
secondDataOfObject2 = o2.insertFloatData();
  • In this program, data2 of o1 and data1 of o2 are not used and contains garbage value.

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