User-Defined Derived Data types There are some derived data types that are defined by the user. These are class , structure , union and enumeration.

Class

• A class represents a group of similar objects. To represent classes , it offers a user-defined data type called class.

• Once a class has been defined , objects belonging to that class can easily be created.

• Class keyword is used to define a class.

• Class can store different data members and member functions of different data types.

• In order to access members of a class , class object is needed.

• By default all members are private in class.

Below given code fragment is a example of class ->

class student {  //student class

public:

int age;            //data members
int rollno;

void display()   //member function 
{
cout<<"Age"<<age;
cout<<"Roll number"<<rollno;
}
};

Structure

• A structure is a collection of variables of different types (different data types) referenced under one name, providing a convenient means of keeping related information together.

• struct keyword is used in defining a structure.

• By default all members are public in structure.

Below given code fragment is a example of structure ->

struct student {  //student structure

int age;            //data members
int rollno;

};

Union

• A Union is a memory location that is shared by two or more different variables generally of different types at different times.

• Defining a union is similar to defining a structure.

• union keyword is used for defining a union.

Below given code fragment is a example of union ->

union share {

int i;
char ch;
};  // both i and ch share the same memory allocation.

Enumeration

• An alternative method for naming integer constants is often more convenient than const. This can be achieved by creating enumeration.

• enum keyword is used for creating enumeration.

Below given code fragment is a example of enumeration ->

enum { start,pause,go };
//defines three integer constants and assigns values to them 
//enum values are by default assigned increasing from 0.
//above declaration is equivalent for writing const int start=0,const int pause=1,const int go=2.