USER DEFINED DATATYPES IN C/C++

There are some derived data types that are defined by the user. These are class, structure, union and enumeration. This article briefly talks about these user-defined derived data types.

USER DEFINED DATATYPES IN C/C++

Class 

A class represents a group of similar objects. To represent classes in C++, it offers a user defined type called class. Once a class has been defined in C++, objects belonging to that class can easily be created. A class bears the same relationship to an object as a data type bears to a variable.

To define a class, we describe what sort of information it can represent and what sort of actions you can perform with that data. For instance, if you want to create class – department, it can represent the information like name of the department, number of its employees, name of its head and the actions that can be performed with this information are : addition, deletion, modification of employee data, printing of department report etc. Now this class will be declared in C++ as :

class department

{

char name[20];

int num_emp ;

char h_o_d [20];

public:

     void add();

     void Delete();

     void modify();

     void print();

};

department sales;       

The above code defines a class department and the last statement creates an object named sales of the department class.

 

Structure

A structure is a collection of variables (of different data types) referenced under one name, providing of convenient means of keeping related information together. For example, a student record is collection of rollno, name, Class, marks, grade etc. A structure definition forms a template that may be used to create structure variables. The variables that make up the structure are called structure elements. The keyword struct is used to construct a structure.

The following code creates a structure for student records :

struct student

{

int rollno;

char name[20];

int Class;

float marks;

char grade;

} ;

student s;

The above declaration declares a structure named student with structure elements rollno, name, Class, marks and grade. The last line creates (i.e. reserves memory) a structure variable named s of type student.

 

As it might seem, both class and structure are similar aspects. You are right. In C++ they only differ depending on their default behaviour. Any member of a structure is by default public, while any member of a class is by default private. In fact, a structure is basically a class defined with struct keyword.

The members of structure or class can be referenced using . (dot operator) as follows :

object.member;

Example,

s.rollno = 12;

sales.add();

 

Here, s.rollno accesses rollno member of the variable s (of type student) and sets its value to 12.

Similarly, sales.add() accesses add() function of the object sales (of class department).

 

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. Following declaration declares a union share having two variables (integer and character type) and creates a union object cnvt of type share :

union share

{

int i;

char ch;

} ;

union share cnvt;

The keyword union is used for declaring and creating a union. In union cnvt, both integer i & character ch share the same memory location. Of course, i being integer takes 4 bytes while ch takes 1 byte. The following figure illustrates the concept :

 

A union is used when the same value is needed in different types. For example, if we need 65 at one point of time and ‘A’ at other we may do it in 2 ways:

First, using the data type casting.

Second, using union.

Using of union is very rare in C++ programs but at times they do serve the purpose.

Let’s see an example:

cnvt.i = 65;

Once cnvt member is assigned a value, this can be used in all the data types included in the union. For instance,

cout<< cnvt.i<<endl;

cout<< cnvt.ch;

Output:

65

A

 

Enumeration 

An alternative method for naming integer constants is often more convenient that const. This can be achieved by creating enumeration using the keyword enum. For example,

enum{START, PAUSE, GO};

The above code defines 3 integer constants, called enumerators, and assigns values to them. Enumerator values are by default assigned increasing from 0, therefore, the above declaration is equivalent to writing

const int START = 0;

const int PAUSE = 1;

const int GO = 2;

An enumeration can be named also

enum status {START, PAUSE, GO};

status i = START;

Here, status acts as a distinct type (const int) and i is initialized with START (i.e. 0).

The enumerator values can be explicitly set by the programmer also.

enum{START=2, PAUSE, GO};

Here, START is initialized with 2 and all the other enumerators are initialized increasingly from 2 (i.e. PAUSE = 3 & GO = 4). The rule is that all enumerators are automatically assigned increasing values till an enumerator is again explicitly initialized.

 

 

 

 

 

 

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