Concept of class

Storage_class struct user_defined_name

{

 datatype1 member1;

datatype2 member2;

..........       .............

.........        .............

 };

above declaration is the general format of a simple structure declaration in C.

where Storage_class refers to the scope of stucture variable it may automatic,resister,static or external.

struct is a keyword which shows the start of a structure.

user_defined_name is structure name defined by the user.

  structure is a collection hetrogenous(different) type of data.structure declaration  forms a template that

may be used to create structure objects(i.e instances of a structure).

once the structure  type has been defined,we can create variables of that type usingdeclarations that are similar to the built-in type

declarations.

e.g

       strucrt home

{

char name[10];

int age;

float income;

} father,mother,son,daughter;

OR

struct home father,mother,son,daughter;

where father ,mother,son,daughter are sturcture variables.

member variables(name,age,income) can be accessed by using dot or period operator as follows:

strcpy(father.name,"omniwas");/* or father.name="omniwas";*/

father.age=49;

father.income=15625.750;

strcpy(son.name,"krishnanand");

son.age=21;

son.income=659.41;

limitations of C structure:

Ø  C does not allow the struct data type to be treated like built-in types.

e.g

struct home

{

float a;

float b;

}s1,s2,s3;

ü  s1,s2,s3 can easily be assigned values using the dot operator,

but can not be added or subtracted  one from other as:

 s3=s1+s2;

 is illegal in C.

Ø  Another important limitation of  C structures is that they do not permit data hiding .

ü  Structure members can be directly accessed by structure variables by any function anywhere in their scope i.e structure members are public members.

 

Extensions to Structures:-

In C++ we expended C structure as

ü  C++ structure can have both variables and functions as members.

ü  C++ structure can also declare some of its members as ‘private’, ’public’ or protected.

ü  The keyword strucrt can be omitted in the declaration of structure variables. e.g we can declare home variable father as:  

              home father;  //this is valid in C++ and make an error in C

ü  C++  expended structure capabilities to suit OOP philosophy (abstraction, encapsulation, polymorphism, inheritance )

C++ incorporates all above extensions in another user-defined type known as class.

 Most of C++ programmers tend to use the structure for holding data type,and classes to hold both data and functions.

The only difference between a structure and a class in c++ is that,

·         By default,the members of a class are private.

·         By default ,the members of a structure are public.

In Java the concept of structure is totally eliminated and the concept of class is implemented , also some modifications in the concept of class have done.
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