Structure in C

1. Structure is a way to group  variables
2. Structure is a collection of dissimilar elements.
3. defining structure means creating new data type.

Structure is a user-defined datatype in C language which allows us to combine data of different types together. Structure helps to construct a complex data type which is more meaningful. ... In structure, data is stored in form of records.

How to create a structure?
‘struct’ keyword is used to create a structure. Following is an example.

struct address {    char name[50];    char street[100];    char city[50];    char state[20];    int pin; };

How to declare structure variables?
A structure variable can either be declared with structure declaration or as a separate declaration like basic types.

// A variable declaration with structure declaration. struct Point {    int x, y; } p1;  // The variable p1 is declared with 'Point'       // A variable declaration like basic data types struct Point {    int x, y; };     int main() {    struct Point p1;  // The variable p1 is declared like a normal variable }

How to initialize structure members?
Structure members cannot be initialized with declaration. For example the following C program fails in compilation.

struct Point {    int x = 0;  // COMPILER ERROR:  cannot initialize members here    int y = 0;  // COMPILER ERROR:  cannot initialize members here };

The reason for above error is simple, when a datatype is declared, no memory is allocated for it. Memory is allocated only when variables are created.

Structure members can be initialized using curly braces ‘{}’. For example, following is a valid initialization.

struct Point {    int x, y; };     int main() {    // A valid initialization. member x gets value 0 and y    // gets value 1.  The order of declaration is followed.    struct Point p1 = {0, 1};  }

How to access structure elements?
Structure members are accessed using dot (.) operator.

#include<stdio.h>    struct Point {    int x, y; };    int main() {    struct Point p1 = {0, 1};       // Accesing members of point p1    p1.x = 20;    printf ("x = %d, y = %d", p1.x, p1.y);       return 0; }

Output:

x = 20, y = 1

What is designated Initialization?
Designated Initialization allows structure members to be initialized in any order. This feature has been added in C99 standard.

#include<stdio.h>    struct Point {    int x, y, z; };    int main() {    // Examples of initializtion using designated initialization    struct Point p1 = {.y = 0, .z = 1, .x = 2};    struct Point p2 = {.x = 20};       printf ("x = %d, y = %d, z = %d\n", p1.x, p1.y, p1.z);    printf ("x = %d", p2.x);    return 0; }

Output:

x = 2, y = 0, z = 1
x = 20

This feature is not available in C++ and works only in C.

What is an array of structures?
Like other primitive data types, we can create an array of structures.

#include<stdio.h>    struct Point {    int x, y; };    int main() {    // Create an array of structures    struct Point arr[10];       // Access array members    arr[0].x = 10;    arr[0].y = 20;       printf("%d %d", arr[0].x, arr[0].y);    return 0; }

Output:

10 20

What is a structure pointer?
Like primitive types, we can have pointer to a structure. If we have a pointer to structure, members are accessed using arrow ( -> ) operator.

#include<stdio.h>    struct Point {    int x, y; };    int main() {    struct Point p1 = {1, 2};       // p2 is a pointer to structure p1    struct Point *p2 = &p1;       // Accessing structure members using structure pointer    printf("%d %d", p2->x, p2->y);    return 0; }

Output:

1 2

Limitations of C Structures

In C language, Structures provide a method for packing together data of different types. A Structure is a helpful tool to handle a group of logically related data items. However, C structures have some limitations.

The C structure does not allow the struct data type to be treated like built-in data types:

We cannot use operators like +,- etc. on Structure variables. For example, consider the following code:

(have 'struct number' and 'struct number')   n3=n1+n2;    */

No Data Hiding: C Structures do not permit data hiding. Structure members can be accessed by any function, anywhere in the scope of the Structure.

Functions inside Structure: C structures do not permit functions inside Structure

Static Members: C Structures cannot have static members inside their body

Access Modifiers: C Programming language do not support access modifiers. So they cannot be used in C Structures.

Construction creation in Structure: Structures in C cannot have constructor inside Structures.