Structures in C

What is a structure?
A structure is a key word that create user defined data type in C/C++. A structure creates a data type that can be used to group items of possibly different types into a single type. 

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

C

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. 

C

// 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
}

Note: In C++, the struct keyword is optional before in declaration of a variable. In C, it is mandatory.

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

C

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. 

C

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. 

C

#include<stdio.h>

struct Point
{
   int x, y;
};

int main()
{
   struct Point p1 = {0, 1};

   // Accessing 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. 

C

#include<stdio.h>

struct Point
{
   int x, y, z;
};

int main()
{
   // Examples of initialization 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. 

C

#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.

C

#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

What is structure member alignment?
See https://www.geeksforgeeks.org/structure-member-alignment-padding-and-data-packing/
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:
  

C

struct number
{
    float x;
};
int main()
{
    struct number n1,n2,n3;
    n1.x=4;
    n2.x=3;
    n3=n1+n2;
    return 0;
}

/*Output:

prog.c: In function 'main':
prog.c:10:7: error:
invalid operands to binary + (have 'struct number' and 'struct number')
  n3=n1+n2;

*/

But we can use arithmetic operation on structure variables like this.

C

// Use of airthmatic operator in structure

#include <stdio.h>

struct number {
    float x;
};
int main()
{
    struct number n1,n2,n3;
    n1.x=4;
    n2.x=3;
    n3.x=(n1.x)+(n2.x);
    printf("\n%f",n3.x);
    return 0;
}

• 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.

We will soon be discussing union and other struct related topics in C. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.