Structure Unions in C Programming

Unions

  • A union is similar to a structure.
  • The only difference between a union and structure is that information can be stored in one field at any one time.
  • They are used for saving the memory.
  • Unions can be used for applications which involve a lot of multiple members, where the values need not be assigned to all the members at one time.

Syntax

union union_name
{
    member 1;
    member 2;
    .
    .
    member n;
};


The individual union variables are declared as
storage-class union union_name var1, var2, …..varn;

The combination is given as

storage-class union union_name
{
    member 1;
    member 2;
    .
    .
    member n;
}var1, var2, …..varn ;


The members are accessed in the same way as the structure members are i.e. by using the '.' or '→' operator.

Syntax:
union_name.member
OR
union_name → member

Example : Demonstration of a simple union

#include <stdio.h>
union student
{
    int roll_no;
    float fees;
};

void main()
{
    union student data;        
    data.roll_no = 5;
    printf( "data.roll_no : %d\n", data.roll_no);
    data.fees = 500.50;
    printf( "data.fees : %f\n", data.fees);
}


Output:
data.roll_no : 5
data.fees : 500.500000

  • Just like structures we can have nested unions, arrays of unions and pointer to union.
  • The pointers can be passed to and from the functions.
  • Unions are initialized with only one value and value of the first type.
  • The operations performed on unions can be the same as structures.
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