The union Keyword
union tag {
union_member(s);
/* additional statements may go here */
}instance;
The union keyword is used for declaring unions. A union is a collection of one or more variables (union_members) that have been grouped under a single name. In addition, each of these union members occupies the same area of memory.
The keyword union identifies the beginning of a union definition. It's followed by a tag that is the name given to the union. Following the tag are the union members enclosed in braces.
An instance, the actual declaration of a union, also can be defined. If you define the structure without the instance, it's just a template that can be used later in a program to declare structures. The following is a template's format:
union tag {
union_member(s);
/* additional statements may go here */
};
To use the template, you would use the following format:
union tag instance;
To use this format, you must have previously declared a union with the given tag.
/* Declare a union template called tag */
union tag {
int num;
char alps;
}
/* Use the union template */
union tag mixed_variable;
/* Declare a union and instance together */
union generic_type_tag {
char c;
int i;
float f;
double d;
} generic;
/* Initialize a union. */
union date_tag {
char full_date[9];
struct part_date_tag {
char month[2];
char break_value1;
char day[2];
char break_value2;
char year[2];
} part_date;
}date = {"09/12/80"};
Let us better understand it with the help of examples:
#include <stdio.h>
int main()
{
union
{
int value; /* This is the first part of the union */
struct
{
char first; /* These two values are the second part of it */
char second;
} half;
} number;
long index;
for (index = 12 ; index < 300000L ; index += 35231L)
{
number.value = index;
printf("%8x %6x %6x\n", number.value,
number.half.first,
number.half.second);
}
return 0;
}
And the output of the program will be displayed as follows:
c c 0
89ab ffab ff89
134a 4a 13
9ce9 ffe9 ff9c
2688 ff88 26
b027 27 ffb0
39c6 ffc6 39
c365 65 ffc3
4d04 4 4d
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