UNIONS

UNIONS

A union is a special data type available in C that allows to store different data
types in the same memory location. You can define a union with many
members, but only one member can contain a value at any given time. Unions
provide an efficient way of using the same memory location for multiple
purpose.

Defining a Union

To define a union, you must use the union statement in the same way as you
did while defining a structure. The union statement defines a new data type with
more than one member for your program. The format of the union statement is
as follows:

union [union tag] { member definition; member definition; ... member definition; } [one or more union variables];

The union tag is optional and each member definition is a normal variable
definition, such as int i; or float f; or any other valid variable definition. At the
end of the union's definition, before the final semicolon, you can specify one or
more union variables, but it is optional. Here is the way you would define a
union type named Data having three members i, f, and str:

union Data { int i; float f; char str[20]; } data;

Now, a variable of Data type can store an integer, a floating-point number, or a
string of characters. It means a single variable, i.e., same memory location, can
18. UNIONS
be used to store multiple types of data. You can use any built-in or user-defined
data types inside a union based on your requirement.
The memory occupied by a union will be large enough to hold the largest
member of the union. For example, in the above example, Data type will occupy
20 bytes of memory space because this is the maximum space which can be
occupied by a character string. The following example displays the total memory
size occupied by the above union:

#include <stdio.h> #include <string.h> union Data { int i; float f; char str[20]; }; int main( ) { union Data data; printf( "Memory size occupied by data : %d\n", sizeof(data)); return 0; }

When the above code is compiled and executed, it produces the following result:

Memory size occupied by data : 20

Accessing Union Members

To access any member of a union, we use the member access operator (.).
The member access operator is coded as a period between the union variable
name and the union member that we wish to access. You would use the keyword
union to define variables of union type. The following example shows how to
use unions in a program:

#include <stdio.h> #include <string.h> union Data { int i; float f; char str[20]; }; int main( ) { union Data data; data.i = 10; data.f = 220.5; strcpy( data.str, "C Programming"); printf( "data.i : %d\n", data.i); printf( "data.f : %f\n", data.f); printf( "data.str : %s\n", data.str); return 0; }

When the above code is compiled and executed, it produces the following result:

data.i : 1917853763 data.f : 4122360580327794860452759994368.000000 data.str : C Programming

Here, we can see that the values of i and f members of union got corrupted
because the final value assigned to the variable has occupied the memory
location and this is the reason that the value of str member is getting printed
very well.

Now let's look into the same example once again where we will use one variable
at a time which is the main purpose of having unions:

#include <stdio.h> #include <string.h> union Data { int i; float f; char str[20]; }; int main( ) { union Data data; data.i = 10; printf( "data.i : %d\n", data.i); data.f = 220.5; printf( "data.f : %f\n", data.f); strcpy( data.str, "C Programming"); printf( "data.str : %s\n", data.str); return 0; }

When the above code is compiled and executed, it produces the following result:

data.i : 10 data.f : 220.500000 data.str : C Programming

Here, all the members are getting printed very well because one member is
being used at a time.