Unions and Enumerations in C programming
Unions
An union is a derived data type—like a structure—with members that share the same storage space. The members of a union can be of any data type.
A union definition has the same format as a structure definition.
union number {
int x; // int member x
double y; // double member y
}; // end union number
The union definition is normally placed in a header and included in all source files that use the union type
The operations that can be performed on a union are:
- assigning a union to another union of the same type.
- taking the address (&) of a union variable.
- and accessing union members using the structure member operator and the structure pointer operator.
Demonstrating Unions
The program uses the variable value of type union number to display the value stored in the union as both an int and a double.
The program output is implementation dependent. The program output shows that the internal representation of a double value can be quite different from the representation of int.
// Displaying the value of a union in both member data types
#include <stdio.h>
// number union definition
union number {
int x;
double y;
}; // end union number
int main( void ) {
union number value; // define union variable
value.x = 100; // put an integer into the union
printf( "%s\n%s\n%s\n %d\n\n%s\n %f\n\n\n",
"Put 100 in the integer member",
"and print both members.",
"int:", value.x,
"double:", value.y );
value.y = 100.0; // put a double into the same union
printf( "%s\n%s\n%s\n %d\n\n%s\n %f\n",
"Put 100.0 in the floating member",
"and print both members.",
"int:", value.x,
"double:", value.y );
} // end main
Output:
Put 100 in the integer member and print both members.
int: 100
double: -92559592117433136000000000000000000000000000000000000
000000000.000000
Put 100.0 in the floating member and print both members.
int: 0
double: 100.000000
Enumeration Constants
An enumeration ( enum ), is a set of integer enumeration constants represented by identifiers. Values in an enum start with 0, unless specified otherwise, and are incremented by 1.
enum months {
//Initialize JAN=1 to number the months 1 to 12
JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG,
SEP, OCT, NOV, DEC }; // end enum months
The identifiers in an enumeration must be unique. Multiple members of an enumeration can have the same constant value.
Demonstrating Enums
The code below show this:
// Using an enumeration
#include <stdio.h>
// enumeration constants represent months of the year
enum months {
JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC
}; // end enum months
int main( void ) {
enum months month; // can contain any of the 12 months
// initialize array of pointers
const char *monthName[] = { "", "January", "February", "March",
"April", "May", "June", "July", "August", "September", "October",
"November", "December" };
// loop through months
for ( month = JAN; month <= DEC; ++month ) {
printf( "%2d%11s\n", month, monthName[ month ] );
} // end for
} // end main
Program output:
1 January
2 February
3 March
4 April
5 May
6 June
7 July
8 August
9 September
10 October
11 November
12 December
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