unions

spx2 · 08-21-2006, 08:58 AM

why would i as a c programmer use union
instead of struct ?

how is union declared right
and how are fields of an union
accesed correctly?

demon_vox · 08-21-2006, 09:06 AM

Hi,
an union is not a struct, so you normally dont want to use on over the other. Its just a matter of needs. The union lets you store different type of value in one variable (but one at a time). Whereas the struct lets you keep a lot of "variable" into one big variable.
Check this post, as an example, where one solution is to use union: http://www.linuxquestions.org/questi...d.php?t=467530

I hope this is useful

Cheers!

Agrouf · 08-21-2006, 09:08 AM

/* object is a type AND a value */
struct object
{
/* type is an int with 3 sensible values 0, 1 or 2 */
enum { INTEGER_TYPE, FLOAT_TYPE, CHAR_TYPE } type;
/* value can be an int OR a float OR a char */
union { int i; float f; char c;} value;
}

struct object ob1;
ob1.type = CHAR_TYPE;
ob1.value.c = 'x';

struct object ob2;
ob2.type = INTEGER_TYPE;
ob2.value.i = 10;

xhi · 08-21-2006, 09:35 AM

>why would i as a c programmer use union instead of struct ?
if you want to have a structure that could potentially be many different things but can only be one thing at a time, they are commonly used for a variant in a structure.

>how is union declared right and how are fields of an union accesed correctly?
a union is declared the same way a struct is, only with the struct keyword replaced by union.

using a union will save you space, adn will also let you do fun stuff at a very low level.

here is an example

Code:

#include <stdio.h>

typedef union int32_bytes
{
    int int_value;
    struct { unsigned char b0, b1, b2, b3; } byte_value;
} INT32_VALUE;

int main()
{
    INT32_VALUE v;
    v.int_value = 123456789;
    printf("Int value: %d\n", v.int_value);
    printf("Bytes are: B0: %d, B1: %d, B2: %d, B3: %d\n", 
           v.byte_value.b0, v.byte_value.b1, v.byte_value.b2, v.byte_value.b3);
    return 0;
}