Hi. I was reading up a bit on the features of C I seldom use, and I came
across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared to
a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first value.
Now I'm just wondering what the use of this is. I'm sure there are lots, so
I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
Thanks.
--
Tim Cambrant
<tim at cambrant dot com>
Nov 13 '05
16  3960 
> Hi. I was reading up a bit on the features of C I seldom use, and I came across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared
to a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first
value.
Now I'm just wondering what the use of this is. I'm sure there are lots,
so I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
Another application of unions is the following. Assume you have a
datacommunicati on application which receives bytes and in your application
you want to access the individual bits in some way. Then you could define a
union in which you have a bitfield of 8 bits and an unsigned char. Now you
can assign incoming bytes to the unsigned char member and access the
individual bits by using the bitfield member.
I've implemented this in the following example: -
#include <stdio.h>
-
-
struct bitfield_s
-
{
-
unsigned int b0:1;
-
unsigned int b1:1;
-
unsigned int b2:1;
-
unsigned int b3:1;
-
-
unsigned int b4:1;
-
unsigned int b5:1;
-
unsigned int b6:1;
-
unsigned int b7:1;
-
};
-
-
union bitfield_e
-
{
-
unsigned char c;
-
struct bitfield_s s;
-
};
-
-
int main ()
-
{
-
union bitfield_e b;
-
unsigned char c = 0x12;
-
int i;
-
-
b.c = c;
-
printf ("%d%d%d%d%d%d%d%d\n", b.s.b7, b.s.b6, b.s.b5, b.s.b4,
-
b.s.b3, b.s.b2, b.s.b1, b.s.b0);
-
-
return 0;
-
}
-
Regards,
Nathan
>Subject: Unions From: "Tim Cambrant" ti*@cambrant.co m.net
Date: 9/24/03 4:23 AM Hawaiian Standard Time
Message-id: <uN************ *******@newsb.t elia.net>
Hi. I was reading up a bit on the features of C I seldom use, and I came
across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared to
a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first value.
Now I'm just wondering what the use of this is. I'm sure there are lots, so
I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
I've used them to represent multidimensiona l data as both linear and
multi-dimensional arrays.
Stuart
>Subject: Unions From: "Tim Cambrant" ti*@cambrant.co m.net
Date: 9/24/03 4:23 AM Hawaiian Standard Time
Message-id: <uN************ *******@newsb.t elia.net>
Hi. I was reading up a bit on the features of C I seldom use, and I came
across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared to
a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first value.
Now I'm just wondering what the use of this is. I'm sure there are lots, so
I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
I've used them to represent multidimensiona l data as both linear and
multi-dimensional arrays.
Stuart
Nathan wrote: Hi. I was reading up a bit on the features of C I seldom use, and I came
across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared
to a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first
value.
Now I'm just wondering what the use of this is. I'm sure there are lots,
so I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
Another application of unions is the following. Assume you have a
datacommunicati on application which receives bytes and in your application
you want to access the individual bits in some way. Then you could define a
union in which you have a bitfield of 8 bits and an unsigned char. Now you
can assign incoming bytes to the unsigned char member and access the
individual bits by using the bitfield member.
I've implemented this in the following example:
- #include <stdio.h>
- struct bitfield_s
- {
- unsigned int b0:1;
- unsigned int b1:1;
- unsigned int b2:1;
- unsigned int b3:1;
- unsigned int b4:1;
- unsigned int b5:1;
- unsigned int b6:1;
- unsigned int b7:1;
- };
- union bitfield_e
- {
- unsigned char c;
- struct bitfield_s s;
- };
- int main ()
- {
- union bitfield_e b;
- unsigned char c = 0x12;
- int i;
-
-
i isn't used.
-
- b.c = c;
- printf ("%d%d%d%d%d%d%d%d\n", b.s.b7, b.s.b6, b.s.b5, b.s.b4,
- b.s.b3, b.s.b2, b.s.b1, b.s.b0);
-
-
This looks like a lot of b.s. to me. :-)
-
-
Isn't this implementation-dependent, i.e. non-portable.
-
BTW, I get '01001000' on my machine.
-
- return 0;
- }
-
--
Tim Hagan
Nathan wrote: Hi. I was reading up a bit on the features of C I seldom use, and I came
across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared
to a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first
value.
Now I'm just wondering what the use of this is. I'm sure there are lots,
so I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
Another application of unions is the following. Assume you have a
datacommunicati on application which receives bytes and in your application
you want to access the individual bits in some way. Then you could define a
union in which you have a bitfield of 8 bits and an unsigned char. Now you
can assign incoming bytes to the unsigned char member and access the
individual bits by using the bitfield member.
I've implemented this in the following example:
- #include <stdio.h>
- struct bitfield_s
- {
- unsigned int b0:1;
- unsigned int b1:1;
- unsigned int b2:1;
- unsigned int b3:1;
- unsigned int b4:1;
- unsigned int b5:1;
- unsigned int b6:1;
- unsigned int b7:1;
- };
- union bitfield_e
- {
- unsigned char c;
- struct bitfield_s s;
- };
- int main ()
- {
- union bitfield_e b;
- unsigned char c = 0x12;
- int i;
-
-
i isn't used.
-
- b.c = c;
- printf ("%d%d%d%d%d%d%d%d\n", b.s.b7, b.s.b6, b.s.b5, b.s.b4,
- b.s.b3, b.s.b2, b.s.b1, b.s.b0);
-
-
This looks like a lot of b.s. to me. :-)
-
-
Isn't this implementation-dependent, i.e. non-portable.
-
BTW, I get '01001000' on my machine.
-
- return 0;
- }
-
--
Tim Hagan
Fred L. Kleinschmidt <fr************ *****@boeing.co m> wrote:
Tim Cambrant wrote:
Hi. I was reading up a bit on the features of C I seldom use, and I came
across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared to
a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first value.
Now I'm just wondering what the use of this is. I'm sure there are lots, so
I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
Thanks.
--
Tim Cambrant
<tim at cambrant dot com>
One common type of usage of a union is the definition of an XEvent:
typedef union _Xevent {
int type;
XAnyEvent xany;
XButtonEvent xbutton;
XKeyEvent xkey;
/* Many more specific event structs */
} XEvent;
Each of the event structs in this union represents one of the possible X
events.
The programmer can thus use
void myfunction( XEvent *event );
in his code, then check the 'type' field to see what kind of event it
is, then
cast the variable to the appropriate struct within the union. For
example,
if ( event->type == KeyPress ) {
keycode = ((XKeyEvent *)event)->keycode;
/* ... do something based on the keycode */
}
....but this isn't using the union-ness of the union at all - you might
as well just be using struct pointers. However, if you write it as:
if (event->xany.type == KeyPress) {
keycode = event->xkey.button;
/* ... do something based on the keycode */
then you're fully using the union (this assumes that XAnyEvent and
XKeyEvent are structs that have a common initial element called "type").
- Kevin.
On Wed, 24 Sep 2003 15:40:43 GMT, "Fred L. Kleinschmidt"
<fr************ *****@boeing.co m> wrote in comp.lang.c:
Tim Cambrant wrote:
Hi. I was reading up a bit on the features of C I seldom use, and I came
across unions. I understand the concept, and that all the contained
variables etc. share the same memory. Thus, when a new value is declared to
a variable in the union, the existing value is overwritten even though the
new value is declared to a different variable than that of the first value.
Now I'm just wondering what the use of this is. I'm sure there are lots, so
I'm not critizising, but I just don't see a use for unions. Could someone
give me a few concrete examples of when to use unions?
Thanks.
--
Tim Cambrant
<tim at cambrant dot com>
One common type of usage of a union is the definition of an XEvent:
typedef union _Xevent {
Of course this is illegal code and produces undefined behavior,
because all identifiers beginning with an underscore followed by
either another underscore or an upper case letter are reserved for the
implementation in all contexts.
int type;
XAnyEvent xany;
XButtonEvent xbutton;
XKeyEvent xkey;
/* Many more specific event structs */
} XEvent;
Each of the event structs in this union represents one of the possible X
events.
The programmer can thus use
void myfunction( XEvent *event );
in his code, then check the 'type' field to see what kind of event it
is, then
cast the variable to the appropriate struct within the union. For
example,
if ( event->type == KeyPress ) {
keycode = ((XKeyEvent *)event)->keycode;
/* ... do something based on the keycode */
}
else if ( event->type == ButtonPress ) {
button = ((XButtonEvent *)event)->button;
/* ... do something based on which button was pressed */
}
thus "myfunction " can be used for any type of event.
....and the code is wrong, too.
If event->type exists in the union, there is no XKeyEvent there.
What you really mean is:
enum message_type
{
MSG_TYPE_1,
MSG_TYPE_2,
/* ... */
};
union
{
struct event_type_1; /* assume previous definition */
struct event_type_2;
} event_types;
struct event_struct
{
enum message_type type;
union event_type event;
};
I have found that proper coding standards forbid assigning both a
typedef and a tag to a union or struct unless absolutely required for
a forward declaration. 99% of all structure and enumeration types do
not need both.
--
Jack Klein
Home: http://JK-Technology.Com
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