union access

[Sean Dolan]

typedef struct ntt {
int type;
union {
int i;
char* s;
};
}nt;

nt n;
n.i = 0;

I found a C example like this and could not get gcc 2.95.4 to compile it
(struct has no member named `i') until I declared an instance of the union:
union {
int i;
char* s;
}u;

and accessed like:
n.u.i = 0;

Is the first example valid? If so, where is the problem?

--
Sean Dolan

[Derrick Coetzee]

Sean Dolan wrote:

typedef struct ntt {
int type;
union {
int i;
char* s;
};
}nt;

This is called an anonymous union and it is a Microsoft extension. While
gcc supports MS extensions with the flag -fms-extensions, you'd do
better converting it to standard C. You can do this as you suggested. If
you want to avoid fixing up all references, you can simply dispose of
the union, costing an extra word or so per object of this type - which
may or may not be a big deal in your app. If the names inside the union
are globally unique, you can use macros to expand them out to their
fully-qualified versions.
--
Derrick Coetzee
I grant this newsgroup posting into the public domain. I disclaim all
express or implied warranty and all liability. I am not a professional.

[Sean Dolan]

On 2004-09-14, Derrick Coetzee <dc****@moonfla re.com> wrote:

This is called an anonymous union and it is a Microsoft extension. While
gcc supports MS extensions with the flag -fms-extensions, you'd do
better converting it to standard C. You can do this as you suggested. If
you want to avoid fixing up all references, you can simply dispose of
the union, costing an extra word or so per object of this type - which
may or may not be a big deal in your app. If the names inside the union
are globally unique, you can use macros to expand them out to their
fully-qualified versions.

Thank you Derrick, that cleared it right up.

--
Sean Dolan

[Keith Thompson]

Sean Dolan <se*****@bellso uth.net> writes:

typedef struct ntt {
int type;
union {
int i;
char* s;
};
}nt;

nt n;
n.i = 0;

I found a C example like this and could not get gcc 2.95.4 to compile it
(struct has no member named `i') until I declared an instance of the union:
union {
int i;
char* s;
}u;

and accessed like:
n.u.i = 0;

Is the first example valid? If so, where is the problem?

No, the first example is not valid. In the member declaration

int type;

"int" is the member type and "type" is the name of the member.

In the (attempted) member declaration

union { int i; char *s; };

"union { int i; char *s; }" is the member type, but there is no member
name. (Sub-unions and sub-structs don't flatten into the namespace of
the surrounding struct or union.)

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[c453___]

one of simple/silly questions:
why use unions when we have classes? what makes union worth of using?

[Ben Pfaff]

c453___ <mu**@nospam.o2 .pl> writes:

one of simple/silly questions:
why use unions when we have classes? what makes union worth of using?

C doesn't have classes.
--
Ben Pfaff
email: bl*@cs.stanford .edu
web: http://benpfaff.org

[c453___]

> C doesn't have classes.
;) why use unions if we have structures?

[Keith Thompson]

c453___ <mu**@nospam.o2 .pl> writes:

one of simple/silly questions:
why use unions when we have classes? what makes union worth of using?

Because we don't have classes. (This is comp.lang.c, not comp.lang.c++.)

And, of course, unions and classes are two different things. The
point of a union is that the members are all stored in the same
location (and reading a member other than the last one written invokes
undefined behavior in most cases).

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Christian Bau]

In article <opsebyi1kjekdo 2r@pieszczoch>, c453___ <mu**@nospam.o2 .pl>
wrote:

one of simple/silly questions:
why use unions when we have classes? what makes union worth of using?

Unions are very useful to organise employees to negotiate higher
salaries, better working conditions etc. Classes are more useful to
learn stuff, like C classes where someone teaches C and people like you
should listen carefully to learn.

There should be Usenet classes as well, where people could learn not to
post C++ questions to comp.lang.c.

[xarax]

"c453___" <mu**@nospam.o2 .pl> wrote in message
news:opsebz20jc ekdo2r@pieszczo ch...

C doesn't have classes.

;) why use unions if we have structures?

All members of a union start at offset zero.

Each successive member of a struct starts at the next
available offset that is appropriate for the alignment
of that member and the preceding (if any) member.