Initializing unions

[Josh Lessard]

Given a union definition:

union problem_t {
int mask[2];
struct {
int indices[2];
int ops[2];
} comp;
};

Is it possible to initialize the struct containing the two arrays when
declaring a variable of this type? I've tried just about every
combination I can think of, but I can't seem to get it to work. For
example:

int main() {
problem_t problem = { {}, { { 0, 0 }, { 0, 0 } } };
}

union.cc: In function `int main()':
union.cc:12: excess elements in aggregate initializer

I've tried omitting the empty array representing the first element of the
union and that doesn't work either. I can't simply reverse the two union
elements, because I initialize it differently in different parts of my
program depending on how I want the union used.

Any ideas? Thanks in advance...

[Andrey Tarasevich]

Re: Initializing unions

Josh Lessard wrote:[color=blue]
> Given a union definition:
>
> union problem_t {
> int mask[2];
> struct {
> int indices[2];
> int ops[2];
> } comp;
> };
>
> Is it possible to initialize the struct containing the two arrays when
> declaring a variable of this type?[/color]

No. You'll have to use assignment instead of initialization

(Unless you have another union, which already holds the desired values.
In that case you can copy initialize the new union from the old one.)

--
Best regards,
Andrey Tarasevich

[Sam Dennis]

Re: Initializing unions

Josh Lessard wrote:[color=blue]
> union problem_t {
> int mask[2];
> struct {
> int indices[2];
> int ops[2];
> } comp;
> };
>
> Is it possible to initialize the struct containing the two arrays when
> declaring a variable of this type?[/color]

With a designator, yes:

union problem_t problem = { .comp = { { 0, 0 }, { 0, 0 } } };

If you want your code to be portable to pre-C99 implementations , though,
you'll just have to use an assignment to achieve this.

--
++acr@,ka"

[Jack Klein]

Re: Initializing unions

On Sat, 6 Mar 2004 02:14:54 +0000 (UTC), Sam Dennis
<sam@malfunctio n.screaming.net > wrote in comp.lang.c++:
[color=blue]
> Josh Lessard wrote:[color=green]
> > union problem_t {
> > int mask[2];
> > struct {
> > int indices[2];
> > int ops[2];
> > } comp;
> > };
> >
> > Is it possible to initialize the struct containing the two arrays when
> > declaring a variable of this type?[/color]
>
> With a designator, yes:
>
> union problem_t problem = { .comp = { { 0, 0 }, { 0, 0 } } };
>
> If you want your code to be portable to pre-C99 implementations , though,
> you'll just have to use an assignment to achieve this.[/color]

There is a version of C, not particularly widely available
unfortunately, that will accept this definition with initialization.

But there is no version of C++ that does, so why post it here?

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[Sam Dennis]

Re: Initializing unions

Jack Klein wrote:[color=blue]
> On Sat, 6 Mar 2004 02:14:54 +0000 (UTC), Sam Dennis
><sam@malfuncti on.screaming.ne t> wrote in comp.lang.c++:[color=green]
>> [C99 feature][/color]
>
> But there is no version of C++ that does, so why post it here?[/color]

My mistake; I read both groups and got confused.

--
++acr@,ka"

[Jack Klein]

Re: Initializing unions

On Sat, 6 Mar 2004 05:47:59 +0000 (UTC), Sam Dennis
<sam@malfunctio n.screaming.net > wrote in comp.lang.c++:
[color=blue]
> Jack Klein wrote:[color=green]
> > On Sat, 6 Mar 2004 02:14:54 +0000 (UTC), Sam Dennis
> ><sam@malfuncti on.screaming.ne t> wrote in comp.lang.c++:[color=darkred]
> >> [C99 feature][/color]
> >
> > But there is no version of C++ that does, so why post it here?[/color]
>
> My mistake; I read both groups and got confused.[/color]

No problem, I make my own share of mistakes, I just didn't want the OP
complaining that his C++ compiler didn't support this and it should...

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[Nick Hounsome]

Re: Initializing unions


"Josh Lessard" <jrlessar@plg2. math.uwaterloo. ca> wrote in message
news:Pine.SOL.4 .44.04030520343 40.425-100000@plg2.mat h.uwaterloo.ca. ..[color=blue]
> Given a union definition:
>
> union problem_t {
> int mask[2];
> struct {
> int indices[2];
> int ops[2];
> } comp;
> };
>
> Is it possible to initialize the struct containing the two arrays when
> declaring a variable of this type? I've tried just about every
> combination I can think of, but I can't seem to get it to work. For
> example:
>
> int main() {
> problem_t problem = { {}, { { 0, 0 }, { 0, 0 } } };
> }
>
> union.cc: In function `int main()':
> union.cc:12: excess elements in aggregate initializer
>
> I've tried omitting the empty array representing the first element of the
> union and that doesn't work either. I can't simply reverse the two union
> elements, because I initialize it differently in different parts of my
> program depending on how I want the union used.
>
> Any ideas? Thanks in advance...
>[/color]

ref 8.5.1
.....the braces shall only contain an initializer for the first member of the
union.

so reorder your union and treat it like it was just the first member.

Alternatively add one or more constructors.

[Josh Lessard]

Re: Initializing unions

On Sat, 6 Mar 2004, Nick Hounsome wrote:
[color=blue]
>
> "Josh Lessard" <jrlessar@plg2. math.uwaterloo. ca> wrote in message
> news:Pine.SOL.4 .44.04030520343 40.425-100000@plg2.mat h.uwaterloo.ca. ..[color=green]
> > Given a union definition:
> >
> > union problem_t {
> > int mask[2];
> > struct {
> > int indices[2];
> > int ops[2];
> > } comp;
> > };
> >
> > Is it possible to initialize the struct containing the two arrays when
> > declaring a variable of this type? I've tried just about every
> > combination I can think of, but I can't seem to get it to work. For
> > example:
> >
> > int main() {
> > problem_t problem = { {}, { { 0, 0 }, { 0, 0 } } };
> > }
> >
> > union.cc: In function `int main()':
> > union.cc:12: excess elements in aggregate initializer
> >
> > I've tried omitting the empty array representing the first element of the
> > union and that doesn't work either. I can't simply reverse the two union
> > elements, because I initialize it differently in different parts of my
> > program depending on how I want the union used.
> >
> > Any ideas? Thanks in advance...
> >[/color]
>
> ref 8.5.1
> ....the braces shall only contain an initializer for the first member of the
> union.
>
> so reorder your union and treat it like it was just the first member.
>
> Alternatively add one or more constructors.[/color]

Thank you very much for your suggestions, but I've already considered
them. Reordering the union members won't help because the size of the
'mask' array is different for each platform I'm implementing my program on
(the size is actually set by a platform specific const int), and there are
times where I need to fill that entire array.

Constructors also won't work because I'm trying to initialize an array of
these unions. I guess I'll just have to make it a struct, and thus use an
extra four bytes for the two arrays of two elements.

Thanks everyone for your suggestions.

[Josh Lessard]

Re: Initializing unions

On Sat, 6 Mar 2004, Josh Lessard wrote:
[color=blue]
> Constructors also won't work because I'm trying to initialize an array of
> these unions. I guess I'll just have to make it a struct, and thus use an
> extra four bytes for the two arrays of two elements.[/color]

Erm...extra 4 *ints* for the two arrays of two elements. Sorry about
that.