Struct alignment

Hi.
Assuming a struct:

struct data{
int, a, b, c, d;
};

Can I assume that the four variables from data are stored in memory in
order given above? Or would it be possible that they look like this in
memory: a, c, d, b?

I'm using the Visual Studio 2005 Pro compiler.

g, Hipo

re: Struct alignment

Hipo wrote:[color=blue]
> Assuming a struct:
>
> struct data{
> int, a, b, c, d;
> };
>
> Can I assume that the four variables from data are stored in memory in
> order given above?[/color]

Yes, that's mandated by the Standard.
[color=blue]
> Or would it be possible that they look like this in
> memory: a, c, d, b?[/color]

No.
[color=blue]
> I'm using the Visual Studio 2005 Pro compiler.[/color]

Shouldn't matter.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

re: Struct alignment

Victor Bazarov schrieb:[color=blue]
> Hipo wrote:[color=green]
>> Assuming a struct:
>>
>> struct data{
>> int, a, b, c, d;
>> };
>>
>> Can I assume that the four variables from data are stored in memory in
>> order given above?[/color]
>
> Yes, that's mandated by the Standard.[/color]

Great, that saves my day :-)
[color=blue][color=green]
>> I'm using the Visual Studio 2005 Pro compiler.[/color]
>
> Shouldn't matter.[/color]

Hope so, standard is not always standard (bad translation of a german
saying) :-)

g, Hipo

re: Struct alignment

Hipo wrote:
[color=blue][color=green][color=darkred]
>>> Can I assume that the four variables from data are stored in memory in
>>> order given above?[/color]
>>
>> Yes, that's mandated by the Standard.[/color]
>
> Great, that saves my day :-)[/color]

You can't assume they aren't padded; that's implementation specific.

(Yes, VC++ probably doesn't pad them.)

Don't rely on it. You weren't going to treat that struct as binary data,
were you?

--
Phlip

re: Struct alignment

Hipo posted:
[color=blue]
> Hi.
> Assuming a struct:
>
> struct data{
> int, a, b, c, d;
> };
>
> Can I assume that the four variables from data are stored in memory in
> order given above? Or would it be possible that they look like this in
> memory: a, c, d, b?
>
> I'm using the Visual Studio 2005 Pro compiler.
>
> g, Hipo[/color]


Long discussion on comp.std.c++ about this right now. Thread Title: "Struct
members -> Array elements".


-Tomás

re: Struct alignment

On Thu, 25 May 2006 22:35:30 GMT, "Tomás" <No.Email@Address> wrote in
comp.lang.c++:
[color=blue]
> Hipo posted:
>[color=green]
> > Hi.
> > Assuming a struct:
> >
> > struct data{
> > int, a, b, c, d;
> > };
> >
> > Can I assume that the four variables from data are stored in memory in
> > order given above? Or would it be possible that they look like this in
> > memory: a, c, d, b?
> >
> > I'm using the Visual Studio 2005 Pro compiler.
> >
> > g, Hipo[/color]
>
>
> Long discussion on comp.std.c++ about this right now. Thread Title: "Struct
> members -> Array elements".
>
>
> -Tomás[/color]

No, this question is only about the order, not about the possibility
of padding between the members.

--
Jack Klein
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re: Struct alignment

Jack Klein posted:

[color=blue]
> No, this question is only about the order, not about the possibility
> of padding between the members.[/color]

I realise that... but the order of the members is of no relevance unless
you try to do something funky like access them with pointers:

data obj;

int *p = &obj.a;

*p = 1; /* We're assuming this is A */

*++p = 2; /* We're assuming this is B */

*++p = 3; /* We're assuming this is C */

*++p = 4; /* We're assuming this is D */


If you don't try something funky like this, then there's no reason to
care about the order of the members.

(Which makes me wonder: Why does C++ even guarantee that the members be
in order one-after-another in a POD struct? We already don't have a
guarantee of the absence of padding... so there's no point in
guaranteeing that they be in order).

Okay... well... there might be ONE use:

bool IsAhead( const int * const a, const int * const b )
{
return b > a;
}

int main()
{
data obj;

IsAhead( &obj.a, &obj.b );
}


-Tomás