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From: jkanze@otelo.ibmmail.com
Subject: Re: Placement of objects in ROM and physical object layout
Date: 1998/03/07
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References: <87lnuqjry7.fsf@lickey.shell4.ba.best.com> <6dm38g$fhc$1@nnrp1.dejanews.com> <34FFC8D2.72B391A8@physik.tu-muenchen.de>
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In article <34FFC8D2.72B391A8@physik.tu-muenchen.de>,
  Christopher Eltschka <celtschk@physik.tu-muenchen.de> wrote:
>
> jkanze@otelo.ibmmail.com wrote:

> > The standard only guarantees ordering for member variables declared
without
> > an intervening access specifier.
>
> Is this also true if the access specifier re-states the current state?
> That is, does
>
> class X
> {
> public:
>   char a;
>   int b;
>   char c;
> };
>
> guarantee the order of a, b, c and
>
> class Y
> {
>   public: char a;
>   public: int b;
>   public: char c;
> };
>
> not do so?

As I understand it, yes.

> If that's true, this would give an interesting optimizing guideline:
> If you are not interested in the exact order (and usually you
> aren't), then restate the access specifier for each member variable.
>
> (With alignment restrictions for int =3D=3D sizeof int =3D=3D 4, you wo=
uld
> have in the first case the layout (_ stands for padding bytes)
> a___bbbbc___ (giving sizeof X =3D=3D 12), while the second could be
> optimized by a good compiler to ac__bbbb (giving sizeof Y =3D=3D 8).
> If ints are only two-byte aligned, you'd even need no padding at all
> for Y!

Well, I've yet to see a compiler that actually rearranges the order
even when it is allowed.  So don't expect much optimization with this
with current compilers.

--
James Kanze    +33 (0)1 39 23 84 71    mailto: kanze@gabi-soft.fr
        +49 (0)69 66 45 33 10    mailto: jkanze@otelo.ibmmail.com
GABI Software, 22 rue Jacques-Lemercier, 78000 Versailles, France
Conseils en informatique orient=E9e objet --
              -- Beratung in objektorientierter Datenverarbeitung

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