From: "Xenos" <dont.spam.me@spamhate.com>
Subject: Re: Zero length Objects
Date: Thu, 1 Jul 2004 12:05:52 -0400
Date: 2004-07-01T12:05:52-04:00 [thread overview]
Message-ID: <cc1cp9$mt89@cui1.lmms.lmco.com> (raw)
In-Reply-To: 1088697459.558708@master.nyc.kbcfp.com
"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:1088697459.558708@master.nyc.kbcfp.com...
> Xenos wrote:
> > But C++ does compensate for this somewhat. Empty base classes (used for
> > traits and the like) will not take any space in derived classes. For
> > example:
> >
> > class Empty {};
> > class Derived : public Empty { char a; };
> > Derived D[1000000];
> >
> > Here, Empty is not increase the size of D at all.
>
> But it's more complicated than that, because C++ does have the
> similar rule that no distinct objects of the same type can have
> the same address. Expanding on your example,
>
> struct Empty1 { }; struct Empty2 { };
> struct Derived : Empty1, Empty2 { char a; };
> Derived D[1000000];
>
> The optimization still applies, and sizeof(Derived)
> can still be one. But do as follows:
>
> struct Empty { };
> struct E1 : Empty { }; struct E2 : Empty { };
> struct Derived : E1, E2 { char a; };
> Derived D[1000000];
>
> Now, Derived has two distinct subobjects both of type
> Empty. They cannot be allocated at the same address, so
> it's likely that sizeof(Derived) is > 1. Although, in
> this case the compiler can get really clever and place
> one of the Empty objects before the char and one after.
> But cases can be concocted where extra pad bytes are
> definitely required to avoid messing up object identity.
>
Which is why I said "somewhat."
next prev parent reply other threads:[~2004-07-01 16:05 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-06-30 5:55 Zero length Objects Robert C. Leif
2004-06-30 20:32 ` Jacob Sparre Andersen
2004-06-30 21:01 ` Frank J. Lhota
2004-07-01 0:02 ` Nick Roberts
2004-07-01 1:28 ` Georg Bauhaus
2004-07-01 10:37 ` Björn Persson
2004-07-01 11:25 ` Larry Kilgallen
2004-07-01 14:11 ` Nick Roberts
2004-07-01 15:42 ` Larry Kilgallen
2004-07-01 14:06 ` Xenos
2004-07-01 15:26 ` Nick Roberts
2004-07-02 1:06 ` Jeffrey Carter
2004-07-01 0:47 ` Brian May
2004-07-01 13:32 ` Frank J. Lhota
2004-07-01 14:52 ` Nick Roberts
2004-07-01 15:03 ` Xenos
2004-07-01 15:57 ` Hyman Rosen
2004-07-01 16:05 ` Xenos [this message]
2004-07-02 15:02 ` Frank J. Lhota
2004-07-02 15:11 ` Adrian Knoth
2004-07-02 15:43 ` Frank J. Lhota
2004-07-02 19:01 ` Vinzent 'Gadget' Hoefler
2004-07-02 19:07 ` Adrian Knoth
2004-07-02 19:25 ` Vinzent 'Gadget' Hoefler
2004-07-02 21:06 ` Xenos
2004-07-02 21:56 ` Vinzent 'Gadget' Hoefler
-- strict thread matches above, loose matches on Subject: below --
2004-07-02 8:30 Christoph Karl Walter Grein
2004-07-06 11:59 ` Nick Roberts
2004-07-06 22:14 ` Randy Brukardt
2004-07-06 22:28 ` Nick Roberts
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