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From: "Xenos" <dont.spam.me@spamhate.com>
Newsgroups: comp.lang.ada
Subject: Re: Zero length Objects
Date: Thu, 1 Jul 2004 12:05:52 -0400
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Date: 2004-07-01T12:05:52-04:00
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"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."