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Date: Thu, 01 Jul 2004 11:57:39 -0400
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: Zero length Objects
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Date: 2004-07-01T11:57:39-04:00
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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.