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From: "Nick Roberts" <nick.roberts@acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Zero length Objects
Date: Thu, 1 Jul 2004 15:52:29 +0100
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Date: 2004-07-01T15:52:29+01:00
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"Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net> wrote in message
news:2oUEc.2$S77.1@nwrdny03.gnilink.net...

> "Brian May" <bam@snoopy.apana.org.au> wrote in message
> news:sa41xjwy2nv.fsf@snoopy.apana.org.au...

> > Is there any reason every object needs to have a unique address?

> The issue is that if A and B share an address, then they are basically
> aliases of each other, for no subprogram could distinguish between
> them. The programmer, however, is declaring A and B as two
> separate objects, so making them aliases would be a violation of the
> author's intentions.

I understand that the C and C++ standards mandate that all types (classes)
must have a size greater than zero, for the reasons Frank gave. To expand on
on his example a little, supposing we wrote a general-purpose termplate
function which had to make sure its two parameters were not aliased:

   template <class C>
   function wibble(C* x, C* y) {
      if (x==y) raise Aliasing_Error;
      ...
   };

if we then had a class with no members:

   class CEmpty {
      ...
   };

   CEmpty a, b;

and tried to execute:

   wibble(&a,&b);

we would wrongly get Aliasing_Error raised if a and b had been allocated to
the same address. The rule that types (and so their objects) cannot be of
zero size prevents this happening.

To my mind, a significant problem remains. If we declared something like:

      CWhatever mem_a[1000000];

somewhere in a template class, where CWhatever was a template parameter, and
then instantiated the class with CEmpty replacing CWhatever, we would get a
mem_a in the instance that took up a whopping amount of memory to store
absolutely nada! Yuk.

PS: Please forgive any minor mistakes, I'm not a C++ expert.

-- 
Nick Roberts