Re: Zero length Objects

["Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net>]

"Brian May" <bam@snoopy.apana.org.au> wrote in message
news:sa41xjwy2nv.fsf@snoopy.apana.org.au...
> >>>>> "Frank" == Frank J Lhota <NOSPAM.lhota.adarose@verizon.net> writes:
>
>     Frank> No object is really of zero length. The reason is that
>     Frank> every object should have a unique address. Therefore, most
>     Frank> Ada compilers will allocate at least one storage element to
>     Frank> each object, in order to insure that if we declare
>
>     Frank> A, B : Empty_Type;
>
>     Frank> then A and B do not both refer to the same location.
>
> 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.

AFAIK C++ takes the same approach. In fact, the first C++ tutorial I went
through (the one that came with Borland's groundbreaking BC++ compiler for
DOS and Windows) included a code sample that was something like this:

    class CEmpty {};
    ...
    CEmpty a, b;

The tutorial recommended generating an assembly listing for this code to
note that even though CEmpty is empty, a and b are allocated at separate
addresses, and that this is important because operations should be able to
distinguish a and b. I tried this with GCC and MSVC. Not only did both
compilers assign a and b separate addresses, but for both of these compilers

    ( sizeof( CEmpty ) == 1 ) && ( sizeof( a ) == 1 )