Re: Enumerations

[wdl39!mab@ford-wdl1.arpa (Mark A Biggar)]

In article <1992Sep17.222636.4782@aio.jsc.nasa.gov> davenpor@sweetpea.jsc.nasa.
gov (Darren Davenport 283-4173) writes:
>In article <9209162110.AA21397@efftoo.boeing.com>, crispen@efftoo.boeing.com (
crispen) writes:
>|> First, the caveat: I *know* that without rep specs all data in Ada is
>|> abstract.
>|> But what I'd like to know is, does anyone know of an Ada compiler
>|> which does not give a machine value of zero for the first element of
>|> an enumeration, one for the second, and so on?
>|> Just idle curiosity, actually, since it came up in the context of a
>|> document I'm writing that's going to say, "Use rep specs for interface
>|> objects and don't trust different compilers to do the same thing".
>You do not need to write that in your document because:
>"The position number of the value of the first listed enumeration literal is
>zero" LRM 3.5.1(4). 

Sorry, but that only says that E'POS(E'FIRST) must be 0, it dosn't say
anything about what bits are actually stored in memory.  Rememeber that even
if there is a rep spec specifing the stored values of the enumeration, the abov
e
identity is still true.  In fact, the only way to determine what bit values
are being used for a given enumeration literal is to use UNCHECKED_CONVERSION
to translate to a suitable integer type.  Now as a Quality Of Implementation
issue I would expect that most Ada compilers, in the absense of a rep spec,
do store the value E'POS(X) for enumeration literal X, but the LRM does not
require it.  I also do not believe that Ada9x gives any relief in this area.

--
Mark Biggar
mab@wdl1.wdl.loral.com