Re: Enumeration representation

[Keith Thompson <kst@cts.com>]

Alex <busato@newtech.it> writes:
> I've written the following example program for testing enumeration
> represetantion clause:
[example deleted]

An enumeration representation clause is designed to change the
internal representation of an enumeration type *without* affecting the
visible semantics of the program.  Thus, the 'Pos, 'Val, 'Image,
'Value, 'Succ, 'Pred attributes, etc., operate as if the enumeration
representation clause weren't there.  The language doesn't define a
good way to get at the internal representation, though GNAT provides
an implementation-defined attribute for the purpose.  (The usual
advice is to use Unchecked_Conversion; the trick is determining what
integer type, if any, is the same size and signedness as your
enumeration type.)

Of course, if you write a value of an enumeration type to an external
interface (a file, a serial line, a call to another language,
whatever), you'll get the internal representation.  If you try to read
such a value from an external interface, it's very difficult to check
that you've gotten a valid value without invoking erroneous execution
(undefined behavior).

Basically, enumeration representation clauses are usually more trouble
than they're worth.  It's usually better to just declare a collection
of integer constants.  (Note: I said usually, not always.)

-- 
Keith Thompson (The_Other_Keith) kst@cts.com  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center           <*>  <http://www.sdsc.edu/~kst>
"Oh my gosh!  You are SO ahead of your time!" -- anon.