Re: Ada Enumerations

[kst@king.cts.com (Keith Thompson)]

Stanley R. Allen (s_allen@hso.link.com) wrote:
> Matthew Heaney wrote:
> > Personally, I feel that if you're always looking at the rep of the
> > enumerands, then you probably don't want an enumeration type anyway.
> 
> That's a dead argument on the face of it.  If the language
> permits me to *specify* the representation, then why doesn't
> it permit me to *query* the specified values?

Because, IMHO, enumeration representation clauses weren't thought out as
well as they should have been when Ada 83 was designed, and the problems
weren't corrected in the Ada 95 revision.  If the language had defined
attributes to convert between an enumeration type and Universal_Integer,
analogous to 'Val and 'Pos but using the underlying representation rather
than the position value, along with a good way to determine whether
an enumeration value from an external source is valid, enumeration
representation clauses would have been much more useful.

There are some partial solutions for these problems.  Unchecked_Conversion
can be used to query the representation; the problem is determining an
integer type that matches a given enumeration type, especially if the
representation includes negative values.  GNAT provides the 'Enum_Rep
attribute (but not 'Enum_Val), but it's not portable.  The 'Valid
attribute should be usable to validate external data, but straightforward
use of it is erroneous by RM95-13.9.1; see also AI95-00167.