Re: Representation clause for enumeratives

[Robert Dewar <dewar@gnat.com>]

In article <27eh6.3787$y03.254014@news.flash.net>,
  "Ken Garlington" <Ken.Garlington@computer.org> wrote:
> By the way, this description
> of the shifting of the allowable range (selecting different
> unbiased representations) would be very useful in the GNAT
> user guide. I couldn't find it.

Information on representation matters is really part of the
language, and in particular is required to be documented by
RM M(43). All such information is in the GNAT reference manual,
not in the users guide (which is about how to use the compiler,
rather than how to write programs). The divisision is not
always perfectly clear, but in this case, there is a chapter
called "Representation Clauses and Pragmas", which a section
called "Enumeration Clauses", that contains this information
as well as other important information.

In particular, one very important item which is (very
unfortunately) left implementation defined is what happens
if you have an array whose index type is an enumeration type
with holes. There are two implementations:

1. Use the representation to index, fast, but can waste space
(a lot of space if you have a rep clause like the one that
started this thread).

2. Use the pos value to index, slow, but compact

GNAT chooses the second, mostly because Verdix chose the
second, and we generally choose to be Verdix Ada 83 compatible
in such matters (in this case, this is also the choice made by
DEC Ada 83 and other Ada 83 compilers).


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