Re: A question re meaning/use of the "for ... use ..."

["Randy Brukardt" <randy@rrsoftware.com>]

"Keith Thompson" <kst-u@mib.org> wrote in message
news:ln8y8932ei.fsf@nuthaus.mib.org...
> "Randy Brukardt" <randy@rrsoftware.com> writes:
> > "Jeffrey Carter" <spam@spam.com> wrote in message
> > news:H1Hsd.1987$yr1.256@newsread3.news.pas.earthlink.net...
> >> Stephen Leake wrote:
> >>
> >> > There is no standard attribute that returns the internal
> >> > representation specified by the enumeration representation clause.
> >> > However, GNAT provides the non-standard 'Enum_Rep for this purpose.
> >>
> >> I have no problem with 'Pos returning the abstract position number, but
> >> do think something like GNAT's 'Enum_Rep should be standard, along with
> >> a conversion the other way, equivalent to 'Val ('Enum_Val?).
> >
> > The ARG discussed this long ago, and concluded that such a facility
isn't
> > needed. That's because Unchecked_Conversion provides the needed support.
> > Indeed, this is one of the few cases where the result of
> > Unchecked_Conversion is defined by the language (using it in this way
will
> > work on all Ada compilers). There was some discussion about syntax
guides
> > that prohibit the use of Unchecked_Conversion, but there is a lot of
> > discomfort about changing the language just because some people's style
> > guides are broken...
>
> How do you portably choose the target type for the Unchecked_Conversion?
>
> The 'Pos attribute returns a result of type universal_integer; there's
> no way to make an Unchecked_Conversion return a universal_integer.

You have to declare a type for that purpose, but otherwise there is no
problem:

    First_Rep : constant := <rep-of-First>;
    Last_Rep : constant := <rep-of-Last>;
    type Enum is (First, ...., Last);
    for Enum use (First => First_Rep, .... Last => Last_Rep);
    type Enum_Rep is range First_Rep .. Last_Rep;
    for Enum_Rep'Size use Enum'Size;
    function To_Rep is new Ada.Unchecked_Conversion (Enum, Enum_Rep);
    function From_Rep is new Ada.Unchecked_Conversion (Enum_Rep, Enum);

All of this is portable, and required for any Annex C compliant compiler.
The only loss here is writing a bit of extra text (these declarations, and
type conversions on the results of the functions). This is a rare enough
need that that doesn't seem too bad.

                  Randy.