Using the GNAT defined attribute: 'Enum_Rep

["Marin David Condic, 561.796.8997, M/S 731-96" <condicma@PWFL.COM>]

    Here's a little problem which I could use some help handling:

    I get external data for my program which is extracted from the
    internal representation of an enumerated type. That is to say, I'm
    getting integers which correspond to the representation clause of
    the enumerated type, not the position. On input, they are read as
    integers and I'd like to convert them to the enumerated type.

    The ARM suggests using Unchecked_Conversion to get from here to
    there, but now you've got to create an integer type that is the
    same size as the enumerated type. Here's a code fragment:

    --
    type Temp_Integer is range Integer'First..Integer'Last ;
    for Temp_Integer'Size use User_Types.CL_Evt_Status'Size ;
                            --^^^^^^^^^^^^^^^^^^^^^^ The enumerated type.

    function
        CVT_Evt_Status
    is new
        Ada.Unchecked_Conversion (
            Source  => Temp_Integer ;
            Target  => User_Types.CL_Evt_Status) ;

    You can now use the CVT_Evt_Status function to get from the
    external integer to the internal representation. However, the
    declaration of Temp_Integer might not work because you don't know
    that the 'Size of the enumerated type is big enough for the range.
    You can easily build a custom one-off that will work if you know
    enough about the type and it's representation, but it would be
    nicer to have a general solution which will work for any
    enumerated type & representation.

    GNAT is nice enough to provide the attribute: 'Enum_Rep which
    returns a Universal_Integer of the internal representation of an
    enumerated object. (I'm surprised this isn't standard!) This is a
    great answer when I'm writing the data out - all I've got to worry
    about is that the enumeral doesn't create a constraint_error when
    assigned to an integer object for output. What I'd like is a
    corresponding "'Integer_Rep" (or some other more appropriate name)
    attribute which converted from a Universal_Integer to the internal
    representation of the enumerated type - risking only
    Constraint_Error if the integer is not a member of the
    representation set.

    Has anyone got a suggestion as to how this can be done more
    elegantly with some other attributes or Unchecked_Conversion? I
    don't see any simple way to do this. (Please don't suggest 'Pos
    and 'Val. These will work If and only if the representation is
    identical to the position numbers - what you get if you don't
    specify a representation. What I've got are representations with
    gaps in them.) Thanks for any suggestions you have to offer.

    MDC

Marin David Condic, Senior Computer Engineer     ATT:        561.796.8997
Pratt & Whitney GESP, M/S 731-96, P.O.B. 109600  Fax:        561.796.4669
West Palm Beach, FL, 33410-9600                  Internet:   CONDICMA@PWFL.COM
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