Re: Direct_IO problem -- generic package problem

["David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>]

I don't believe the gnat pragma will help in this situation, because
the restriction is on Ada95 that was not in Ada83.  The real
solution is to fix the code which is very simple to do, viz.:

This sounds like the restriction that was put on generic formal type
declarations with no discriminant part that they be instantiated
only with definite subtypes

If the generic is to be instantiated with indefinite subtypes, then the
formal type declaration of the generic must include the unknown
discriminant part (<>).

For example, if the declaration of Element_Type is

    type Element_Type is private;

then changing the declaration to

    type Element_Type (<>) is private;

will allow (but not require) instantiation to be with indefinite
types.

This is discussed in section 31. of the document found at
http://www.adaic.org/docs/compat-guide/compat-guide6-0.txt

This document deals with the other Ada83/Ada95 compatibility
issues, as well.

----- Original Message -----
From: "nicolas" <n.brunot@cadwin.com>
Newsgroups: comp.lang.ada
To: <comp.lang.ada@ada.eu.org>
Sent: July 12, 2001 4:19 AM
Subject: Re: Direct_IO problem -- generic package problem


> If you are using gnat, you can add
> pragma Ada_83;
> in this body to enforce Ada_83 restrictions (provided you don't use specific
> Ada95 features in this body)
> But this is really a bug of Ada83 cerrected in Ada95
> Nicolas
>
> "Magnus Sparf" <magnus-s@dsv.su.se> a �crit dans le message news:
> 9ijobe$qb4$1@mercur.foa.se...
> > I�m new to ADA (hopefully picking up track fast) and this summer I have
> the
> > task of converting an old Ada83 application (VAX environment) to Ada95 (PC
> > environment).
> > In the very heart of this application there is a homemade database that
> > compiles without problem in the VAX environment. When I compile it on GNAT
> > 3.13p in the PC environment. I get the following error:
> >
> >  db.adb:58:49: actual for "Element_Type" must be a definite subtype
> :::(the
> > last line in the extract)
> >
> > Extract of code (DB is a generic package and used to create a lot of
> > different databases):
> > --------------------------------------------------------------------------
> --
> > -----------
> > package body DB is
> >
> >       type FILLED_TABLES is array (SMALL_NAT range <>) of BOOLEAN;
> >    pragma pack (FILLED_TABLES);
> >
> >       type LINE_COL is
> >          record
> >             X_V : SMALL_NAT := 1;
> >             Y_V : SMALL_NAT := 1;
> >          end record;
> >
> >       type TABLE_VALUES  is array (SMALL_NAT range <>) of LINE_COL;
> >
> >       type M_T             is array (SMALL_NAT range <>,
> >       SMALL_NAT range <>) of ITEM;
> >       type B_T             is array (SMALL_NAT range <>,
> >       SMALL_NAT range <>) of BOOLEAN;
> >
> >
> >       type MATRIX (MAX : SMALL_NAT;
> >       FIRST_RECORD : BOOLEAN;
> >       ROWS_FIRST     : SMALL_NAT;
> >       ROWS_LAST      : SMALL_NAT;
> >       COLUMNS_FIRST  : SMALL_NAT;
> >       COLUMNS_LAST   : SMALL_NAT) is
> >
> >          record
> >             case FIRST_RECORD is
> >                when TRUE =>
> >                -- description of entire file with tables
> >                   FILLED              : FILLED_TABLES(2..MAX) :=
> >                   (others => FALSE);
> >                   DISCRIMANT_VALUES : TABLE_VALUES(2..MAX) :=
> >                   (others => (1, 1));
> >                when FALSE =>
> >                -- tables
> >                   VALUE        : M_T (ROWS_FIRST..ROWS_LAST,
> >                   COLUMNS_FIRST..COLUMNS_LAST);
> >                   ITEM_PUT  : B_T (ROWS_FIRST..ROWS_LAST,
> >                   COLUMNS_FIRST..COLUMNS_LAST);
> >                   RAD_INDEX    : R_T (ROWS_FIRST..ROWS_LAST);
> >                   COLUMN_INDEX : C_T (COLUMNS_FIRST..COLUMNS_LAST);
> >             end case;
> >          end record;
> >
> >       package MY_DIRECT_IO is new Ada.DIRECT_IO(MATRIX);
> > --------------------------------------------------------------------------
> --
> > -------------
> >
> > If I give type MATRIX default values it compiles with warnings about
> Storage
> > constraints errors in runtime (that also occurs...). MATRIX is used at a
> lot
> > of other places further down.
> >
> > Are there any quick (not dirty) solutions to this problem? My own ideas is
> > to create separate databases for all eight databases that uses DB (CAR_DB,
> > BOAT_DB, HOUSE_DB...) with specified values for MATRIX, but that seems
> like
> > a lot of work and not very objectoriented.
> > I�ve read the GNAT RM about the problem (Ada95 <-> Ada83) "Indefinite
> > subtypes in generics" but don`t seem to understand, especially when
> there�s
> > a standard package involved.
> >
> > All you experienced ones out there leave the "Ada is dead"-discussions and
> > pleaseeeeee help me out...
> >
> > Magnus Sparf
> > magnus-s@dsv.su.se
> >
> >
> >
>
>
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