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From: Scott Renfro <srenfro@sirinet.net>
Subject: Discriminant as default initial value
Date: 1997/03/29
Message-ID: <333DB640.59EA@sirinet.net>#1/1
X-Deja-AN: 229351145
Organization: Sirius Systems Group, Inc.
Newsgroups: comp.lang.ada
Date: 1997-03-29T00:00:00+00:00
List-Id: <comp.lang.ada>


Greetings,

I'm new to Ada and have scoured the FAQ, RM, and Rationale, but haven't
found an answer to my question that I understand.

I want to use a discriminant in a record as both a bound in the index
constraint of a component declaration and as a default initial value. 
I'm not sure that this is allowed by the RM.

The following example, while contrived, exhibits behavior that I
definitely do not understand.  Using GNAT 3.09 for NT with Max defined
before the Position Matrix, the component record is initialized as
expected.  When Max is defined after the Position Matrix within the
record, Max is initialized to a large value that is not even within the
subtype's range.  No exception is raised.

Is this expected behavior?  Is it permissible to use the same
discriminant as both an index bound and a default initial value?  Should
the behavior change when the order of declaration within the record
changes?

Thanks in advance,
Scott

-----------------------------------------------------
--
-- Test_Discrim.adb
--
-----------------------------------------------------
with Ada.Text_IO, Ada.Command_Line;
use  Ada.Text_IO;

procedure Test_Discrim is

   type Matrix is array (Integer range <>, Integer range <>) of Integer;
   subtype Matrix_Range is Natural range 1 .. 80;
   
   type Matrix_Record (Columns : Matrix_Range) is
      record
--         Max      : Natural := Columns;   -- if declared here, all is
well
         Position : Matrix (1 .. Columns, 1 .. Columns);
         Max      : Matrix_Range := Columns;   -- if declared here, max
improper
      end record;

   width : Matrix_Range := 1;
   
begin
   width := Integer'value (Ada.Command_Line.Argument (1));
   declare
      Temp_Matrix : Matrix_Record (Width);
   begin
      Put_Line (Integer'Image (Temp_Matrix.Max));
   end;
end test_discrim;