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From: GDAU100@BGUVM.BITNET ("Jonathan B. Owen")
Newsgroups: comp.lang.ada
Subject: Variable Length strings...
Message-ID: <8906031720.AA05393@ajpo.sei.cmu.edu>
Date: 4 Jun 89 07:03:21 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
X-Unparsable-Date: Sat, 03 Jun 89 20:05:44 IST
List-Id: <comp.lang.ada>

Thomas Hoyt writes that having the following:

   generic
         Max_len : Integer;
   package VSTR is

         subtype Len_range is integer range 0..Max_len;

   etc...

DOES imply a maximum length for a discriminant of type Len_range, be it
integer'LAST.   Also, he suggests using a fixed upper limit to Max_len,
say 20000.

   First, as far as I observed, on Vax Ada, the actuall value for Max_len
for a particular instantiation is taken as the upper limit for a disc.
of type Len_range.   I see no reason why Verdix Ada should not do it.

   Secondly, I do believe defining the range of Max_len to be limited to
20000.   Still, I think it would be unreasonable to allocate 20K for each
V_string defined, when typicalalues for Max_len would be much smaller
(say 15, 80 and such).

   It seems to me Verdix Ada handles Dynamically defined types/Subtypes
differently than the Vax Ada.

   Come to think of it, I ran into a related problem as the above, in
a non-generic package.

   Consider the following:

       type Record_header is
           record
               key  : Key_type;
               time : Time_type;
           end record;

       Max_data_len : constant integer := 2048-Record_header'SIZE/8;

       subtype Data_len_range is integer range 0..Max_data_len;

       type Record_type( data_len : Data_len_range := 0 ) is
           record
               header : Record_header;
               data   : String(1..data_len);
           end record;

Say the Record_header occupies 48 bytes.  The purpose of the above is
to define a record which would occupy 2K at the most, including the
header.   The above works find on Vax-Ada but fails on Verdix with
the same problem of not recognizing the upper limit of Data_len_range.

                      I hope the shortcoming of Verix is now obvious...

                                                   Jonathan

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