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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Behavior of Stream Attributes On Access Types.
Date: Tue, 11 Jun 2002 16:56:25 -0500
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Date: 2002-06-11T16:56:25-05:00
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Ted Dennison wrote in message
<4519e058.0206110547.526d2369@posting.google.com>...
>"Marin David Condic" <dont.bother.mcondic.auntie.spam@[acm.org> wrote
in message news:<ae2v7f$r12$1@nh.pace.co.uk>...
>> My inclination is to think that the sensible thing would be to call
the
>> 'Read or 'Write for the thing pointed to by the access type, but this
has
>
>You can pretty much count on it *not* doing that. For instance, what
>would happen in that case with a circularly-referenced structure like
>a doubly-linked list?
>
>
>> implications for dynamically allocated objects. Will it write/read a
>> (totally useless) access value?
>
>Pretty much. It isn't *totally* useless. It will work just fine within
>the same execution (assuming you don't deallocate the memory).
>However, it is next to useless.


There is one more thing you can do with it: test it for nullness.
Although that is not guarenteed to work, it would be an unusual system
indeed where it did not. This can be very useful. The Janus/Ada compiler
uses this to store and reconstruct linked lists: (Note that Janus/Ada,
as an Ada 83 app., doesn't use the stream attributes themselves but a
very similar implementation specific package)

              type Data;
              type Ptr is access Data;
              type Data is record
                  ...
                  Next : Ptr;
              end record;

              List : Ptr;


              procedure Write_List (To : Stream_Access) is
                   Walk : Ptr := List;
              begin
                   Ptr'Write (To, Walk);
                   while Walk /= null loop
                         Data'Write (To, Walk.all);
                         Walk := Walk.Next;
                   end loop;
              end Write_List;

              procedure Read_List (From : Stream_Access) is
                   Work : Ptr;
              begin
                   Ptr'Read (From, Work);
                   if Work /= null then
                        List := new Data;
                        Work := List;
                   else
                        List := null;
                        return; -- Nothing to read.
                   end if;
                   loop
                          Data'Read (From, Work.all);
                          if Work.Next = null then
                                return;
                          else
                                Work.Next := new Data;
                                Work := Work.Next;
                         end;
                   end loop;
             end Read_List;

(There's probably a simpler way to read the list, but you get the
idea...)

               Randy.