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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Separate type for byte arrays
Newsgroups: comp.lang.ada
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Organization: cbb software GmbH
References: <87irolhpzo.fsf@mid.deneb.enyo.de>
Date: Thu, 4 May 2006 18:17:44 +0200
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Date: 2006-05-04T18:17:28+02:00
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On Thu, 04 May 2006 15:35:39 +0200, Florian Weimer wrote:

> Does it make sense to provide low-level I/O interfaces in terms of
> octet arrays as well as in terms of strings?  In other words,
> 
>   procedure Read
>      (Ch : in out Input_Channel;
>       Item : out Byte_Array;
>       Length: out Byte_Count);
> 
> in addition to 
> 
>   procedure Read
>      (Ch : in out Input_Channel;
>       Item : out String;
>       Length: out Natural);
> 
> ?  This is a bit cumbersome.  Unfortunately, it's not possible to make
> Read itself generic because it's a dispatching subprogram.  If you
> parameterize the whole I/O subsystem on the element and array type,
> you still have the problem that no efficient communication is possible
> between two subsystems which store data in different array types
> (without using Unchecked_Conversion).  The other alternative is to
> create some kind of buffer object which can be accessed as octet and
> character array.  I've implemented this, and it doesn't look quite
> right, either.
> 
> Any suggestions?  I'm aiming for something similar java.nio.*.

The requirements are somewhat unclear. The transport layer deals with
octets and the application layer wants strings? If so, then I see nothing
wrong with your buffer object approach. Or something with like this:

package Abstract_Channel is
   type Input_Channel is abstract ...;

   procedure Read
      (Ch : in out Input_Channel;
       Item : out Byte_Array;
       Length: out Byte_Count) is abstract; -- To be implemented

   procedure Read
      (Ch : in out Input_Channel'Class;
       Item : out String;
       Length: out Natural); -- Uses octet Read internally

I would also suggest a function like:

   function Read_Packet (Ch : access Input_Channel'Class) return String;

used as:

... loop
   declare
      Packet : String renames Read_Packet (Channel'Access);
   begin
      -- Process the packet
      ...
   end;
end loop;

in hope that the compiler wouldn't do an extra copy. In Ada 200Y you could
use an anonymous access result, I suppose.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de