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From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: AdaYY wish list from current projects
Date: 22 Feb 2001 12:08:44 +0100
Organization: Enyo's not your organization
Message-ID: <87u25nnfer.fsf@deneb.enyo.de>
References: <96ulad$r9e$1@belenus.iks-jena.de>
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Date: 2001-02-22T12:08:44+01:00
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lutz@belenus.iks-jena.de (Lutz Donnerhacke) writes:

> I have three sad problems (and a bonus problem) from my current projects.
> If they can fixed by a compiler it would be very fine:
> 
> Problem 1: Pointer of component => Pointer to aggregate
> 
>   In order to mixin a generic list with head nodes not mixed into any other
>   user defined structure I'd need to link not the base data but the mixin
>   itself. Doing so I need to regenerate the whole aggregate pointer from
>   a pointer of a component. Current (not tested) implementation:

For limited types, there is another approach:

   generic
      type Base is tagged limited private;

   package Mix_In is

      type Mixed is new Base with private;

      type Mix_In_Type (Ref : access Mixed) is tagged limited private;

      type Mix_In_Access is access all Mix_In_Type'Class;
      function Cast (B : Mixed) return Mix_In_Access;

   private

      type Mix_In_Type (Ref : access Mixed) is tagged limited record
         null;
      end record;

      type Mixed is new Base with record
         Mix_In : aliased Mix_In_Type (Mixed'Access);
      end record;

   end Mix_In;

   package body Mix_In is

      function Cast (B : Mixed) return Mix_In_Access is
      begin
         return B.Mix_In.Ref.Mix_In'Access;
      end Cast;

   end Mix_In;


It appears that Ada is not suitable for this kind of design.

> Problem 2: Defining Byte_Order of record representations
> 
>    My low level networking application has to deal with low and big endian
>    values on the net I want to handle with record representations clauses
>    in order to get the benefits of compiler generated I/O functions.

For my OpenPGP implementation, I've written my own stream I/O
functions.  That's certainly the way to go if you don't need a
specific in-memory representation.

I don't think an additional attribute is really necessary.  Instead,
vendors could provide packages like Interfaces.Little_Endian and
Interfaces.Big_Endian which contain the Integer_* and Unsigned_*
types from package Interfaces, with the corresponding in-memory and
stream representation.  If you make these types private and provide
conversion functions, you can implement these packages even yourself.

> Problem 3: Static expressions of discriminants in record representations
> 
>    Trying to may a simple data structure like a Pascal string is not possible
>    with Ada95.

The following seems to work in some cases.

   subtype Pascal_Length is Integer range 0 .. 255;

   type Pascal_String (Length : Pascal_Length) is record
      Data : String (1 .. Length);
   end record;
   pragma Pack (Pascal_String);