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From: lutz@iks-jena.de (Lutz Donnerhacke)
Newsgroups: comp.lang.ada
Subject: Re: Ada 200X Assertions
Date: Wed, 5 Dec 2001 15:16:45 +0000 (UTC)
Organization: IKS GmbH Jena
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* Matthew Heaney wrote:
>"Lutz Donnerhacke" <lutz@iks-jena.de> wrote in message
>> Problem 1: Pointer of component => Pointer to aggregate
[...]
>>    Of course this Conversion may return a Pointer to an invalid aggregate.
>
>What on Earth are you trying to do here?

I've a list of degenerated compound instantiations for which only a single
component is relevant. More than this, this same fixed component is shared
by multiple compound instantiations.

Typical example 1: A common termination node of linked lists.
Typical example 2: A head node of a linked list contains a next pointer which
                   should be handled exactly as the next pointer of the real
		   list nodes, but no payload at all.

It would be fine to generate a pointer from this component to a 'virtual'
aggregate of the given type.

Example:
   struct node {
     ... data ...
     struct node * next;
   }
   struct head {
     struct node * next;
   }
   
   struct head h;
   h.next = &h - sizeof (node) + sizeof (head);

AdaYY should be able to represent such structures.

>Of course the language already allows you to move from a pointer to
>component to a pointer to aggregate: just use an access discriminant.

This generates a completely different structure.

>   procedure Iterate (List : in out List_Type) is
>      Item : List_Item_Class_Access := List.Head;
>   begin
>      while Item /= null loop
>         Process (Item);
>         Item := Item.Next;
>      end loop;
>   end;

and end in a different algorithm:

  procedure Iterate (List : in out List_Type) is
     Item : List_Item_Class_Access := List.Next;
  begin
     while Item /= Item.Next loop
        Process (Item);
        Item := Item.Next;
     end loop;
  end;