Re: A novel design of linked lists (was: Address of an object)

["Adam Beneschan" <adam@irvine.com>]

Dmitry A. Kazakov wrote:
> [ It seems to me that some people have an impression that I am looking for
> something extraordinary when asking for the object address, so I feel
> necessary to provide an elaborated example where it were needed. ]
>
> Let's consider referential double-linked lists as an example. The drawbacks
> of the Ada.Containers design are obvious. A typical design of such lists in
> other languages would be adding Prev, Next pointers to the elements. Note
> also, that we wished to have linked lists of elements which might
> participate in many lists simultaneously. We also liked to statically check
> the types of such lists, so that paths in different lists could never
> converge.
>
> There are two ways to do add links Prevs and Nexts. One of them is to do it
> upon inheritance. Another is by using new elements containing pointers to
> the "true" elements.
>
> 1. The variant with inheritance is not an option in current Ada, because of
> lack of MI. Even with MI it faces the problem that links has to be added to
> the base type. That would require downcasting later, all over the program.
> Adding links in the leaves of the type hierarchy would break it apart and
> also freeze the types for future derivation. Ad-hoc supertypes could help,
> but, firstly, they are not supported and, secondly, they barely could have
> referential semantics. So the elements will require copying.
>
> 2. The variant with pointers is not a solution at all, because, honestly,
> it would be a list of pointers rather than of the elements.

I'll have to admit that I don't see what "problem" you have that this
isn't a solution to.  In fact, I'm not really clear on why it's
important to you to adopt the approach you're adopting; your
explanation is pretty sketchy.

Part of my problem is that you seem to want to define a type, Item_Ptr,
that gives you more than it's supposed to give you.  Your generic
declares

   type Item_Ptr is access List_Type_Item;

where List_Type_Item is whatever type you're making a list out of.  I
see a type like this, and I think it's just an access to the
List_Type_Item, but there's actually additional baggage in this
pointer, i.e. a "next" and a "previous" pointer.  So if I see code that
calls Next on this Item_Ptr, my first thought is "Whooaa!  Item_Ptr is
just an access to my Employee record, so where the heck is it going to
get a pointer to another Employee?"  It would be pretty confusing to
me. Personally, if I'm going to have something that contains additional
information, I'd prefer that it be a different type.  The need for
downcasting, type conversion, or a Deref function in your generic that
takes an Item_Ptr and returns the dereferenced item that I would use
instead of .all, doesn't bother me at all.  So I don't see what problem
there is that needs solving.

Plus, you introduce a problem.  Assuming the actual for List_Type_Item
is T, you've defined Item_Ptr as an access to T; what happens if
somewhere else in the program, there is another type that is also an
access to T?  The language will let you convert between the two access
types, but the result would be an absolute disaster.  But I just know
that someone who doesn't really understand that Item_Ptr isn't just an
access to T is going to try it.

Put simply, you're trying to trick your Ada compiler into allowing
programmers to use Ada syntax to write code in some other language than
Ada, and someone who looks at the Ada code and assumes it's Ada is
likely to be pretty darn confused.

Anyway, now that I've given you my tirade on why I think this is a bad
approach, I'm going to give you an idea of how you might implement your
bad approach.  The problem with your Next function is that if
List_Item_Type is an unconstrained array, there will be extra data
between the thing Item_Ptr points to and the Item_Header.  Since you've
already admitted that this is going to be highly
implementation-dependent, then you can probably assume that the size of
this extra data is going to be the same always (for a given
List_Item_Type), and then all you have to do is figure out what that
size is the first time Next [or some other routine] is called.  Declare
a variable in your package body:

    Dope_Offset : Storage_Offset := -1;

Fix your Allocate so that it saves the resulting Storage_Address in
some global Global_Storage_Address in the package body before it
returns.  Inside Next:

   X : Item_Ptr;
   ...
   if Dope_Offset = -1 then
     X := new List_Item_Type' (Item.all);
     Dope_Offset := To_Address(X) - Global_Storage_Address;
     Free(X);
  end if;

where To_Address is from an instance of
System.Address_To_Access_Conversions.  Now once Dope_Offset has been
set up, you can use it as an offset and subtract it from your
From_Item_Ptr calls.  

                         -- Adam