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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: List Container Strawman 1.4
Date: Fri, 28 Dec 2001 14:49:09 -0500
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Date: 2001-12-28T14:49:09-05:00
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"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C2CA143.6030006@mail.com...
> Ted Dennison wrote:
>
> > Actually, I was thinking along the lines of an Containers.Lists.Fixed
>
>  > package that works with arrays.
>
> I think this kind of thing is where C++'s iterators and algorithms
> approach shows its superiority. Why should a List have any notion
> of other data structures to which its elements can be copied?

You're right, it shouldn't.  Iterators allow you to abstract away the
differences among different containers.  That's the whole point.

The app I will be submitting to the Ada-Belgium competition includes a list
container type that allows you to copy its items using iterators, like this:

List : Integer_Lists.List_Type;
...
declare
   Items : array (1 .. Length (List)) of Integer;
   Iter : Iterator_Type := First (List);
begin
   for I in Items'Range loop
      Items (I) := Item (Iter);
      Next (Iter);
   end loop;
end;

(My list type has selectors that return iterators designating First, Last,
Front, and Back.  The latter two are sentinals.)

It would be easy to generalize this algorithm to copy items from any
container type with an iterator:

generic
   type Iterator_Type (<>) is private;
   type Item_Type is private;
   type Item_Array is array (Positive range <>) of Item_Type;
   with function "=" (L, R : Iterator_Type) return Boolean is <>;
procedure Generic_Copy
  (First  : in      Iterator_Type;
   Back : in      Iterator_Type;
   Items : in out Items_Type);

procedure Generic_Copy (...) is
   Iter : Iterator_Type := First;
   I : Positive := Items'First;
begin
   while Iter /= Back loop
      Items (I) := Item (Iter);
      Next (Iter);
      I := I + 1;
   end loop;
end Generic_Copy;

If you wanted, you could generalize this further (as the STL does) to
abstract away the target type (in the example above we've hard-coded an
array type as the target type).  To do this for arrays you'd need an
appropriate iterator type; maybe Interfaces.C.Pointers would work.