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Date: Wed, 13 Jul 2005 03:15:57 +0200
From: Georg Bauhaus <bauhaus@futureapps.de>
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Newsgroups: comp.lang.ada
Subject: Re: GCC 4.0 Ada.Containers Cursor danger.
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Date: 2005-07-13T03:16:01+02:00
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MMM wrote:

>>Cursors are among the means to choose different
>>containers when the need arises, ideally without disturbing the rest of
>>the program.
> 
> I can't say I understand you here, but anyway, how cursors could
> help to change container type without disturbing the rest of the
> program?
> [...]
> Care to provide an example of such a generic algorithm?

Here is a silly example demonstrating a subprogram that is
independent of any specific container. That is, you can choose
any instance in place of My_Container.
 As arguments to the following function, use cursors
returned by Floor and Ceiling (to mark a range of
associations in a map.) Use First and Last of a set to express
forall. Do the same using a vector.


with My_Container;
   -- some instance of some Ada.Container ("abstract" if you wish)

function gen_find
  (first,
   last: My_Container.Cursor;
   item: My_Container.Cursor) return My_Container.Cursor

--  The first cursor of the sequence from `first` to `last` that designates
-- an elemenent equal to the element designated by `item`.
-- `No_Element` if not found.
is
   use My_Container;

   p: Cursor := first;
   off: constant Cursor := Next(last);
begin
   loop
      exit when p = off or else Element(p) = Element(item);
      next(p);
   end loop;

   if p = off then
      return No_Element;
   else
      return p;
   end if;

end gen_find;