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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Counter-proposal for variable arrays
Date: Mon, 06 Oct 2003 15:12:29 +0200
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Date: 2003-10-06T15:12:29+02:00
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On Sun, 05 Oct 2003 05:52:06 GMT, Hyman Rosen <hyrosen@mail.com>
wrote:

>Dmitry A. Kazakov wrote:
>> Pointer arithmetics is not yet an array.
>
>No. But an array is an array. What are you talking about?

Mmm I guess array is something that one can pass as a parameter. For
example:

void Erase (char X [20])
{
   memset (X, 0, sizeof (X)); // How many element will be set?
}

>> an array of class-wide elements forces use of all sorts of pointers.
>
>Yeah. So what? What exactly would the implementation of an array of
>class-wide elements look like, anyway?

For example:

-- This is not Ada, Syntax is imaginary.
type Window is tagged ...;
type Child_Window is new Window with ...;
type Pop_Up_Window is new Child_Window with ...; 

type Container (From, To : Tag) is
   array (Integer range <>) of
      Window'Class (From..To);

Wild_Array : Container  (1..20);
   -- Compile error, not constrained

Children_And_Pop_Up_Array :
   Container 
   (  1..20, -- Constrain the bounds
      From => Child_Window'Tag, -- Constrain the elements
      To => Pop_Up_Window'Tag
   );
   -- This is OK

The array contains 20 elements of types between Child_Window and
Pop_Up_Window. Note that because all possible types are known, the
upper bound of the element size is also known. So the array could be
allocated as usual.

Observe, that this is in full conformance with notorious Ada's concept
of unconstrained types. You can have them, but all instances have to
be constrained. Thus nothing prevents us from having an array of
class-wide elements, IF there were a way to constrain both the array
bounds (we already have this in Ada) and the elements (we do not have
this).

My proposal is to allow discriminants for array to have a way to
constrain array elements.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de