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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Proposal: Auto-allocation of Indefinite Objects
Date: Fri, 31 Jul 2020 12:20:07 +0200
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On 31/07/2020 11:25, Stephen Davies wrote:
> On Friday, 3 April 2020 at 23:48:43 UTC+1, Stephen Davies wrote:
>> type Node_Type is record
>> Item : new Element_Type;
>> Prev : Node_Ptr_Type;
>> Next : Node_Ptr_Type;
>> end record;
> 
> Perhaps a better way to think of this is as a discriminated record without
> an explicit discriminant? In which case maybe a different syntax than "new"
> would be appropriate?

That is right, but it would be a much larger problem to resolve than 
your original proposal.

First, Ada lacks necessary discriminants, e.g.

   type Node_Type (Descendant : Ada.Tags.Tag) is record
      Item : Element_Type (Tag => Descendant);
      Prev : Node_Ptr_Type;
      Next : Node_Ptr_Type;
   end record;

Here Element_Type is a specific type from Element_Type'Class which tag 
is Descendant.

Second, Ada's object initialization process has no means to determine 
discriminants. One extra step is missing before a call to allocator to 
provide constraints of a type. [I proposed a record type T'Constraints 
that would contain all constraints and no data to be passed to allocator 
to determine the object constraints and thus its size.]

Third, there is still the problem that task components do not work, as 
well as all other cases when the Rosen trick is used to work-around full 
multiple inheritance by making record members out of intended parents.

All in one, the crude work-around has always been an access type, which 
your original proposal would handle with much more grace.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de