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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Is this actually possible?
Date: Wed, 11 Dec 2019 20:00:58 -0600
Organization: JSA Research & Innovation
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Date: 2019-12-11T20:00:58-06:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:qsrncj$u95$1@gioia.aioe.org...
> On 2019-12-11 22:12, Lucretia wrote:
...
> Why? You must know all variants in order to compute the array element 
> size. In Ada you cannot have it either:
>
>    type Not_an_Array is array (Positive range <>) of T'Class;
>
> This is illegal.

This is the purpose of the Bounded_Indefinite_Holders, because this is a 
useful thing to do sometimes.

    package My_Holders is new Ada.Containers.Bounded_Indefinite_Holders 
(T'Class, 100);

    type An_Array is array (Positive range <>) of My_Holders.Holder;

Here, a Holder will hold up to 100 storage units. If some extension of 
T'Class is bigger than that, setting the holder to a value of that extension 
raises Program_Error. (As with most checks, this one can be suppressed, and 
if it would fail and is suppressed, the program is erroneous - presumably 
some other object memory would be overwritten. So don't do that. ;-)

This is probably one of the best (and simplest) ideas of Ada 202x. Thanks to 
Niklas Holsti for the original idea (although I think the actual resulting 
package isn't much like Niklas' original idea).

                         Randy.