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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Discriminants of tagged types
Date: Wed, 27 Oct 2010 09:52:48 -0400
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Date: 2010-10-27T09:52:48-04:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Wed, 27 Oct 2010 05:34:26 -0700 (PDT), Ludovic Brenta wrote:
>
>> The tag has no default value and cannot have one, for that would
>> allow changing the type of an object at run time (through assignment
>> of an aggregate).
>
> No, it would not because you cannot assign a value of another type.

I think Ludovic means that the Tag could change.  That is, tags
are sort of like discriminants, so if Tags could have defaults,
then we can guess that they would inherit the strange rule "can change
if defaulted".  So this would work:

    A : T1;
    B : T'Class := A;
    C : T2;
    
    B := C; -- Raises an exception.

>> Therefore, *additional* discriminants cannot have default values either.
>
> I think there were concerns about implementation difficulties when the
> derived type adds new discriminants and/or defines some inherited ones.
>
> Anyway the limitation is indeed annoying.

Then you won't like my opinion, which is that this feature should
never have existed even for untagged types.  It's useful, but
it's just not worth the trouble, in terms of programmer confusion,
and implementation difficulty.  Also nonportability:

    type T(Length: Natural := 0) is
        record
            S: String(1..Length);
        end record;

This works on some implementations, and not others.

- Bob