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From: Andre Spiegel <spiegel@berlin.informatik.uni-stuttgart.de>
Subject: Re: Changing discriminants at run-time: erroneous execution?
Date: 1996/08/08
Message-ID: <lhmivatq4dz.fsf@berlin.berlin.informatik.uni-stuttgart.de>#1/1
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sender: spiegel@berlin.berlin.informatik.uni-stuttgart.de
references: <lhmg25z8v2x.fsf@berlin.berlin.informatik.uni-stuttgart.de>
organization: University of Stuttgart, Germany
newsgroups: comp.lang.ada
Date: 1996-08-08T00:00:00+00:00
List-Id: <comp.lang.ada>


Thanks to all who replied to my question, both here and in private
e-mail.  I think I understand the issue now.  In summary, it *is*
possible to "resize" arrays via discriminants, but compilers normally
achieve this by allocating an object of the maximum possible size.  So
using an array index of type Natural almost certainly fails, leading
to a stack overflow.  I should use a subtype, maybe of range 1..500 or
some such.

We got bitten by a yet-unimplemented feature in GNAT; so that the
problem was not reported by the run-time system -- which should
normally happen.

Anyway, I normally allocate dynamic arrays on the heap, or declare
them only after the bounds are known, so I hadn't come across this
problem yet.  Also, for dynamic _strings_, the standard libraries
Ada.Strings.Bounded/Unbounded are the way to go, of course.  The issue
was raised by programs that somebody else in our group had written; I
looked at these programs, and I really had no idea why they failed so
mysteriously.

Thanks again to everyone,

Andre Spiegel
University of Stuttgart, Germany