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From: stt@inmet.inmet.com
Newsgroups: comp.lang.ada
Subject: Re: assigning variant records
Message-ID: <20600073@inmet>
Date: 19 Nov 90 23:16:00 GMT
References: <13567@june.cs.washington.edu>
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Nf-From: inmet.inmet.com!stt    Nov 19 18:16:00 1990
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Re:  assigning variant records

> /* Written 6:48pm Oct 29,1990 by simon@cs.washington.edu */
> 
> He-e-e-elp!
> 
>      I have gotten the following Ada code to compile on a Verdix compiler
> used by the P-3 project at Boeing A&E, and have run it several times.  Now
> that I'm a grad student at the University of Washington it still compiles,
> but when I try to run it, inputting into "get_line" a string of length 1 or
> more (as shown in the following debugging sessions), I get an
> '"Illegal instruction" [4]' message, followed by
> '--> Software trap: range check', as soon as I try to assign "Y.all := X;".

Your problem is that objects created by an allocator are always
constrained, even if the declaration of the record
type has defaults for all discriminants.

RM 4.8:5 states: "If the type of the created object is an array type
  or a type with discriminants then the created
  object is always constrained.  If the allocator includes a subtype
  indication, the created object is constrained either by
  the subtype or by the default discriminant values."

In your case, you had "Y := new XX;".  This creates
an object which is constrained by the default discriminant value,
which is 0.  It rejects your attempt to assign a value into
it with a different discriminant.

>      Can anybody make any sense out of this?  What am I doing wrong?  I'd
> really appreciate any advice anyone can give me.  Thanks,
> 
>                                              ---Kevin Simonson

I hope this helped.

S. Tucker Taft
Intermetrics, Inc.
Cambridge, MA  02138