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From: reagan@hiyall.enet.dec.com (John R. Reagan)
Newsgroups: comp.lang.pascal
Subject: Re: UNtagged variant records and JPAM
Message-ID: <17209@shlump.nac.dec.com>
Date: 13 Nov 90 15:08:24 GMT
Sender: newsdaemon@shlump.nac.dec.com
Organization: Digital Equipment Corporation
Lines: 30


In article <225.273b14fa@vger.nsu.edu>, g_harrison@vger.nsu.edu writes...
> 
>1.	Although I can't find it in Wirth and Jansen (ISO STANDARD), is there 
>a guarantee that this will give the bit implementation of the integer?  

First of all J&W is not the same as the ISO 7185 unextended Pascal
standard.  Several things are different (procedural parameters, conformant
arrays, etc.)  Second, the standard does not guarantee this functionality.
You have an invalid program.  It tends to work for most compilers due to
popular allocation schemes to save on memory and the fact that detecting
the use of undefined variants is expensive to implement.  Since most compilers
don't check for this error (including VAX Pascal), you get lucky most of the
time.  The standard does not say how a compiler should allocate fields
in a variant record.  There are cases in VAX Pascal where variant parts
do not overlap (those which contain file variables).  The standard is
even flexible enough for a compiler to not allocate fields that are
never referenced (if it can determine such a fact).

Untagged variants have much the same uses as tagged variants.  They can
describe "generic" records which can mutate forms (variants allocated
with the NEW(p,c1,...cn) form cannot mutate) during the program execution.

---
John Reagan, VAX Pascal Project Leader and X3J9 Secretary
Digital Equipment Corporation
reagan@hiyall.enet.dec.com
Disclaimer:  The opinions and statements expressed by me are not
             necessarily those of Digital Equipment Corporation.
---