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From: jbn@lulea.trab.se (Johan Bengtsson)
Newsgroups: comp.std.c++
Subject: Re: Zero-length structures and pointer comparisons
Message-ID: <5458@holden.lulea.trab.se>
Date: 21 Dec 92 19:30:51 GMT
References: <9235622.25587@mulga.cs.mu.OZ.AU>
Organization: Telia Research AB, Aurorum 6, 951 75 Lulea, Sweden
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Cc: jbn
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Fergus James HENDERSON (fjh@munta.cs.mu.OZ.AU) wrote:
: jbn@lulea.trab.se (Johan Bengtsson) writes:

: >Markku Sakkinen (sakkinen@jyu.fi) wrote:
: >: BTW, actually total order is
: >: a simple thing within an OODB _because_ the identity of each object
: >: is assured not to change.
: >
: >It would impose severe restrictions on how the virtual address space
: >could be used though.  The OODBMS may end up in a situation where
: >it can't get a persistent object into memory, not because it has used up
: >all virtual memory addresses, but because all unused virtual memory
: >adresses would violate the total ordering property.

: I thought that the idea was that for OODBMSs, you DON'T compare virtual
: addresses for pointers to persistent objects, instead you compare the object
: ids. Maybe someone could explain the problems in a little more detail?

As Stuart MacMartin said, if the OODBMs uses objects of class Handle
(never mind the name) to represent references to persistent objects
you will need to have

ptrcmp(Handle,Handle), if you want templates to compare Handles the
same way they compare pointers.  The OODBMs should provide that.
Also it should provide ptrcmp(Handle,void*) and ptrcmp(void*,Handle).

_Some_ OODBMs use void* as their Handles (they play tricks by
using virtual memory mapping to persistent storage), so they must
provide a ptrcmp(void*,void*) that can tell the difference between
pointers to persistent objects and normal memory pointers.  For persistent
objects, it then finds the object ids and makes the comparison.

My point is that this may be difficult, or slow (although I don't
really have any data on that).  The alternative is to simply compare
the void pointers, even for pointers to persistent objects, however
that imposes the unacceptable restrictions I described in my previous
posting.

A more difficult issue is what do you do if there is more than one
OODBMs in use, or more realistically, one OODBM, and one distributed
object manager (that similarly uses Handles to distributable objects).
Who is responsible for providing the needed ptrcmp(OODB_Handle,Distr_Handle)
and ptrcmp(Distr_Handle,OODB_Handle) functions?  This may become
totally unmanageable if handle-based schemes become common.

Hope this is a little bit clearer.

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| Johan Bengtsson, Telia Research AB, Aurorum 6, S-951 75 Lulea, Sweden  |
| Johan.Bengtsson@lulea.trab.se; Voice:(+46)92075471; Fax:(+46)92075490  |
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| Johan Bengtsson, Telia Research AB, Aurorum 6, S-951 75 Lulea, Sweden  |
| Johan.Bengtsson@lulea.trab.se; Voice:(+46)92075471; Fax:(+46)92075490  |
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