Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!apollo!mrst!sdti!turner
From: turner@sdti.SDTI.COM (Prescott K. Turner)
Newsgroups: comp.lang.c++
Subject: Re: Objects and Shared Memory
Summary: surrogate solution problematic
Message-ID: <448@sdti.SDTI.COM>
Date: 11 Apr 89 22:17:00 GMT
References: <5462@cbnews.ATT.COM> <9171@alice.UUCP>
Reply-To: turner@sdti.UUCP (0006-Prescott K. Turner, Jr.)
Organization: Software Development Technologies, Sudbury MA
Lines: 31

In article <9171@alice.UUCP> shopiro@alice.UUCP (Jonathan Shopiro) writes:
>I think the best way to deal with data in shared memory is with
>what I like to call ``surrogates.''  ...
>
>If you can't have real pointers in shared memory because of address
>mapping considerations, it is simple to put whatever transformations
>are necessary between the data in Actual_shared_data and regular
>pointers into access functions in Shared_data.  Shared_data can also
>have whatever virtual functions are appropriate. 

At SDTI we have been dealing with a similar problem using a "surrogate"
solution.  Our most significant objects go in the memory which can't have
real pointers.  Even though Shared_data can have virtual functions,
and can refer to the Actual_shared_data object, it will behave according
to how the Shared_data object was constructed, not according to how the
Actual_shared_data object was constructed.  This has been so restrictive
that I wonder if I'm getting any experience with real object-oriented design
in my area of application.

I've been tempted to hack at Shared_data's constructors.  When a Shared_data
object is constructed which refers to a pre-existing Actual_shared_data
object, the underlying vtbl pointer of the constructed Shared_data object
would be modified.  It would then refer to the class derived from Shared_data
which corresponds correctly to the Actual_share_data object.

Anyone care to comment on the level of danger of such a hack?
--
Prescott K. Turner, Jr.
Software Development Technologies, Inc.
P.O. Box 366, Sudbury, MA 01776 USA         (508) 443-5779
UUCP: ...{harvard,mit-eddie}!sdti!turner    Internet: turner@sdti.sdti.com