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From: thomasw@hpcupt1.HP.COM (Thomas Wang)
Newsgroups: comp.std.c++
Subject: Re: Casting within MI Graphs
Message-ID: <52390001@hpcupt1.HP.COM>
Date: 28 Aug 90 16:48:01 GMT
References: <48168@brunix.UUCP>
Organization: Hewlett Packard, Cupertino
Lines: 27

/ hpcupt1:comp.std.c++ / sdm@cs.brown.edu (Scott Meyers) / 12:23 pm  Aug 27, 1990 /
In article <CLINE.90Aug27124642@cheetah.ece.clarkson.edu> cline@sun.soe.clarkson.edu (Marshall Cline) writes:

>Unfortunately, I don't have access to the C++ Report, but the commentary of
>E&S outlines an implementation for virtual base classes that makes it
>impossible to do downward casts such as I described, and I suspect this is
>the implementation discussed in the C++ Report.  As the discussion in E&S
>implies, there are alternative implementations for virtual bases that allow
>for well-defined downward casts, but they require more memory per object.

The current C++ implementation of MI have yet another defect.  The location
of data members inside a virtual object is undefined until the object is
initialized.  This means that functions such as print_all_data_member(), when
called during initialization phase, would segment fault because it would
likely access address 0.

The function mark_all_data_member(), and sweep_all_data_member() would not
work when called during construction phase.  These two subroutines are used to
implement mark & sweep memory management.  They will be called anytime memory
is nearly exhausted.

> Scott

 -Thomas Wang
              (Everything is an object.)
                                                     wang@hpdmsjlm.cup.hp.com
                                                     thomasw@hpcupt1.cup.hp.com