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From: rfg@NCD.COM (Ron Guilmette)
Newsgroups: comp.std.c++
Subject: Re: type/member tags (was Re: asking an object for its type)
Message-ID: <4332@lupine.NCD.COM>
Date: 10 Mar 91 03:20:58 GMT
References: <70877@microsoft.UUCP> <10721@dog.ee.lbl.gov> <11792@pasteur.Berkeley.EDU>
Organization: Network Computing Devices, Inc., Mt. View, CA
Lines: 22


In article <11792@pasteur.Berkeley.EDU> jbuck@galileo.berkeley.edu (Joe Buck) writes:
+In article <10721@dog.ee.lbl.gov>, beard@ux5.lbl.gov (Patrick C Beard) writes:
+|> The information about what class an object is can be easily obtained by using
+|> the pointer to the object's virtual function table as the class id, which
+|> whill always be unique for every class...
+
+False.  Several reasons:
...
+2).  You can't count on testing two objects for the same type by comparing
+their vtbl pointers anyway.  The compiler may have had to duplicate the
+vtbl; for example, because of incremental linking or because it didn't
+have enough information to avoid generating the table more than once.

And this happens quite often!  Addresses of vtables CANNOT BE USED as
globally unique class identifiers.  Period.

-- 

// Ron Guilmette  -  C++ Entomologist
// Internet: rfg@ncd.com      uucp: ...uunet!lupine!rfg
// New motto:  If it ain't broke, try using a bigger hammer.