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From: pal@xanadu.wpd.sgi.com (Anil Pal)
Newsgroups: comp.lang.c++
Subject: Re: Need help, missing virtual tables, Sun C++ 2.0
Keywords: Sun
Message-ID: <1991May9.173442.5946@dragon.wpd.sgi.com>
Date: 9 May 91 17:34:42 GMT
References: <1991May8.224321.12580@cs.utk.edu>
Sender: news@dragon.wpd.sgi.com (CNews Account)
Reply-To: pal@wpd.sgi.com
Organization: Silicon Graphics, Inc.
Lines: 35

In article <1991May8.224321.12580@cs.utk.edu>, mullens@jamsun.ic.ornl.gov (James A. Mullens) writes:
|> First, would anyone posting an answer also point me to a "frequently asked
|> questions" list and a Sun C++ 2.0 compiler bug list?

Sorry, I don't know where to find such a beast.  However, I will post an answer
regardless :-)

|> The problem is that the loader complains of undefined symbols such as
|> ___vtbl__23SGob__BaseGob__ViewMngr__7WinMngr, meaning (I think) the virtual
|> function table for class WinMngr (a class derived from ViewMngr, BaseGob, and
|> SGob).

The trick here is understanding where the translator puts the virtual
function table.  In order to reduce the number of copies of the virtual
function table, cfront-based translators (I believe Sun C++ 2.0 is one)
use the following strategy:

    The virtual function table for a class is output in a compilation
    stream if the compilation stream contains the *definition* for the
    first non-inline virtual function *declared* in that class.  Simple :-)

So, how do you use this information?  Well, look at the class in
question (and its base classes) and determine which is the first
non-inline virtual function declared (perhaps one of your base classes
has a virtual destructor?).  Then check to see that you have included
in your link the appropriate definition for that function in your
class.  In my experience, this is the most common cause of undefined
vtables at link time.  Note that if a pure virtual function is declared
in a derived class, it will not generate any warnings.  You may still
have forgotten to provide an implementation for a declared function,
and if no-one calls it, you will not be told that it is undefined.

-- 
Anil A. Pal,	Silicon Graphics, Inc.
pal@wpd.sgi.com	(415)-335-7279