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From: chased@rbbb.Eng.Sun.COM (David Chase)
Newsgroups: comp.std.c++
Subject: Re: Tags, typecodes, experience with these things.
Message-ID: <9997@exodus.Eng.Sun.COM>
Date: 18 Mar 91 20:39:55 GMT
References: <1399@culhua.prg.ox.ac.uk> <27D3E544.619A@tct.uucp> <4338@lupine.NCD.COM> <1991Mar12.123811.13701@kodak.kodak.com> <9661@exodus.Eng.Sun.COM> <71305@microsoft.UUCP>
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Organization: Sun Microsystems, Mt. View, Ca.
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In article <71305@microsoft.UUCP> jimad@microsoft.UUCP (Jim ADCOCK) writes:
>In article <9661@exodus.Eng.Sun.COM> chased@rbbb.Eng.Sun.COM (David Chase) writes:
>|I also think that the language system should support these features,
>|but that is probably a religious argument.  Certainly, if nifty
>|features are confined to objects with vptrs the costs can be kept low.
>
>Help me out.  The obvious implementations to me would be either to 
>store such information in a given vtable slot [slot "0" say] or
>indirect via a vtable function that is always automatically defined
>[for classes that support runtime type info.]  Either approach would
>seem to defeat the not-too-uncommon situation in C++ where two classes
>can share a vtable in common.  Or am I missing something?

No, you're not missing anything.  That's the technique I'd use.  I
wasn't aware that space consumed by a vtables was a big deal, except
in those cases where the compilation-and-linking system produced many
vtables for a single class (I've heard about that).  How big do
vtables usually get, how often can they be shared, and (given a
one-per-class implementation) how much space does this sharing usually
save?

David Chase
Sun