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From: ericg@ucschu.ucsc.edu (Eric Goodman)
Newsgroups: comp.object,comp.lang.c++
Subject: Virtual Functions (was: Inheritance and Information Hiding)
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Date: 1 Feb 91 20:54:58 GMT
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References:<1991Jan23.224203.3206@runx.oz.au> <1991Jan24.214652.18515@Think.COM> <TYNOR.91Jan25110611@hydra.gatech.edu> <27A44871.5586@tct.uucp> <1991Jan29.221121.20642@odin.corp.sgi.com>

In article <1991Jan29.221121.20642@odin.corp.sgi.com> linton@sgi.com (Mark 
Linton) writes:
> As for virtual functions, if you can't live with cost of a virtual 
function call
> you are unlikely to be happy with the cost of a direct function call.  A 
virtual
> function call typically adds a few memory references on top of what is 
likely
> a considerably more expensive operation, involving register save/restore 
and
> a branch.

There is more than the indirect reference.  There is the virtual function 
table which must be maintained for each object.  If you are dealing with 
many (thousands) of small (2-4 int) objects, by using virtual functions 
you are adding at least 25%-50% extra size to your objects (assuming ints 
and the index into the virtual function table to be equal in size - it's 
probably worse than that).  

This leads into my question: when I declare a class with virtual functions 
how much memory am I losing per object?  I was originally given to 
understand that for each virtual function a class maintains an index into 
a virtual function table, meaning extra memory size required is roughly 
proportional to the number of virtual functions.  But thinking about it, 
that sounds too inefficient, so it should be something like a pointer to a 
virtual function table is created, meaning each object created uses extra 
memory for having virtual functions, but the amount of memory lost per 
object is constant whether one or several virtual functions are used in 
the class.  

Who wants to straighten me out?  E-mail replies appreciated, or post and 
e-mail (I don't always get postings).

Thanks 

Eric Goodman, UC Santa Cruz

ericg@ucschu.ucsc.edu                              ericg@ucschu.bitnet
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