Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!bu-cs!encore!multimax
From: pierson@multimax (Dan Pierson)
Newsgroups: comp.lang.c++
Subject: Re: Efficiency of C++ relative to C??
Message-ID: <9962@multimax.Encore.COM>
Date: 19 Sep 89 15:04:46 GMT
References: <16493@watdragon.waterloo.edu>
Sender: news@Encore.COM
Reply-To: pierson@multimax (Dan Pierson)
Organization: Encore Computer Corp
Lines: 43

In article <16493@watdragon.waterloo.edu>, dvadura@watdragon (Dennis Vadura) writes:
>	Then ask how much of the cost of the C++ program is due to the
>	use of virtual functions, and what is the call overhead for
>	a virtual function when compared to the call overhead for a
>	normal call in C?

To paraphrase one of Danny Bobrow's comments when people raise the
analogous question about the Common Lisp Object System:

It isn't realistic to compare the cost of a virtual mentod with the
cost of a normal function call; if all you wanted was a normal
function call you would use one.  A relevant question is: what is cost
of a virtual function call compared to the cost of the C code you
would use to do the same thing the virtual function call is trying to
do?  Such code might look like:

void print (object *thing)
{
    switch (thing->type) {
      case OBJ_PICTURE:
    	print_picture(thing);
        break;
      ...
    }
}

In this case, a virtual function call should be compared to:
    C call + structure reference via pointer + switch statement

I suspect that you will find that C++ virtual methods compare rather
well to the above.

Caveat: Of course it is possible to write C++ code that uses virtual
methods when normal methods would do the job.  It's also possible to
write C++ code that uses normal methods when inline methods would be
more appropriate.  It's possible to write poor code in C too, but that
shouldn't be the point of a language comparison.
-- 
                                            dan

In real life: Dan Pierson, Encore Computer Corporation, Research
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