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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: Messaging overhead
Message-ID: <9216@alice.UUCP>
Date: 18 Apr 89 19:25:08 GMT
References: <1357@stl.stc.co.uk>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 51

In article <1357@stl.stc.co.uk>, dsr@stl.stc.co.uk (David Riches) writes:

> Speed is a very important factor when considering
> the advantages and disadvantages of routing algorithms,
> and as a result I am interested in the overhead due to 
> messaging in OOP when compared to "conventional" programming. 

C++ does `messaging' by calling virtual functions.
Calling a virtual function in C++ is only a little
slower than calling an ordinary member function.

How much slower depends, of course, on your machine
and C compiler.  To get some specific evidence, I tried
it on my home machine (a MicroVAX II):

	struct Foo {
		void f();
		virtual void g();
	};

	main()
	{
		register Foo* fp;
		fp->f();
		fp->g();
	}

The code generated for fp->f() looks like this:

	pushl	r11
	calls	$1,<Foo::f>

and the code for fp->g() looks like this:

	movl	(r11),r0
	cvtwl	8(r0),r0
	addl3	r0,r11,-(sp)
	movl	(r11),r0
	calls	$1,*12(r0)

So the virtual function call takes three extra instructions
and three extra memory references.

On this particular machine, a subroutine call/return instruction
pair take a lot longer than three memory references, so the extra
cost for a virtual function call is insignificant.  The overhead
would be relatively greater on a machine with extremely fast
call/return instructions, but is still small in absolute terms.
-- 
				--Andrew Koenig
				  ark@europa.att.com