Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!ucsd!sdcsvax!ucsdhub!hp-sdd!hplabs!hp-pcd!hplsla!jima
From: jima@hplsla.HP.COM (Jim Adcock)
Newsgroups: comp.lang.c++
Subject: Re: Messaging overhead
Message-ID: <6590103@hplsla.HP.COM>
Date: 20 Apr 89 17:30:42 GMT
References: <1357@stl.stc.co.uk>
Organization: HP Lake Stevens, WA
Lines: 67

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

Hm, I guess I'd like to argue a little this definition of
"messaging" as applied to C++.

I'd like to claim that whenever I use a construct similar
to:

	myObject.aMessage(withParams);   -- or --
	myObjectP->aMessage(withParams);

-- then I've sent a message to an object in C++, regardless
of the code that actually gets generated to perform this 
task.  [Why should I, the class user, care how the class
writer, or the compiler conspire to make this message 
happen -- as long as its damned fast!?]

C++ gives the writer of the class that myObject belongs
to several choices on how the request for a message
to be sent to an object gets turned into code to do
the job.  Depending on what needs to be done, the
"messaging" request may compile into:

						X times faster

an indirect function call			10-100
a function call					 5-50
inline code [example: a simple assignment]      100-1000
nothing				    		infinitely

-- where the "X times faster" column is intended to give
a rough feel of the decimal order of magnitude times faster
C++ is for this coding of a "message" verses what might
be expected from other OOP languages that use hashed look-up
schemes.

Perhaps, most importantly are C++'s advantages in the last
two cases.  Frequently in OOP a message request simply
sets or returns a simple value from inside an object:

	[myComplexNumber imagPart: 0.0];

		or

       double imag = [myComplexNumber imagPart];


Using inline functions in C++, these simple "assignment"-type
methods which are so common in OOP get turned into the same
code as for C simple assignments:

	myComplexNumber.im = 0.0;

		or

	double imag = myComplexNumber.im;


Finally, a good optimizing back end code generator may well discover
that the inline code that is generated serves no useful purpose, and
so removes it.

Thus sometimes, no code whatsoever need be generated in the implementation 
of a C++ messaging request.  Clearly, you cannot expect these levels of 
optimization in other OOPLs.