Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!apple.com!arn
From: arn@apple.com (Arn Schaeffer)
Newsgroups: comp.lang.c++
Subject: Const member function semantics
Message-ID: <2943@internal.Apple.COM>
Date: 20 Jul 89 21:39:08 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 34

I think the addition of const member functions is a powerful and necessary 
addition to the language; however, the semantics of a const member 
function declaration is essentially a statement about storage.  That is, a 
const member function will not cause the actual bits in the object to 
change.  For a simple non-virtual "getter," a const member function is an 
easy statement to make.  Unfortunately, providing virtual const member 
functions is a bit tricky.  It forces the class designer to make an 
implementation decision for all subclasses.  

Sometimes this is necessary because you need (or want) to make a statement 
about storage.  Sometimes, you just want to make a statement that the 
object hasn't changed in any way that is perceptible through the 
interface.  For example, I might have a member function that returns some 
information and caches the result in the object.  Obviously, this can't be 
a const member function using the current definition of const, yet there 
is an important statement that the class designer would like to make (this 
object does not change in any way perceptible through the interface) that 
cannot be made.

I understand the motivation about making const a statement about storage.  
I also think that it is tremendously valuable for non-virtual "getters."  
There are many general classes that I have developed which I am reluctant 
to make such a strong statement about storage (I don't want to tie the 
hands of subclasses) yet I'd be very willing to make an important 
statement that no perceptible change has occured to the object.  

Is it (will it be) possible for me to make the statement I'd like to make? 
In my work it seems that most of the time I don't want to make the 
statement about storage just about interface.

The opinions expressed are not necessarily those of Apple Computer.
Arnold Schaeffer
Apple Computer, Inc.
arn@apple.com