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From: rgreen@bbn.com (Robert W. Green)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Re: const is not object-oriented
Message-ID: <60760@bbn.BBN.COM>
Date: 10 Nov 90 00:33:54 GMT
References: <1990Nov9.181408.23110@odin.corp.sgi.com>
Sender: news@bbn.com
Reply-To: rgreen@spcpv3.bbn.com (Bob Green)
Followup-To: comp.lang.c++
Organization: Bolt Beranek and Newman Inc., Cambridge MA
Lines: 38

In article <1990Nov9.181408.23110@odin.corp.sgi.com> linton@sgi.com (Mark Linton) writes:
>The "const" keyword in C++ specifies a property of storage, and
>as such is not particularly useful in an object-oriented program.
>Declaring a parameter or a member function as const requires
>knowledge about the implementation of a class.

> ... example deleted

>The bottom line is if you are defining a C++ class, especially
>for a library, do not use const parameters or const member functions.
>Const means storage, not behavior, so it is by definition
>an implementation/representation concept.

Whoa, I think you are throwing out the baby with the bath water. The ability to
declare methods as const is just too valuable for the design of large systems to
just throw out. The language specification may need some tuning but I wouldn't
just toss it. Consider your example. It can be implemented using a cast of the
form:

        ((IntVector *const) this)->current_sum = /* sum calculation */

This works with g++. I haven't tried it with cfront but I think it should be
legal c++ code (I lifted the type spec. from page 177 of E&S which defines the
declaration of this).

Now I would agree that use of a cast is fairly ugly but I have difficulty
thinking of a mechanism which doesn't paper over a similiar hole in the
type system. As ugly as it is, an occasional cast is a big improvement
over dropping use of const everywhere.

-Bob

PS. I have implemented a class which does exactly the caching of statistical
calculations in your example. However, in my implementation, the cached
statistics are maintained as a side effect of the update methods defined for the
data vector.  Since changing the data vector is by definition not a const member
function, I was able to update the cached statistics without violating the type
system or using a ugly cast. Again, dropping use of const seems to be overkill.