Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!caen!spool.mu.edu!uunet!sdl.mdcbbs.com!alanb
From: alanb@sdl.mdcbbs.com
Newsgroups: comp.lang.c++
Subject: Re: Better Encapsulation:Overload Keywords
Message-ID: <1991Jun25.103644.1@sdl.mdcbbs.com>
Date: 25 Jun 91 09:36:44 GMT
Article-I.D.: sdl.1991Jun25.103644.1
References: <1991Jun23.223737.31644@kuhub.cc.ukans.edu> <1991Jun24.163530.14631@Think.COM>
Organization: Shape Data Ltd. (McDonnell Douglas M&E, Cambridge UK)
Lines: 48
Nntp-Posting-Host: shapeg
Nntp-Posting-User: alanb

In article <1991Jun24.163530.14631@Think.COM>, barmar@think.com (Barry Margolin) writes:
> In article <1991Jun23.223737.31644@kuhub.cc.ukans.edu> blythe@kuhub.cc.ukans.edu writes:
>>	One of the best things about C++ is its ability to overload an
>>operator for a certain combination of objects.  These operators are then
>>implemented as a function by the compiler.  Why not keywords also?  In
>>specific, the selection keywords (if...else and switch) and the iteration
>>keywords (while, do, and for) are the only keywords I see having good
>>potential when overloaded.
> 
> If C++ had a real boolean type, and required the test expression of an
> "if", "while", "do", and "for" statements to be of this type (perhaps with
> a built-in conversion from integral and pointer types, for
> back-compatibility) then much of this could be done by defining a
> conversion from the class to boolean.
>
[Discussion of "for" omitted. CLU style iterators would be nice though.]

We have a C preprocessor that provides a boolean type and enforces the rule
that test expressions must be booleans (e.g. if (pointer != NULL), not
if (pointer)). 
(Now we use C++, we just have a coding convention).

If you allow conversion from integral and pointer types, you have lost the 
type enforcement you might want, so why not just define
operator (void *) for the classes you want to use in while loops?
Something like

{ if (my_status) return this; else return NULL );

if (anobject) { foo(); }
is now equivalent to
if (anobject.status()) { foo(); }
and while uses the same test as if.

(this assumes you never want to coerce to a pointer other than when your
if/while condition is true - in many cases this is reasonable.)

(The preprocessor also provided function prototypes before ANSI C was around, 
and workarounds for assorted platform compiler bugs - I'm not claiming that
just having a boolean type is necessarily worth the effort).

> Barry Margolin, Thinking Machines Corp.
> 
> barmar@think.com
> {uunet,harvard}!think!barmar

Alan Braggins       alanb@sdl.mdcbbs.com
"Any resemblance between these opinions,..."