Newsgroups: comp.lang.c++
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!barmar
From: barmar@think.com (Barry Margolin)
Subject: Re: Better Encapsulation:Overload Keywords
Message-ID: <1991Jun24.163530.14631@Think.COM>
Sender: news@Think.COM
Reply-To: barmar@think.com
Organization: Thinking Machines Corporation, Cambridge MA, USA
References: <1991Jun23.223737.31644@kuhub.cc.ukans.edu>
Date: Mon, 24 Jun 91 16:35:30 GMT
Lines: 51

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.

The "for" statement is a bit more complex if you want the overloading to be
able to take over the initialization and stepping as well.  You'd probably
need an alternative syntax; maybe:

	for (<lval> = <expression>)
	  <statement>

where <expression> should evaluate to a class object.  The interface would
probably be similar to iterators in CLU.  I'm not sure how you could
combine traditional "for" loops with the overloaded kind in one statement.

I'm not sure what an overloaded "switch" statement would be like.  Would
the labels still be integer constants?  If so, then a conversion from the
class to int would be sufficient.

Of course, all these suggestions about using conversions only work if the
same conversion is useful in all the contexts.  If you want the test in a
"while" statement to be true in a different case from the test in an "if"
statement, it won't work.  However, I think that would be a very bad idea.
The equivalence between

	while (<expr>) <statement>

and

<label>: if (<expr>) {
	    <statement>
	    goto <label>;
	 }

is generally considered fundamental; choices between the various forms of
control flow are supposed to be based on style, not semantics.
-- 
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar