Xref: utzoo comp.lang.c++:13756 comp.std.c++:945
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!sunic!lth.se!newsuser
From: dag@control.lth.se (Dag Bruck)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Re: Boolean confusion
Summary: How you could introduce `boolean' into the language
Message-ID: <1991May30.060200.6590@lth.se>
Date: 30 May 91 06:02:00 GMT
References: <HAYDENS.91May23103233@natasha.juliet.ll.mit.edu> <332@nazgul.UUCP> <1991May29.145100.18519@watson.ibm.com> <1991May29.171209.14760@neon.Stanford.EDU>
Sender: newsuser@lth.se (LTH network news server)
Distribution: comp.lang.c++
Organization: Department of Automatic Control, Lund, Sweden
Lines: 57

In article <1991May29.171209.14760@neon.Stanford.EDU> philip@pescadero.stanford.edu writes:
>.... But how many programs would break? I for one would be prepared
>to pay the price for the long-term gain, but I generally try to program
>as if boolean and int are distinct types, so the price wouldn't be
>very high for me.

The "break old code" argument against any changes is the one you hear
most often at the C++ standardization meetings.  It is of course
valid, but often over-used.  Even if there were incompatible changes,
every compiler would have a switch to get the old behaviour.

One of the strongest arguments in favour of a new boolean data type is
that you could do function overloading:

	void f(int);
	void f(boolean);

I also think a boolean data type would increase type safety in C++, at
least the way I program (:-).

I once tried to implement `class boolean' and failed.  Firstly there
was a performance problem, which I think good compilers could
overcome.  Secondly, I don't think I gained anything in type safety
and little in convenience: to be at all useful I needed implicit type
conversions int->boolean and boolean->int.  I think a boolean data
type should _not_ be another kind of integer (at least eventually) which
can be implicitly converted to/from int.  It should instead be a
unique enumerated type, but that doesn't work until all standard
functions (e.g., operator == (int, int)) return a boolean and not
an int.

The transition to a better world (:-) should be done in two phases:

	1.  Introduce the new built-in data type `boolean' (or
	whatever you call it), and specify that all standard
	operator == (etc.) return a boolean.

	2.  Include implicit type conversions between boolean and
	int, to minimize the number of broken programs.  I think
	this automatic conversion will help in most cases.

	3.  Warn about the automatic conversions and label them as
	anachronisms.

	4.  (Some years later) Turn the warnings of (3) into errors.

We had a similar case with C++ 2.0 with respect to handling of `enum',
and I think the transition was relatively painless.  The most common
problem was that people had used an enum for booleans, which caused
warning messages under the new interpretation.

Dag Br!"uck
--
Department of Automatic Control		E-mail: dag@control.lth.se
Lund Institute of Technology
P. O. Box 118				Phone:	+46 46-104287
S-221 00 Lund, SWEDEN			Fax:    +46 46-138118