Xref: utzoo comp.lang.c++:4758 comp.lang.c:22165
Path: utzoo!attcan!uunet!mcsun!sunic!Urd!newsuser
From: newsuser@lth.se (LTH network news server)
Newsgroups: comp.lang.c++,comp.lang.c
Subject: Re: Time to standardize "true" and "false"
Keywords: true false C C++
Message-ID: <1989Sep22.073138.19684@lth.se>
Date: 22 Sep 89 07:31:38 GMT
References: <13730@well.UUCP>
Reply-To: dag@Control.LTH.Se (Dag Bruck)
Distribution: comp
Organization: Dept. of Automatic Control, Lund Inst. of Technology, Sweden
Lines: 124

In article <13730@well.UUCP> nagle@well.UUCP (John Nagle) writes:
>
>     I would like to suggest that the time has come to standardize the
>Boolean values in C.

I completely agree that a boolean data type is needed in C++.  I
think the definition should define the following properties:

	1.  The data type is called "boolean".
	2.  The allowed values are "false" and "true".
	3.  int(false) = 0 and int(true) = 1
	4.  boolean(0) = false, other values are true

I do not know if implicit conversion boolean <==> int should be allowed.

In the time before C++ 2.0, it was quite common to define a boolean type
as an enumeration:

	enum boolean {false, true};

Unfortunately (in this case), C++ no longer allows implicit conversion
int ==> enum, so the result of a comparison must be explicitly type cast:

	boolean b;
	b = (i == 3);		// warning
	b = boolean(i == 3);	// ok

This is rather clumsy.  We cannot define our own operator functions on
the boolean data type, because one of the arguments must be a class object.

	boolean operator == (boolean x, boolean y) {...}	// error

I do not understand exactly why this restriction is required, but apparently
it is.  Please enlighten me.

What remains is to define a class for boolean.  This is not easy to
make as efficient as the built in types, but the problem is mostly in
the lack of optimization in the code generators.  I have tried; you
will find my attempt to define a boolean below.  Note that I have not
defined the operators && and || -- I do not think we can achieve the
"short circuit" evaluation we're used to with a user-defined operator.

Dag Michael Bruck
--
Department of Automatic Control		Internet:  dag@control.lth.se
Lund Institute of Technology
P. O. Box 118				Phone:	+46 46-108779
S-221 00 Lund, SWEDEN			Fax:    +46 46-138118

==============================================================================

// Boolean data type

enum {false, true};

class Boolean {
public:
  Boolean()		{ val = false; }
  Boolean(int i)	{ val = (i != false); }
  Boolean(const Boolean& b)	{ val = b.val; }
  // Any non-zero value is true; default value is false.

  operator int()	{ return val; }
  // Type cast boolean => integer.

  void operator &= (const Boolean& b)	{ val &= b.val; }
  void operator |= (const Boolean& b)	{ val |= b.val; }
  void operator ^= (const Boolean& b)	{ val ^= b.val; }
  // Operator assignment.

  friend Boolean operator & (const Boolean&, const Boolean&);
  friend Boolean operator | (const Boolean&, const Boolean&);
  friend Boolean operator ^ (const Boolean&, const Boolean&);
  friend Boolean operator ! (const Boolean&);
  friend Boolean operator == (const Boolean&, const Boolean&);
  friend Boolean operator != (const Boolean&, const Boolean&);
  // These operators need access to the internal representation.

private:
  int	val;
  Boolean(long i) { val = int(i); }	// no check
};

inline Boolean operator & (const Boolean& p, const Boolean& q)
{ return long(p.val & q.val); }

inline Boolean operator | (const Boolean& p, const Boolean& q)
{ return long(p.val | q.val); }

inline Boolean operator ^ (const Boolean& p, const Boolean& q)
{ return long(p.val ^ q.val); }

inline Boolean operator ! (const Boolean& p)
{ return long(!p.val); }

inline Boolean operator == (const Boolean& p, const Boolean& q)
{ return long(p.val == q.val); }

inline Boolean operator != (const Boolean& p, const Boolean& q)
{ return long(p.val != q.val); }

// Note: there are no && and || operators.


Boolean f(Boolean b)
{
  return !b;
}


main()
{
  Boolean p, q, r;

  r &= p;
  r = p & q;
  r = f(p);
}
-- 
Department of Automatic Control		Internet:  dag@control.lth.se
Lund Institute of Technology
P. O. Box 118				Phone:	+46 46-108779
S-221 00 Lund, SWEDEN			Fax:    +46 46-138118