From 1098483408559780978 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,7b33df5790e7ad92 X-Google-Attributes: gidf78e5,public From: jcoffin@taeus.com (Jerry Coffin) Subject: Re: complex design flaw (#1) Date: 1999/03/15 Message-ID: #1/1 X-Deja-AN: 455391634 X-NNTP-Posting-Host: 166-93-76-49.rmi.net Approved: stephen.clamage@sun.com (comp.std.c++) References: <01be6e30$aeae59e0$7d9cd4c7@danielp.interlog.com> X-UID: 0000000001 X-Status: $$$T X-Complaints-To: abuse@rmi.net X-Trace: news1.rmi.net 921532327 18820 166.93.76.49 (15 Mar 1999 21:12:07 GMT) Organization: TAEUS NNTP-Posting-Date: 15 Mar 1999 21:12:07 GMT Newsgroups: comp.std.c++ Originator: clamage@taumet In article <01be6e30$aeae59e0$7d9cd4c7@danielp.interlog.com>, danielp@no_spam.com says... [ ... ] > Incidently, I'm a little rusty on the bit layout of a floating point > number; can I rely on > > double x = 0.0; > double y; > > memset( &y, 0, sizeof(double) ); > > x == y ? > memcmp( &x, &y, sizeof(double) ) == 0 ? No, to both questions. If you were restricting things to IEEE floating point, I _believe_ (though I'd have to do some looking to be sure) that setting all bits to zero will result in a value of zero. The reverse is NOT necessarily true -- in particular, any number in which the mantissa is zero has a value of zero, even if the exponent has non-zero bits. Likewise, floating point will fairly frequently use 1's complement for the mantissa, and a bias representation for the exponent. This means that all bits set to one in the mantissa is really a zero. All bits set to zero in the exponent is an exponent of -128, and a zero exponent is what would normally be viewed as the value 128. However, all of this is merely common practice: the C++ standard doesn't mandate that you use IEEE floating point (or anything particularly close to it), and there are still quite a few machines in the world that don't but may have C++ implementations anyway. I haven't tried to figure out for sure, but I believe you could do a perfectly legal implementation of C++ using something like BCD, and (for example) use the value zero in a nibble to represent "Negative", "NaN" or something similar. In the end, setting all bits of a floating point number to zero has only one effect according to the standard: undefined results. You typically won't see floating-point traps simply by declaring something as floating point and setting it to an invalid bit-pattern, but there's nothing to stop the machine from doing this if it wants to. [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://reality.sgi.com/austern_mti/std-c++/faq.html ]