| -----Original Message----- | From: boost-bounces@lists.boost.org | [mailto:boost-bounces@lists.boost.org] On Behalf Of Janek Kozicki | Sent: 15 March 2006 18:47 | To: boost@lists.boost.org | Subject: Re: [boost] [serialization] | Serialisation/deserialisation offloating-point values | | Paul A Bristow said: (by the date of Wed, 15 Mar 2006 | 15:23:50 -0000) | | > | > | This suggests that it is a broken feature of Microsoft's | > | > | operator>> for doubles read from filestreams. | > | | > | | > | why not just read the string, ans use | boost::lexical_cast<double> ? | > | | > | -- | > | Janek Kozicki | > | > I fear that lexical_cast uses the same method of reading | from a stringstream | > :-(( | > | > And that our testing of lexical_cast is insufficient to catch it. | > | > Some very rough tests of lexical_cast suggest that several | percent of not | > very random double values fail to loopback ( 1 bit wrong), | but I don't have | > time to work on this at present. | | but if during saving lexical_cast<string>(some_double_value) is used | IMHO no data should be lost at all. operator<< for string | cannot make a | mistake. Also operator>> for string shouldn't make a mistake, | otherwise | copying around textual data with << and >> should lead to | data corruption. | (think copying text file using microsoft's operators << and >> ) | | bear in mind that lexical_cast<string>(foo) converts double to string | without any data loss (that's the exact purpose of lexical_cast). Then | later if this data is treated as string then it's not possible to loss | any information (otherwise we have a plain corruption of | textual data). | | so during save you convert double to string, then save. | when you load it - you load a string, then convert it to double. std::stringstream stream; double num; stream << std::setprecision(3 + std::numeric_limits<double>::digits * 3030/10000); stream << orig_value; stream >> num; // <<<<<<<<<< This is where I believe it goes wrong, sometimes, by one 1 bit :-(( so orig_value != num. Or am I misunderstanding your suggestion? Paul -- Paul A Bristow Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB Phone and SMS text +44 1539 561830, Mobile and SMS text +44 7714 330204 mailto: pbristow@hetp.u-net.com http://www.hetp.u-net.com/index.html http://www.hetp.u-net.com/Paul%20A%20Bristow%20info.html