| -----Original Message----- | From: boost-bounces@lists.boost.org | [mailto:boost-bounces@lists.boost.org] On Behalf Of Caleb Epstein | Sent: 15 March 2006 23:20 | To: boost@lists.boost.org | Subject: Re: [boost] [serialization] | Serialisation/deserialisationoffloating-point values | | On 3/15/06, Janek Kozicki <janek_listy@wp.pl> wrote: | > | > | > try following modifications in above code. If you still get | a mistake by | > one bit, then .... well, I'd be very surprised. | | | Prepare to be suprised. | | Here's the exact code I compiled: | | #include <iostream> | #include <sstream> | #include <cassert> | #include <boost/lexical_cast.hpp> | | int main () | { | std::stringstream stream; | double orig_value = 0.0019075645054089487; | stream << boost::lexical_cast<std::string> (orig_value); | double num = boost::lexical_cast<double> (stream.str()); | assert (num == orig_value); | } | | On gcc + Linux this fails: | | lc: lc.cpp:12: int main(): Assertion `num == orig_value' failed. | | Breakpoint 1, main () at lc.cpp:12 | 12 assert (num == orig_value); | (gdb) print num | $3 = 0.0019075645054089489 | (gdb) print orig_value | $4 = 0.0019075645054089487 | | On MSVC 8, the program also asserts and the values are | similarly mismatched: | | orig_value 0.0019075645054089487 double | num 0.0019075645054089489 double | | Note that boost::lexical_cast uses a precision of | std::numeric_limits<T>::digits10 | + 1 in its T-to-string conversions. For double, this is 16 | which would | probably explain the mismatch on the 17th significant digit | on two separate | platforms. Well I have pointed this mistake out over two years ago, but it still hasn't been changed. I have to say I think that this is a bit poor. Our testing of this very widely used utility is also not up to Boost standards either. Sadly though, I fear this is not the only problem. I think that there is also a problem in the Microsoft input string to double, even with enough decimal digits, for a small proportion of decimal digits strings. There testing / quality aspriation is obviously not brilliant either. I may get round to checking this out more fully later. 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