| -----Original Message----- | From: boost-bounces@lists.boost.org | [mailto:boost-bounces@lists.boost.org] On Behalf Of Gennaro Prota | Sent: 04 August 2006 13:14 | To: boost@lists.boost.org | Subject: Re: [boost] Spurious warning in lexical_cast.hpp | | On Fri, 4 Aug 2006 12:16:03 +0100, "Paul A Bristow" | <pbristow@hetp.u-net.com> wrote: | | >And in rely to Gennaro's question, | > | >| stream.precision(2 + std::numeric_limits<Source>::digits | * 301/1000); | > | >| Does that work for non IEEE 754 floating points too? | My question was just if it (the formula) requires IEEE754 or not. IMO, it does NOT require IEEE754 - it only requires numeric_limits<T>::digits to be correct for that type T. So, it works for NTL quad_float which is NOT IEEE754. | As to the docs, this is what I would add. Feedback welcome. | | lexical_cast<> offers the following guarantees: | | - in the absence of overflow/underflow, | | * if a "decimal string" with at most numeric_limits<T>::digits10 | significant digits is converted to a floating type T and back | to a string the result will equal the original string value. | | * if a floating point value having type F is converted to a | string allowing at least max_digits10 [or our formula here] | significant decimal digits and back to F the result will | be the original number. Well in principle yes, but the C++ Standard is not clear if the second is REQUIRED, or is just a Good Quality Implementation. At present, I only know for certain that MSVC 8.0 (and earlier?) does not QUITE achieve this - there is a tiny range of double (and of course long double which are the same) values for which one third of values are one bit wrong. (For float it is true). When I reported this as a bug, it was deemed a feature, but I would not be surprised to see it fixed, since a similar problem existed for float and was also deemed a fixture, but was fixed at 8.0. So sadly, it is a desirata not a guarantee :-(( Paul --- Paul A Bristow Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB +44 1539561830 & SMS, Mobile +44 7714 330204 & SMS pbristow@hetp.u-net.com