Curiously I have just posted a description of what may be the cause of this.
Attached... My suggested remedy relies on the correct value for numeric_limits::digits (not digits10) Paul Paul A Bristow, Prizet Farmhouse, Kendal, Cumbria, LA8 8AB UK +44 1539 561830 Mobile +44 7714 33 02 04 mailto:[EMAIL PROTECTED] | -----Original Message----- | From: [EMAIL PROTECTED] | [mailto:[EMAIL PROTECTED] Behalf Of Aleksandr Golovanov | Sent: Saturday, August 16, 2003 2:24 AM | To: [EMAIL PROTECTED] | Subject: [boost] Re: lexical_cast | | | | "Ross Smith" <[EMAIL PROTECTED]> wrote in message | news:[EMAIL PROTECTED] | > Aleksandr Golovanov wrote: | > > "Ross Smith" <[EMAIL PROTECTED]> wrote in message | > > news:[EMAIL PROTECTED] | > > | > >>Aleksandr Golovanov wrote: | > >> | > >>>Yesterday, I ran into a small problem, lexical_cast accepts copy | instead of | > >>>(const)? reference to a source. I have a class which I would prefer to | be | > >>>noncopyable and castable with laxical_cast at the same time. | > >> | > >>Wrap the object in boost::cref(). | > > | > > Unfortunately, cref won't work because lexical_cast propagates the | source | > > type as a template parameter to various helper/standard templates: | > | > You're wrong; it works perfectly well. I tried it before I posted the | > suggestion. | > | | Following working example shows one possible case when application of | boost::cref leads to a wrong result. | Compiler VC6 SP5; lexical_cast from 1.30.0 boost release. | | #include "boost/lexical_cast.hpp" | #include "boost/ref.hpp" | #include <iostream> | #include <string> | #include <limits> | | class big_decimal | { | public: | big_decimal() : m_val( 1.23456789 ) {} | public: | double m_val; | }; | | namespace std { | class numeric_limits<big_decimal> | : public numeric_limits<double> | { | }; | } | | std::ostream& operator<<( std::ostream& s, big_decimal const& arg ) | { | return s << arg.m_val; | } | | int main() | { | big_decimal dec; | std::cout << boost::lexical_cast<std::string>( dec ) << "\n"; | std::cout << boost::lexical_cast<std::string>( boost::cref( dec ) ) << | "\n"; | return 0; | } | | Result of execution: | | 1.23456789 | 1.23457 | | Conclusion: usage of boost::cref with lexical_cast may lead to wrong results | in the current implementation. | | -- | Thanks. | Aleksandr Golovanov, | MetaCommunications Engineering. | | | | _______________________________________________ | Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost | |
--- Begin Message ---I note that the 'precision' number of digits in lexical cast is obtained from digits10 +1 if(std::numeric_limits<Target>::is_specialized) { stream.precision(std::numeric_limits<Target>::digits10 + 1); } If, as I believe correct, the objective is to get all digits that can be significant, and can be read back in without loss of precision, this isn't always quite right according to: "Lecture notes on the status of IEEE 754 standard for binary floating point arithmetic" William Kahan http://http.cs.berkley.edu/~wkahan/ieee754status/ieee754.ps gives formula for number of decimal digits which are guaranteed to be correct on output and required for input to achieve maximum possible precision as a function of the number of significand bits (given by std::number_limits<FPType>::digits). In C++ the full formula is: int significant_digits10 = int(ceil(1 + float_significand_digits * log10Two)); and using this formula : std::numeric_limits<float>::digits = 24 significand bits std::numeric_limits<float>::digits10 = 6 floor(float_significand_digits -1) * log10(2) = 6 ceil(1 + float_significand_digits * log10(2) = 9 all significant bits (note that the existing code gives 7 here, which is 2 too few) std::numeric_limits<double>::digits = 53 std::numeric_limits<double>::digits10 = 15 floor(double_significand_digits -1) * log10(2) = 15 ceil(1 + double_significand_digits * log10(2)) = 17 (note that the existing lecial_cast.hpp code gives 16 here, which is 1 too few) 32 significand bits digits10 = 6 significant_digits10 = 9 53 significand bits digits10 = 15 significant_digits10 = 17 64 significand bits digits10 = 18 significant_digits10 = 21 106 significand bits digits10 = 31 significant_digits10 = 33 113 significand bits digits10 = 33 significant_digits10 = 36 128 significand bits digits10 = 38 significant_digits10 = 40 (note that the rest are a few too few) I have proposed before that numeric limits should have another item called, perhaps, significant_digits10 returning these useful values, but meanwhile I suggest that following the style of boost/limits.h BOOST_STL_DECLARE_LIMITS_MEMBER(int, digits10, (digits * 301) / 1000); // log 2 = 0.301029995664... The integer fraction 301/1000 is needed to avoid suggeating to the compiler that it should do a floating point calculation (which silently fails!) so the following formula is used instead: int const sig_digits10 = 2 + std::numeric_limits<FPType>::digits * 301/1000; // log10(2.) This gives the same result without using the ceil function which might not be computed at compile time. So in lexical_cast, substitute for example the above fragment with: if(std::numeric_limits<Target>::is_specialized) { // Show all significant decimal digits, 2 or 3 more than digits10. stream.precision(2 + std::numeric_limits<Target>::digits * 301/1000); } // & similarly for Source A suggested revision and test attached, for example showing float & double now have extra decimal digits. Boost release 30 outputs: 1.414214 1.414213562373095 Revised version outputs: 1.41421354 1.4142135623730951 And it is thus now possible to convert float & double to a string and back again to give exactly the same as the original float & double (which the current version sometimes does not - a pit for the unwary). Paul Paul A Bristow, Prizet Farmhouse, Kendal, Cumbria, LA8 8AB UK +44 1539 561830 Mobile +44 7714 33 02 04 mailto:[EMAIL PROTECTED]
test_lexical_cast.cpp
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