Re: [Boost-users] Numeric Cast from integer to floating point

2014-09-08 17:53 GMT+04:00 :

Hi,

Quoting the docs for boost::numeric_cast: "There are several situations where conversions are unsafe: [...] Conversions from floating point types to integral types."

What about conversions from integral types to floating point types? E.g. from 64bit int to double.

The following example shows what I mean:

#include <iostream> #include <cmath> #include

int main() { const uint64_t i = 123445678911188878; std::cout << "i=" << i << std::endl;

const double d = i; std::cout << "d=" << std::fixed << d << std::endl; std::cout << "next=" << std::fixed << std::nextafter(d, std::numeric_limits<double>::max() ) << std::endl; std::cout << "prev=" << std::fixed << std::nextafter(d, -std::numeric_limits<double>::max()) << std::endl;

// I'd expect the following cast to fail const double dd = boost::numeric_cast<double>(i); std::cout <<"dd=" << std::fixed << dd << std::endl;

return 0; }

prints

i=123445678911188878

d=123445678911188880.000000

next=123445678911188896.000000

prev=123445678911188864.000000

dd=123445678911188880.000000

because that integer cannot be represented in double precision

Is there something in Boost to help here?

That's an interesting question. numeric_cast function was designed to detect positive and negative overflows, not precision loss. Consider the example: std::cout << boost::numeric_cast<int>(1.1999); // OK: 1 std::cout << boost::numeric_cast<int>(1e100); // exception: positive overflow Your example is related to the first line, where 1.1999 is converted to int. When uint64_t i = 123445678911188878; is converted to double there is no positive/negative overflow, there's only a precision loss. There is no out-of-the-box solution for your problem, but you can try to so something by your own using the numeric converter (code was not tested): // See: http://www.boost.org/doc/libs/1_56_0/libs/numeric/conversion/doc/html/boost_... template<class Traits> struct precise_converter { typedef typename Traits ::result_type result_type ; typedef typename Traits::argument_type argument_type ; static result_type low_level_convert ( argument_type s ) { const result_type res = static_cast(s) ; if (std::abs(s - res) >= 1) { throw std::runtime_error("precision loss"); } return res; }} ; template inline Target precise_numeric_cast( Source arg ) // use this instead of numeric_cast { typedef numeric::conversion_traits conv_traits; typedef numeric::numeric_cast_traits cast_traits; typedef boost::numeric::converter < Target, Source, conv_traits, typename cast_traits::overflow_policy, typename cast_traits::rounding_policy, /* only the following line differs from the original numeric_cast*/ precise_converter< conv_traits >, typename cast_traits::range_checking_policy > converter; return converter::convert(arg); } -- Best regards, Antony Polukhin