id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12327	cpp_rational::convert_to<double>() does not return the nearest number	komakisen@…	John Maddock	"Hello,

I tested with the following program to see if boost::multiprecision::cpp_rational::convert_to<double>() returns the nearest double number to the exact value. I expected it to print only ""true""s, but it actually prints some ""false""s too. Is it a bug or (un)documented behavior? 

{{{#!c++
#include <boost/multiprecision/cpp_int.hpp>

typedef boost::multiprecision::cpp_rational NT;

void test_convert_to_double(double d1, double d2)
{
   NT r1(d1);
   NT r2(d2);

   NT two(2);
   NT three(3);

   NT r112 = (two * r1 + r2) / three;
   NT r12 = (r1 + r2) / two;
   NT r122 = (r1 + two * r2) / three;

   double rd1 = r1.convert_to<double>();
   double rd112 = r112.convert_to<double>();
   double rd12 = r12.convert_to<double>();
   double rd122 = r122.convert_to<double>();
   double rd2 = r2.convert_to<double>();

   std::cout << std::boolalpha;
   std::cout << (rd1 == d1) << std::endl;
   std::cout << (rd112 == d1) << std::endl;
   std::cout << (rd12 == d2) << std::endl;
   std::cout << (rd122 == d2) << std::endl;
   std::cout << (rd2 == d2) << std::endl;
}

int main()
{
   volatile double a = 0.099999999999999992;
   volatile double b = 0.10000000000000001;
   volatile double c = 0.10000000000000002;

   // prints true, false, true, true, true
   test_convert_to_double(a, b);

   // prints true, true, false, false, true
   test_convert_to_double(b, c);

   return 0;
}
}}}

I compiled this program with MSVC 14.0 and ICL 16.0.3, both for x64 Debug build. They give the same results.
"	Bugs	closed	Boost 1.62.0	multiprecision	Boost 1.61.0	Problem	fixed		komakisen@…