Precision rounding problem with boost multiprecision

I want to multiply number 123456789.123456789 by 1000000000.0 and as a result of this operation I expect 123456789123456789 as int or float 123456789123456789.0, but I got:

``````res: 123456789123456791.04328155517578125
int_res: 123456789123456791
``````

Should I do it in other way ?

``````#include <iostream>

#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>

namespace bmp = boost::multiprecision;

int main()
{
bmp::cpp_dec_float_100 scalar{1000000000.0};
bmp::cpp_dec_float_100 a{123456789.123456789};
bmp::cpp_dec_float_100 res = a * scalar;
bmp::cpp_int int_res = res.convert_to<bmp::cpp_int>();
std::cout << "    res: " << res.str() << std::endl;
std::cout << "int_res: " << int_res.str() << std::endl;
return 0;
}
``````

>Solution :

You are initialising `a` with a `double` literal. `123456789.123456789` can’t be represented by a `double` so you get the closest approximation which is `123456789.12345679104328155517578125`. If you want to initialise precisely use a string literal instead:

``````bmp::cpp_dec_float_100 a{"123456789.123456789"};
``````