higher precision floating point using boost lib (higher then 16 digits)

Question

I am running a simulation of physical experiments, so I need really high floating point precision (more than 16 digits). I use Boost.Multiprecision, however I can\'t get a p

Answer 1

Your issue is here:

cpp_dec_float_50 my_num = cpp_dec_float_50(0.123456789123456789123456789);
                                            ^ // This number is a double!

The compiler does not use arbitrary-precision floating point literals, and instead uses IEEE-754 doubles, which have finite precision. In this case, the closest double to the number you have written is:

0.1234567891234567837965840908509562723338603973388671875

And printing it to the 50th decimal does indeed give the output you are observing.

What you want is to construct your arbitrary-precision float from a string instead (demo):

#include 
#include 
#include 
#include 

using boost::multiprecision::cpp_dec_float_50;

int main() {
    cpp_dec_float_50 my_num = cpp_dec_float_50("0.123456789123456789123456789");
    std::cout.precision(std::numeric_limits::digits10);
    std::cout << my_num << std::endl;
}

Output:

0.123456789123456789123456789