So it will be probably limitation/bug of "float" type parser. Try to use double_ parser.
#include<iostream>
#include<iomanip>
#include<string>
#include<boost/spirit/include/qi.hpp>
int main()
{
std::cout.precision(20);
//float x=219721.03839999999f;
//std::cout << x*1.0f << std::endl;
//gives 219721.03125
double resultD;
std::string arg="219721.03839999999";
auto itBeg = arg.begin();
auto itEnd = arg.end();
if(!boost::spirit::qi::parse(itBeg, itEnd,boost::spirit::qi::double_,resultD) || itBeg != itEnd)
std::cerr << "Cannot convert from std::string to double" << std::endl;
else
std::cout << "qi::double_:" << resultD << std::endl;
float resultF;
itBeg = arg.begin();
itEnd = arg.end();
if(!boost::spirit::qi::parse(itBeg, itEnd,boost::spirit::qi::float_,resultF) || itBeg != itEnd)
std::cerr << "Cannot convert from std::string to float" << std::endl;
else
std::cout << "qi::float_ :" << resultF << std::endl;
return 0;
}
Output:
qi::double_:219721.03839999999036
qi::float_:219721.109375