Boost spirit floating number parser precision

问题

There is something strange I noticed when comparing boost::lexical_cast and boost spirit parsing. I'm trying to parse a string into float. for some reason spirit gives very imprecise result. for example: when parsing string "219721.03839999999" using lexical_cast i get 219721.03 which is more or less OK. but when I use spirit (see code below) I get "219721.11" which is far from bein OK. Any idea why it happens?

template<>
inline float LexicalCastWithTag(const std::string& arg)
{
    float result = 0;

    if(arg.empty())
    {
        throw BadLexicalCast("Cannot convert from to std::string to float");
    }

    auto itBeg = arg.begin();
    auto itEnd = arg.end();

    if(!boost::spirit::qi::parse(itBeg, itEnd, boost::spirit::qi::float_, result) || itBeg != itEnd)
    {
        throw BadLexicalCast("Cannot convert from to std::string to float");
    }

    return result;
}

回答1:

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

来源：https://stackoverflow.com/questions/17391348/boost-spirit-floating-number-parser-precision