#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

int main()
{
    const std::string INPUT_DATA = "12e-1 + 3.4 - .67";
    typedef std::string::const_iterator iterator_type;
    iterator_type begin = std::begin(INPUT_DATA);
    iterator_type end   = std::end(INPUT_DATA);

    namespace qi    = boost::spirit::qi;
    namespace ascii = boost::spirit::qi::ascii;
    namespace phx   = boost::phoenix;

    bool ok;
    double result;
    {
        using namespace qi;

        auto parser = 
               double_                  [_val  = _1]      // (1)
            >> *(   (lit('+') >> double_[_val += _1])     // (2)
                  | (lit('-') >> double_[_val -= _1])     // (3)
                );

        ok = phrase_parse(begin, end, parser, ascii::space, result);
    }

    if (ok  && begin == end)
        std::cout << "parsed, result = " << result << std::endl;
    else
        std::cout << "not parsed" << std::endl;
}