http://stackoverflow.com/questions/8780604/how-to-add-qisymbols-in-grammariterator-double

test.cpp

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

#include <iostream>
#include <string>

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

    struct vars_ : qi::symbols<char, double> {
        vars_() {
            add("ans" , 0);
        }
    } vars;

    template <typename Iterator>
    struct parser : qi::grammar<Iterator, double()>
    {
        parser() : parser::base_type(function)
        {
            using qi::eps;
            using qi::lit;
            using qi::_val;
            using qi::_1;
            using ascii::char_;
            using qi::double_;
            using qi::string;
            using qi::lexeme;
            using boost::phoenix::if_;
            using qi::alpha;
            using qi::alnum;

            MAX = lexeme[string("max") | string("MAX")]; //define max symbol

            MIN = lexeme[string("min") | string("MIN")]; //define min symbol

            D   = lexeme[string("d") | string("D")];     //define distance symbol

            ANS = lexeme[string("ans") | string("ANS")]; //not working yet

            function =
                expression                                      [_val = _1]
                | declaration
                | assignment
                | ( MAX >> "(" >> function                      [_val = _1] >> 
                        +(',' >> function                       [if_(_1 > _val)[_val = _1]]) >> ')') // call : max(function,...)
                | ( MIN >> "(" >> function                      [_val = _1] >>
                        +(',' >> function                       [if_(_1 < _val)[_val = _1]]) >> ')') // call : min(function,...)
                | ( D   >> "(" >> (function >> ',' >> function) >> ')');                             // call : d(point1,point2) not implemented yet

            expression =
                term                            [_val = _1]
                >> *(   ('+' >> term            [_val += _1])
                    |   ('-' >> term            [_val -= _1]));

            term =
                factor                          [_val = _1]
                >> *(   ('*' >> factor          [_val *= _1])
                    |   ('/' >> factor          [_val /= _1]));
            factor = 
                double_                         [_val = _1]
                | (vars                         [_val += _1] )
                |   '(' >> expression           [_val = _1] >> ')'
                |   ('-' >> factor              [_val = -_1])
                |   ('+' >> factor              [_val = _1])
                | declaration;
            ;

            assignment =
                vars >> '=' >> function;

            var_decl =
                lexeme [ qi::raw [  ( alpha >> *( alnum | '_' ) ) - vars  ] ] 
                    [ phx::bind(vars.add, _1) ]
                    ;


            declaration =
                "var " >> var_decl >> *( ',' >> var_decl );
        }

        qi::rule<Iterator, double()> MAX, MIN, D, ANS, expression, term, factor, 
                                        function, assignment, var_decl, declaration;
    };
}

///////////////////////////////////////////////////////////////////////////////
//  Main program
///////////////////////////////////////////////////////////////////////////////
int main()
{
    std::cout << "**********************************************************" << std::endl;
    std::cout << "*                                                        *" << std::endl;
    std::cout << "*          Command interface for VideoTraction4          *" << std::endl;
    std::cout << "*                                                        *" << std::endl;
    std::cout << "**********************************************************" << std::endl << std::endl;
    std::cout << "Type an expression...or [q or Q] to quit" << std::endl << std::endl;

    typedef std::string::const_iterator iterator_type;
    typedef client::parser<iterator_type> parser;

    parser _parser; // Our grammar

    std::string str;
    double result;
    while (std::getline(std::cin, str))
    {
        if (str.empty() || str[0] == 'q' || str[0] == 'Q')
            break;

        std::string::const_iterator iter = str.begin();
        std::string::const_iterator end = str.end();

        bool r = parse(iter, end, _parser, result);

        if (r && iter == end)
        {
            std::cout << "-------------------------\n";
            std::cout << "Parsing succeeded\n";
            std::cout << "result = " << result << std::endl;
            std::cout << "-------------------------\n";
            client::vars.remove("ans");
            client::vars.add("ans",result);
        }
        else
        {
            std::string rest(iter, end);
            std::cout << "-------------------------\n";
            std::cout << "Parsing failed\n";
            std::cout << "stopped at: \": " << rest << "\"\n";
            std::cout << "-------------------------\n";
        }
    }

    std::cout << "Bye... :-) \n\n";
    return 0;
}

T:\>cl /EHsc /I "c:\Program Files (x86)\boost\boost_1_47" test.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.30319.01 for 80x86
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cpp
Microsoft (R) Incremental Linker Version 10.00.30319.01
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test.exe
test.obj

T:\>.\test.exe
**********************************************************
*                                                        *
*          Command interface for VideoTraction4          *
*                                                        *
**********************************************************

Type an expression...or [q or Q] to quit

3*4+7
-------------------------
Parsing succeeded
result = 19
-------------------------