Hello all, I want to write a const-expression parser which calculate the result while parsing. When do it with double and int, I don't know how to do, my code like: namespace b=boost; namespace bs=boost::spirit; namespace bsq=boost::spirit::qi; namespace bp=boost::phoenix; bsq::real_parser<double,bsq::strict_real_policies<double> > bnf_double; bsq::int_parser<int, 8, 1, -1> bnf_int8; bsq::int_parser<int, 10, 1, -1> bnf_int10; bsq::int_parser<int, 16, 1, -1> bnf_int16; template <typename Iterator> struct const_expression: bsq::grammar<Iterator,b::variant<int,double>()> { typedef b::variant<int,double> result_type; const_expression() : const_expression::base_type(rstart) { using bsq::_val; using bsq::_1; using bsq::_a; result_type result; rstart = bnf_double[_val=_1] >> *( bsq::lit('+') >> bnf_double[how to write here] ) | rint [_val=_1] ; rint = bsq::lit('0') >> bsq::char_("xX") >> bnf_int16[_val=_1] | bsq::lit('0') >> bnf_int8[_val=_1] | bnf_int10[_val=_1] ; } bsq::rule<Iterator, result_type()> rstart; bsq::rule<Iterator, int()> rint; }; Thanks yours