Question

I want to parse a floating point number in a text file and insert it in a symbol table; the parser and the symbol table are provided by spirit::qi.

Here is my code:

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

#include <stdint.h>
#include <iostream>
#include <string>
template<typename VTYPE>
struct VTable : boost::spirit::qi::symbols<char, VTYPE> {
    VTable() {} // empty
};

int main() {
using boost::spirit::qi::_1;
using boost::spirit::qi::eps;
using boost::spirit::qi::rule;
using boost::spirit::qi::ascii::space;
using boost::spirit::qi::space_type;
using boost::spirit::qi::real_parser;
using boost::spirit::qi::int_parser;
using boost::spirit::qi::strict_real_policies;
    VTable<double> floatDecs;
    floatDecs.add("hallo", 15.26)("duDa", 18.5);

const std::string some = "some";
    rule<std::string::iterator, double> value = real_parser<double, strict_real_policies<double>>() [ boost::phoenix::bind(&VTable<double>::add, floatDecs, boost::phoenix::cref(some), _1) ];

    std::cout << boost::spirit::qi::phrase_parse(test.begin(), test.end(), value, space);
    return 0;
}

The problem in here lies in boost::phoenix::bind, but I do not know, how to fix that (I am fairly new to this library).

Answer 1

Here’s a working version. After, I’ll list what was wrong with yours.

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

#include <stdint.h>
#include <iostream>
#include <string>
template<typename VTYPE>
struct VTable : boost::spirit::qi::symbols<char, VTYPE> {
    VTable() {} // empty
};

int main() {
using boost::spirit::qi::_1;
using boost::spirit::qi::eps;
using boost::spirit::qi::rule;
using boost::spirit::qi::ascii::space;
using boost::spirit::qi::space_type;
using boost::spirit::qi::real_parser;
using boost::spirit::qi::int_parser;
using boost::spirit::qi::strict_real_policies;

    VTable<double> floatDecs;
    floatDecs.add("hallo", 15.26)("duDa", 18.5);

    const char some[] = "some";

    rule<std::string::iterator, double()> value =
            real_parser<double, strict_real_policies<double> >()
            [ boost::phoenix::bind(floatDecs.add, boost::phoenix::val(some), _1) ];

    std::string test("10.6");

    std::string::iterator b(test.begin()), e(test.end());

    std::cout << boost::spirit::qi::phrase_parse(b, e, value, space);

    return 0;
}

First, the variable you want to use as an argument to add() needs to be fixed.

const char some[] = "some";

Next, you need to fix the syntax in your rule template parameters:

rule<std::string::iterator, double()>

Note the brackets, to make the double a functor declaration rather than a vanilla double, for lazy evaluation.

Next, you need to use the syntax above for your semantic action.

[ boost::phoenix::bind(floatDecs.add, boost::phoenix::val(some), _1) ];

Honestly I am not sure why this works as I am not a phoenix expert, but the link in ‘sehe’s’ comment above provided the answer. Also, you were barking up the wrong tree with cref, there’s no indication from add()‘s declaration that a reference is what’s required. val is what you need to turn the some variable into a phoenix-happy lazy argument.

Finally, you just need to supply some reasonable test data.

The code above should work for you now.