boost spirit: parameter type when using semantic actions and phoenix

Question

I use boost spirit to parse mathematical expressions and have run into a problem that I extracted into the following code.

There is a simple lexer with one token, having an attribute holding the matched string. The parser defines a single rule which is meant to take the token's attribute and call a function with it. The result of the function call should be the attribute value of the rule.

This fails to compile (calc_something: cannot convert parameter 1 from const boost::spirit::_1_type to const std::string &) - clearly because the type of qi::_1 is not inferred correctly. However, changing the action to a simple "cout << qi::_1" works.

I am fairly new to boost spirit, but have managed to get my grammar to behave correctly. Now that I need to get at parsed values, I am stuck here and would appreciate any help I can get.

// spiritTest.cpp : Defines the entry point for the console application.
//

#include <stdio.h>
#include <tchar.h>

#include <string>
#include <iostream>

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/lex_lexertl.hpp>

namespace qi = boost::spirit::qi;
namespace lex = boost::spirit::lex;
namespace phoenix = boost::phoenix;

template <typename Lexer>
class TestLexer : public lex::lexer<Lexer>
{
public:
    TestLexer()
    {
        number = "(\\d*\\.)?\\d+([eE][-+]?\\d+)?";      
        self = number;
    }

    lex::token_def<std::string> number;
};

int calc_something(const std::string & s)
{
    return 5;
}

template <typename Iterator>
class Parser : public qi::grammar<Iterator, int>
{
public:
    template <typename TokenDef>
    Parser(const TokenDef& tok) : Parser::base_type(value)
    {   
        // the following line causes error C2664: 'calc_something' : cannot convert parameter 1 from 'const boost::spirit::_1_type' to 'const std::string &'    
        value = tok.number [qi::_val = calc_something(qi::_1)];         

        // the following line works as expected
        //value = tok.number [std::cout << qi::_1 << std::endl];            
    }

    qi::rule<Iterator, int> value;
};

int _tmain(int argc, _TCHAR* argv[])
{
    typedef const char* base_iterator_type;
    typedef lex::lexertl::token<base_iterator_type> token_type;
    typedef lex::lexertl::lexer<token_type> lexer_type;
    typedef TestLexer<lexer_type> TestLexer;
    typedef TestLexer::iterator_type iterator_type;
    typedef Parser<iterator_type> Parser;

    TestLexer lexer;
    Parser parser(lexer);

    const char * formula = "530";
    bool result = lex::tokenize_and_parse(formula, formula + strlen(formula), lexer, parser);

    return 0;
}

eddi eddi · Accepted Answer · 2012-03-15T15:25:11

I don't have experience with spirit lex, but I assume it's similar to qi and so you'd want to use a phoenix function to do that:

#include <boost/spirit/include/phoenix_function.hpp>

struct calc_something_impl
{
  template <typename T1>
  struct result { typedef int type; };

  int operator()(const std::string & s) const
  {
    return 5;
  }
};

boost::phoenix::function<calc_something_impl> calc_something;

boost spirit: parameter type when using semantic actions and phoenix

2 Answers