source: downloads/boost_1_34_1/boost/spirit/dynamic/impl/conditions.ipp @ 29

/*=============================================================================
    Copyright (c) 2002-2003 Martin Wille
    http://spirit.sourceforge.net/

    Use, modification and distribution is subject to the Boost Software
    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
    http://www.boost.org/LICENSE_1_0.txt)
=============================================================================*/
#ifndef BOOST_SPIRIT_CONDITIONS_IPP
#define BOOST_SPIRIT_CONDITIONS_IPP

///////////////////////////////////////////////////////////////////////////////
#include <boost/spirit/meta/parser_traits.hpp>
#include <boost/spirit/core/composite/epsilon.hpp>

///////////////////////////////////////////////////////////////////////////////
namespace boost { namespace spirit {

    namespace impl {

///////////////////////////////////////////////////////////////////////////////
//
// condition evaluation
//
///////////////////////////////////////////////////////////////////////////////
    //////////////////////////////////
    // condition_parser_selector, decides which parser to use for a condition
    // If the template argument is a parser then that parser is used.
    // If the template argument is a functor then a condition parser using
    // the functor is chosen

    template <typename T> struct embed_t_accessor
    {
        typedef typename T::embed_t type;
    };

#if defined(BOOST_MSVC) && BOOST_MSVC <= 1300
    template <> struct embed_t_accessor<int>
    {
        typedef int type;
    };
#endif

    template <typename ConditionT>
    struct condition_parser_selector
    {
        typedef
            typename mpl::if_<
                    is_parser<ConditionT>,
                    ConditionT,
                    condition_parser<ConditionT>
                >::type
            type;

        typedef typename embed_t_accessor<type>::type embed_t;
    };

    //////////////////////////////////
    // condition_evaluator, uses a parser to check wether a condition is met
    // takes a parser or a functor that can be evaluated in boolean context
    // as template parameter.

    // JDG 4-15-03 refactored
    template <typename ConditionT>
    struct condition_evaluator
    {
        typedef condition_parser_selector<ConditionT>       selector_t;
        typedef typename selector_t::type                   selected_t;
        typedef typename selector_t::embed_t                cond_embed_t;

        typedef typename boost::call_traits<cond_embed_t>::param_type
            param_t;

        condition_evaluator(param_t s) : cond(s) {}

        /////////////////////////////
        // evaluate, checks wether condition is met
        // returns length of a match or a negative number for no-match
        template <typename ScannerT>
        std::ptrdiff_t
        evaluate(ScannerT const &scan) const
        {
            typedef typename ScannerT::iterator_t iterator_t;
            typedef typename parser_result<selected_t, ScannerT>::type cres_t;
            iterator_t save(scan.first);
            cres_t result = cond.parse(scan);
            if (!result)            // reset the position if evaluation
                scan.first = save;  // fails.
            return result.length();
        }

        cond_embed_t cond;
    };

///////////////////////////////////////////////////////////////////////////////
    } // namespace impl

}} // namespace boost::spirit

#endif