/*=============================================================================    Copyright (c) 1998-2003 Joel de Guzman    Copyright (c) 2001 Daniel Nuffer    Copyright (c) 2002 Hartmut Kaiser    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)=============================================================================*/#if !defined(BOOST_SPIRIT_OPTIONAL_HPP)#define BOOST_SPIRIT_OPTIONAL_HPP#include <boost/spirit/core/parser.hpp>#include <boost/spirit/core/primitives/primitives.hpp>#include <boost/spirit/core/composite/composite.hpp>#include <boost/spirit/meta/as_parser.hpp>namespace boost { namespace spirit {    ///////////////////////////////////////////////////////////////////////////    //    //  optional class    //    //      Handles expressions of the form:    //    //          !a    //    //      where a is a parser. The expression returns a composite    //      parser that matches its subject zero (0) or one (1) time.    //    ///////////////////////////////////////////////////////////////////////////    struct optional_parser_gen;        template <typename S>    struct optional    :   public unary<S, parser<optional<S> > >    {        typedef optional<S>                 self_t;        typedef unary_parser_category       parser_category_t;        typedef optional_parser_gen         parser_generator_t;        typedef unary<S, parser<self_t> >   base_t;            optional(S const& a)        : base_t(a) {}            template <typename ScannerT>        typename parser_result<self_t, ScannerT>::type        parse(ScannerT const& scan) const        {            typedef typename parser_result<self_t, ScannerT>::type result_t;            typedef typename ScannerT::iterator_t iterator_t;            iterator_t save = scan.first;            if (result_t r = this->subject().parse(scan))            {                return r;            }            else            {                scan.first = save;                return scan.empty_match();            }        }    };        struct optional_parser_gen    {        template <typename S>        struct result         {            typedef optional<S> type;        };            template <typename S>        static optional<S>        generate(parser<S> const& a)        {            return optional<S>(a.derived());        }    };        template <typename S>    optional<S>    operator!(parser<S> const& a);}} // namespace boost::spirit#endif#include <boost/spirit/core/composite/impl/optional.ipp>