core/composite/optional.hpp

0001 /*=============================================================================
0002     Copyright (c) 1998-2003 Joel de Guzman
0003     Copyright (c) 2001 Daniel Nuffer
0004     Copyright (c) 2002 Hartmut Kaiser
0005     http://spirit.sourceforge.net/
0006
0007   Distributed under the Boost Software License, Version 1.0. (See accompanying
0008   file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
0009 =============================================================================*/
0010 #if !defined(BOOST_SPIRIT_OPTIONAL_HPP)
0011 #define BOOST_SPIRIT_OPTIONAL_HPP
0012
0013 #include <boost/spirit/home/classic/namespace.hpp>
0014 #include <boost/spirit/home/classic/core/parser.hpp>
0015 #include <boost/spirit/home/classic/core/primitives/primitives.hpp>
0016 #include <boost/spirit/home/classic/core/composite/composite.hpp>
0017 #include <boost/spirit/home/classic/meta/as_parser.hpp>
0018
0019 namespace boost { namespace spirit {
0020
0021 BOOST_SPIRIT_CLASSIC_NAMESPACE_BEGIN
0022
0023     ///////////////////////////////////////////////////////////////////////////
0024     //
0025     //  optional class
0026     //
0027     //      Handles expressions of the form:
0028     //
0029     //          !a
0030     //
0031     //      where a is a parser. The expression returns a composite
0032     //      parser that matches its subject zero (0) or one (1) time.
0033     //
0034     ///////////////////////////////////////////////////////////////////////////
0035     struct optional_parser_gen;
0036
0037     template <typename S>
0038     struct optional
0039     :   public unary<S, parser<optional<S> > >
0040     {
0041         typedef optional<S>                 self_t;
0042         typedef unary_parser_category       parser_category_t;
0043         typedef optional_parser_gen         parser_generator_t;
0044         typedef unary<S, parser<self_t> >   base_t;
0045
0046         optional(S const& a)
0047         : base_t(a) {}
0048
0049         template <typename ScannerT>
0050         typename parser_result<self_t, ScannerT>::type
0051         parse(ScannerT const& scan) const
0052         {
0053             typedef typename parser_result<self_t, ScannerT>::type result_t;
0054             typedef typename ScannerT::iterator_t iterator_t;
0055             iterator_t save = scan.first;
0056             if (result_t r = this->subject().parse(scan))
0057             {
0058                 return r;
0059             }
0060             else
0061             {
0062                 scan.first = save;
0063                 return scan.empty_match();
0064             }
0065         }
0066     };
0067
0068     struct optional_parser_gen
0069     {
0070         template <typename S>
0071         struct result
0072         {
0073             typedef optional<S> type;
0074         };
0075
0076         template <typename S>
0077         static optional<S>
0078         generate(parser<S> const& a)
0079         {
0080             return optional<S>(a.derived());
0081         }
0082     };
0083
0084     template <typename S>
0085     optional<S>
0086     operator!(parser<S> const& a);
0087
0088 BOOST_SPIRIT_CLASSIC_NAMESPACE_END
0089
0090 }} // namespace BOOST_SPIRIT_CLASSIC_NS
0091
0092 #endif
0093
0094 #include <boost/spirit/home/classic/core/composite/impl/optional.ipp>