1 /*=============================================================================
2     Copyright (c) 1998-2003 Joel de Guzman
3     Copyright (c) 2001 Daniel Nuffer
4     Copyright (c) 2002 Hartmut Kaiser
5     http://spirit.sourceforge.net/
6 
7   Distributed under the Boost Software License, Version 1.0. (See accompanying
8   file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
9 =============================================================================*/
10 #if !defined(BOOST_SPIRIT_OPTIONAL_HPP)
11 #define BOOST_SPIRIT_OPTIONAL_HPP
12 
13 #include <boost/spirit/home/classic/namespace.hpp>
14 #include <boost/spirit/home/classic/core/parser.hpp>
15 #include <boost/spirit/home/classic/core/primitives/primitives.hpp>
16 #include <boost/spirit/home/classic/core/composite/composite.hpp>
17 #include <boost/spirit/home/classic/meta/as_parser.hpp>
18 
19 namespace boost { namespace spirit {
20 
21 BOOST_SPIRIT_CLASSIC_NAMESPACE_BEGIN
22 
23     ///////////////////////////////////////////////////////////////////////////
24     //
25     //  optional class
26     //
27     //      Handles expressions of the form:
28     //
29     //          !a
30     //
31     //      where a is a parser. The expression returns a composite
32     //      parser that matches its subject zero (0) or one (1) time.
33     //
34     ///////////////////////////////////////////////////////////////////////////
35     struct optional_parser_gen;
36 
37     template <typename S>
38     struct optional
39     :   public unary<S, parser<optional<S> > >
40     {
41         typedef optional<S>                 self_t;
42         typedef unary_parser_category       parser_category_t;
43         typedef optional_parser_gen         parser_generator_t;
44         typedef unary<S, parser<self_t> >   base_t;
45 
optionalboost::spirit::optional46         optional(S const& a)
47         : base_t(a) {}
48 
49         template <typename ScannerT>
50         typename parser_result<self_t, ScannerT>::type
parseboost::spirit::optional51         parse(ScannerT const& scan) const
52         {
53             typedef typename parser_result<self_t, ScannerT>::type result_t;
54             typedef typename ScannerT::iterator_t iterator_t;
55             iterator_t save = scan.first;
56             if (result_t r = this->subject().parse(scan))
57             {
58                 return r;
59             }
60             else
61             {
62                 scan.first = save;
63                 return scan.empty_match();
64             }
65         }
66     };
67 
68     struct optional_parser_gen
69     {
70         template <typename S>
71         struct result
72         {
73             typedef optional<S> type;
74         };
75 
76         template <typename S>
77         static optional<S>
generateboost::spirit::optional_parser_gen78         generate(parser<S> const& a)
79         {
80             return optional<S>(a.derived());
81         }
82     };
83 
84     template <typename S>
85     optional<S>
86     operator!(parser<S> const& a);
87 
88 BOOST_SPIRIT_CLASSIC_NAMESPACE_END
89 
90 }} // namespace BOOST_SPIRIT_CLASSIC_NS
91 
92 #endif
93 
94 #include <boost/spirit/home/classic/core/composite/impl/optional.ipp>
95