include/boost/url/grammar/optional_rule.hpp

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1		//
2		// Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
3		// Copyright (c) 2022 Alan de Freitas (alandefreitas@gmail.com)
4		//
5		// Distributed under the Boost Software License, Version 1.0. (See accompanying
6		// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7		//
8		// Official repository: https://github.com/boostorg/url
9		//
10
11		#ifndef BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
12		#define BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
13
14		#include <boost/url/detail/config.hpp>
15		#include <boost/url/optional.hpp>
16		#include <boost/url/error_types.hpp>
17		#include <boost/url/grammar/type_traits.hpp>
18		#include <boost/core/empty_value.hpp>
19		#include <boost/core/detail/static_assert.hpp>
20		#include <boost/assert.hpp>
21
22		namespace boost {
23		namespace urls {
24		namespace grammar {
25
26		namespace implementation_defined {
27		template<class Rule>
28		struct optional_rule_t
29		: private empty_value<Rule>
30		{
31		using value_type = boost::optional<
32		typename Rule::value_type>;
33
34		BOOST_URL_CXX14_CONSTEXPR
35		system::result<value_type>
36		parse(
37		char const*& it,
38		char const* end) const;
39
40		constexpr
41	2139	optional_rule_t(
42		Rule const& r) noexcept
43		: empty_value<Rule>(
44		empty_init,
45	2139	r)
46		{
47	2139	}
48		};
49		} // implementation_defined
50
51		/** Match a rule, or the empty string
52
53		Optional BNF elements are denoted with
54		square brackets. If the specified rule
55		returns any error it is treated as if
56		the rule did not match.
57
58		@par Value Type
59		@code
60		using value_type = optional< typename Rule::value_type >;
61		@endcode
62
63		@par Example
64		Rules are used with the function @ref grammar::parse.
65		@code
66		system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );
67		@endcode
68
69		@par BNF
70		@code
71		optional = [ rule ]
72		@endcode
73
74		@par Specification
75		@li <a href="https://datatracker.ietf.org/doc/html/rfc5234#section-3.8"
76		>3.8. Optional Sequence (rfc5234)</a>
77
78		@param r The rule to match
79		@return The adapted rule
80
81		@see
82		@ref alpha_chars,
83		@ref parse,
84		@ref optional,
85		@ref token_rule.
86		*/
87		template<BOOST_URL_CONSTRAINT(Rule) R>
88		auto
89		constexpr
90	2139	optional_rule(
91		R const& r) ->
92		implementation_defined::optional_rule_t<R>
93		{
94		BOOST_CORE_STATIC_ASSERT(grammar::is_rule<R>::value);
95	2139	return { r };
96		}
97
98		} // grammar
99		} // urls
100		} // boost
101
102		#include <boost/url/grammar/impl/optional_rule.hpp>
103
104		#endif
105