libs/url/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		//
4		// Distributed under the Boost Software License, Version 1.0. (See accompanying
5		// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6		//
7		// Official repository: https://github.com/boostorg/url
8		//
9
10		#ifndef BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
11		#define BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
12
13		#include <boost/url/detail/config.hpp>
14		#include <boost/url/optional.hpp>
15		#include <boost/url/error_types.hpp>
16		#include <boost/url/grammar/type_traits.hpp>
17		#include <boost/core/empty_value.hpp>
18		#include <boost/core/detail/static_assert.hpp>
19		#include <boost/assert.hpp>
20
21		namespace boost {
22		namespace urls {
23		namespace grammar {
24
25		namespace implementation_defined {
26		template<class Rule>
27		struct optional_rule_t
28		: private empty_value<Rule>
29		{
30		using value_type = boost::optional<
31		typename Rule::value_type>;
32
33		system::result<value_type>
34		parse(
35		char const*& it,
36		char const* end) const;
37
38		constexpr
39	2123	optional_rule_t(
40		Rule const& r) noexcept
41		: empty_value<Rule>(
42		empty_init,
43	2123	r)
44		{
45	2123	}
46		};
47		} // implementation_defined
48
49		/** Match a rule, or the empty string
50
51		Optional BNF elements are denoted with
52		square brackets. If the specified rule
53		returns any error it is treated as if
54		the rule did not match.
55
56		@par Value Type
57		@code
58		using value_type = optional< typename Rule::value_type >;
59		@endcode
60
61		@par Example
62		Rules are used with the function @ref grammar::parse.
63		@code
64		system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );
65		@endcode
66
67		@par BNF
68		@code
69		optional = [ rule ]
70		@endcode
71
72		@par Specification
73		@li <a href="https://datatracker.ietf.org/doc/html/rfc5234#section-3.8"
74		>3.8. Optional Sequence (rfc5234)</a>
75
76		@param r The rule to match
77		@return The adapted rule
78
79		@see
80		@ref alpha_chars,
81		@ref parse,
82		@ref optional,
83		@ref token_rule.
84		*/
85		template<BOOST_URL_CONSTRAINT(Rule) R>
86		auto
87		constexpr
88	2123	optional_rule(
89		R const& r) ->
90		implementation_defined::optional_rule_t<R>
91		{
92		BOOST_CORE_STATIC_ASSERT(grammar::is_rule<R>::value);
93	2123	return { r };
94		}
95
96		} // grammar
97		} // urls
98		} // boost
99
100		#include <boost/url/grammar/impl/optional_rule.hpp>
101
102		#endif
103