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//
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//
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// Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
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// Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
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//
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//
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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//
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// Official repository: https://github.com/boostorg/url
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// Official repository: https://github.com/boostorg/url
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//
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//
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#ifndef BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
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#ifndef BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
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#define BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
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#define BOOST_URL_GRAMMAR_OPTIONAL_RULE_HPP
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#include <boost/url/detail/config.hpp>
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#include <boost/url/detail/config.hpp>
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#include <boost/url/optional.hpp>
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#include <boost/url/optional.hpp>
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#include <boost/url/error_types.hpp>
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#include <boost/url/error_types.hpp>
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#include <boost/url/grammar/type_traits.hpp>
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#include <boost/url/grammar/type_traits.hpp>
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#include <boost/core/empty_value.hpp>
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#include <boost/core/empty_value.hpp>
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#include <boost/core/detail/static_assert.hpp>
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#include <boost/core/detail/static_assert.hpp>
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#include <boost/assert.hpp>
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#include <boost/assert.hpp>
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namespace boost {
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namespace boost {
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namespace urls {
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namespace urls {
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namespace grammar {
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namespace grammar {
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namespace implementation_defined {
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namespace implementation_defined {
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template<class Rule>
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template<class Rule>
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struct optional_rule_t
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struct optional_rule_t
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    : private empty_value<Rule>
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    : private empty_value<Rule>
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{
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{
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    using value_type = boost::optional<
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    using value_type = boost::optional<
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        typename Rule::value_type>;
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        typename Rule::value_type>;
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    system::result<value_type>
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    system::result<value_type>
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    parse(
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    parse(
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        char const*& it,
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        char const*& it,
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        char const* end) const;
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        char const* end) const;
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    constexpr
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    constexpr
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    optional_rule_t(
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    optional_rule_t(
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        Rule const& r) noexcept
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        Rule const& r) noexcept
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        : empty_value<Rule>(
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        : empty_value<Rule>(
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            empty_init,
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            empty_init,
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            r)
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            r)
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    {
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    {
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    }
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    }
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};
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};
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} // implementation_defined
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} // implementation_defined
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/** Match a rule, or the empty string
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/** Match a rule, or the empty string
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    Optional BNF elements are denoted with
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    Optional BNF elements are denoted with
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    square brackets. If the specified rule
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    square brackets. If the specified rule
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    returns any error it is treated as if
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    returns any error it is treated as if
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    the rule did not match.
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    the rule did not match.
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    @par Value Type
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    @par Value Type
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    @code
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    @code
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    using value_type = optional< typename Rule::value_type >;
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    using value_type = optional< typename Rule::value_type >;
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    @endcode
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    @endcode
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    @par Example
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    @par Example
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    Rules are used with the function @ref grammar::parse.
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    Rules are used with the function @ref grammar::parse.
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    @code
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    @code
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    system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );
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    system::result< optional< core::string_view > > rv = parse( "", optional_rule( token_rule( alpha_chars ) ) );
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    @endcode
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    @endcode
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    @par BNF
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    @par BNF
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    @code
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    @code
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    optional     = [ rule ]
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    optional     = [ rule ]
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    @endcode
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    @endcode
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    @par Specification
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    @par Specification
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    @li <a href="https://datatracker.ietf.org/doc/html/rfc5234#section-3.8"
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    @li <a href="https://datatracker.ietf.org/doc/html/rfc5234#section-3.8"
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        >3.8.  Optional Sequence (rfc5234)</a>
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        >3.8.  Optional Sequence (rfc5234)</a>
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    @param r The rule to match
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    @param r The rule to match
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    @return The adapted rule
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    @return The adapted rule
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    @see
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    @see
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        @ref alpha_chars,
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        @ref alpha_chars,
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        @ref parse,
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        @ref parse,
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        @ref optional,
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        @ref optional,
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        @ref token_rule.
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        @ref token_rule.
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*/
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*/
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template<BOOST_URL_CONSTRAINT(Rule) R>
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template<BOOST_URL_CONSTRAINT(Rule) R>
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auto
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auto
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constexpr
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constexpr
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optional_rule(
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optional_rule(
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    R const& r) ->
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    R const& r) ->
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        implementation_defined::optional_rule_t<R>
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        implementation_defined::optional_rule_t<R>
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{
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{
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    BOOST_CORE_STATIC_ASSERT(grammar::is_rule<R>::value);
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    BOOST_CORE_STATIC_ASSERT(grammar::is_rule<R>::value);
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    return { r };
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    return { r };
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}
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}
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} // grammar
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} // grammar
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} // urls
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} // urls
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} // boost
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} // boost
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#include <boost/url/grammar/impl/optional_rule.hpp>
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#include <boost/url/grammar/impl/optional_rule.hpp>
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#endif
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#endif