//
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// Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
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// Copyright (c) 2020 Krystian Stasiowski (sdkrystian@gmail.com)
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//
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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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//
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// Official repository: https://github.com/boostorg/json
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//
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#ifndef BOOST_JSON_VALUE_TO_HPP
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#define BOOST_JSON_VALUE_TO_HPP
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#include <boost/json/detail/config.hpp>
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#include <boost/json/value.hpp>
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#include <boost/json/detail/value_to.hpp>
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BOOST_JSON_NS_BEGIN
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/** Customization point tag type.
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This tag type is used by the function
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@ref value_to to select overloads
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of `tag_invoke`.
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@note This type is empty; it has no members.
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@see @ref value_from, @ref value_from_tag, @ref value_to,
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<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
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tag_invoke: A general pattern for supporting customisable functions</a>
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*/
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template<class T>
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struct value_to_tag;
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/** Convert a @ref value to an object of type `T`.
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This function attempts to convert a @ref value
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to `T` using
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@li one of @ref value's accessors, or
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@li a library-provided generic conversion, or
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@li a user-provided overload of `tag_invoke`.
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In all cases, the conversion is done by calling
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an overload of `tag_invoke` found by argument-dependent
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lookup. Its signature should be similar to:
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@code
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T tag_invoke( value_to_tag<T>, value );
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@endcode
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The object returned by the function call is
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returned by @ref value_to as the result of the
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conversion.
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@par Constraints
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@code
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! std::is_reference< T >::value
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@endcode
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@par Exception Safety
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Strong guarantee.
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@tparam T The type to convert to.
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@returns `jv` converted to `T`.
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@param jv The @ref value to convert.
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@see @ref value_to_tag, @ref value_from,
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<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
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tag_invoke: A general pattern for supporting customisable functions</a>
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*/
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#ifdef BOOST_JSON_DOCS
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template<class T>
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T
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value_to(const value& jv);
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#else
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template<class T, class U
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, typename std::enable_if<
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! std::is_reference<T>::value &&
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std::is_same<U, value>::value>::type*
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>
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T
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value_to(const U& jv)
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{
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return detail::value_to_impl(
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value_to_tag<typename std::remove_cv<T>::type>(), jv);
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}
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#endif
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/** Determine a @ref value can be converted to `T`.
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If @ref value can be converted to `T` via a
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call to @ref value_to, the static data member `value`
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is defined as `true`. Otherwise, `value` is
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defined as `false`.
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@see @ref value_to
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*/
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#ifdef BOOST_JSON_DOCS
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template<class T>
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using has_value_to = __see_below__;
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#else
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template<class T, class>
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struct has_value_to
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: std::false_type { };
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template<class T>
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struct has_value_to<T, detail::void_t<decltype(
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detail::value_to_impl(
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value_to_tag<detail::remove_cvref<T>>(),
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std::declval<const value&>())),
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typename std::enable_if<
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! std::is_reference<T>::value>::type
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> > : std::true_type { };
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#endif
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BOOST_JSON_NS_END
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#endif
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