import quincy beta 17.1.0

[ceph.git] / ceph / src / boost / boost / json / value_to.hpp

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+//
+// Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
+// Copyright (c) 2020 Krystian Stasiowski (sdkrystian@gmail.com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/json
+//
+
+#ifndef BOOST_JSON_VALUE_TO_HPP
+#define BOOST_JSON_VALUE_TO_HPP
+
+#include <boost/json/detail/config.hpp>
+#include <boost/json/value.hpp>
+#include <boost/json/detail/value_to.hpp>
+
+BOOST_JSON_NS_BEGIN
+
+/** Customization point tag type.
+
+    This tag type is used by the function
+    @ref value_to to select overloads
+    of `tag_invoke`.
+
+    @note This type is empty; it has no members.
+
+    @see @ref value_from, @ref value_from_tag, @ref value_to,
+    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
+        tag_invoke: A general pattern for supporting customisable functions</a>
+*/
+template<class T>
+struct value_to_tag;
+
+/** Convert a @ref value to an object of type `T`.
+
+    This function attempts to convert a @ref value
+    to `T` using
+
+    @li one of @ref value's accessors, or
+
+    @li a library-provided generic conversion, or
+
+    @li a user-provided overload of `tag_invoke`.
+    
+    In all cases, the conversion is done by calling
+    an overload of `tag_invoke` found by argument-dependent
+    lookup. Its signature should be similar to:
+    
+    @code
+    T tag_invoke( value_to_tag<T>, value );
+    @endcode
+
+    The object returned by the function call is
+    returned by @ref value_to as the result of the
+    conversion.
+
+    @par Constraints
+    @code
+    ! std::is_reference< T >::value
+    @endcode
+
+    @par Exception Safety
+    Strong guarantee.
+
+    @tparam T The type to convert to.
+
+    @returns `jv` converted to `T`.
+
+    @param jv The @ref value to convert.
+
+    @see @ref value_to_tag, @ref value_from,
+    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
+        tag_invoke: A general pattern for supporting customisable functions</a>
+*/
+#ifdef BOOST_JSON_DOCS
+template<class T>
+T
+value_to(const value& jv);
+#else
+template<class T, class U
+    , typename std::enable_if<
+        ! std::is_reference<T>::value &&
+    std::is_same<U, value>::value>::type*
+>
+T
+value_to(const U& jv)
+{
+    return detail::value_to_impl(
+        value_to_tag<typename std::remove_cv<T>::type>(), jv);
+}
+#endif
+
+/** Determine a @ref value can be converted to `T`.
+
+    If @ref value can be converted to `T` via a 
+    call to @ref value_to, the static data member `value`
+    is defined as `true`. Otherwise, `value` is
+    defined as `false`.
+
+    @see @ref value_to
+*/
+#ifdef BOOST_JSON_DOCS
+template<class T>
+using has_value_to = __see_below__;
+#else
+template<class T, class>
+struct has_value_to
+    : std::false_type { };
+
+template<class T>
+struct has_value_to<T, detail::void_t<decltype(
+    detail::value_to_impl(
+        value_to_tag<detail::remove_cvref<T>>(),
+        std::declval<const value&>())),
+    typename std::enable_if<
+        ! std::is_reference<T>::value>::type
+    > > : std::true_type { };
+#endif
+
+BOOST_JSON_NS_END
+
+#endif