ceph/src/boost/boost/json/value_to.hpp

   1 //
   2 // Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
   3 // Copyright (c) 2020 Krystian Stasiowski (sdkrystian@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/json
   9 //
  10
  11 #ifndef BOOST_JSON_VALUE_TO_HPP
  12 #define BOOST_JSON_VALUE_TO_HPP
  13
  14 #include <boost/json/detail/config.hpp>
  15 #include <boost/json/value.hpp>
  16 #include <boost/json/detail/value_to.hpp>
  17
  18 BOOST_JSON_NS_BEGIN
  19
  20 /** Customization point tag type.
  21
  22     This tag type is used by the function
  23     @ref value_to to select overloads
  24     of `tag_invoke`.
  25
  26     @note This type is empty; it has no members.
  27
  28     @see @ref value_from, @ref value_from_tag, @ref value_to,
  29     <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
  30         tag_invoke: A general pattern for supporting customisable functions</a>
  31 */
  32 template<class T>
  33 struct value_to_tag;
  34
  35 /** Convert a @ref value to an object of type `T`.
  36
  37     This function attempts to convert a @ref value
  38     to `T` using
  39
  40     @li one of @ref value's accessors, or
  41
  42     @li a library-provided generic conversion, or
  43
  44     @li a user-provided overload of `tag_invoke`.
  45
  46     In all cases, the conversion is done by calling
  47     an overload of `tag_invoke` found by argument-dependent
  48     lookup. Its signature should be similar to:
  49
  50     @code
  51     T tag_invoke( value_to_tag<T>, value );
  52     @endcode
  53
  54     The object returned by the function call is
  55     returned by @ref value_to as the result of the
  56     conversion.
  57
  58     @par Constraints
  59     @code
  60     ! std::is_reference< T >::value
  61     @endcode
  62
  63     @par Exception Safety
  64     Strong guarantee.
  65
  66     @tparam T The type to convert to.
  67
  68     @returns `jv` converted to `T`.
  69
  70     @param jv The @ref value to convert.
  71
  72     @see @ref value_to_tag, @ref value_from,
  73     <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
  74         tag_invoke: A general pattern for supporting customisable functions</a>
  75 */
  76 template<class T>
  77 T
  78 value_to(const value& jv)
  79 {
  80     BOOST_STATIC_ASSERT(! std::is_reference<T>::value);
  81     return detail::value_to_impl(
  82         value_to_tag<typename std::remove_cv<T>::type>(), jv);
  83 }
  84
  85 /** Convert a @ref value to an object of type `T`.
  86
  87     This overload is **deleted** and participates in overload resolution only
  88     when `U` is not @ref value. The overload exists to prevent unintented
  89     creation of temporary @ref value instances, e.g.
  90
  91     @code
  92     auto flag = value_to<bool>(true);
  93     @endcode
  94 */
  95 template<class T, class U
  96 #ifndef BOOST_JSON_DOCS
  97     , class = typename std::enable_if<!std::is_same<U, value>::value>::type
  98 #endif
  99 >
 100 T
 101 value_to(U const& jv) = delete;
 102
 103 /** Determine a @ref value can be converted to `T`.
 104
 105     If @ref value can be converted to `T` via a
 106     call to @ref value_to, the static data member `value`
 107     is defined as `true`. Otherwise, `value` is
 108     defined as `false`.
 109
 110     @see @ref value_to
 111 */
 112 #ifdef BOOST_JSON_DOCS
 113 template<class T>
 114 using has_value_to = __see_below__;
 115 #else
 116 template<class T, class>
 117 struct has_value_to
 118     : std::false_type { };
 119
 120 template<class T>
 121 struct has_value_to<T, detail::void_t<decltype(
 122     detail::value_to_impl(
 123         value_to_tag<detail::remove_cvref<T>>(),
 124         std::declval<const value&>())),
 125     typename std::enable_if<
 126         ! std::is_reference<T>::value>::type
 127     > > : std::true_type { };
 128 #endif
 129
 130 BOOST_JSON_NS_END
 131
 132 #endif