json_view: from_json() of the macros reads views directly

get<T>() and get_to() of a view call T's from_json() with the view itself
when that from_json() accepts views but not an unrelated probe type, and
fall back to materialize() otherwise. The from_json() functions of the
NLOHMANN_DEFINE_TYPE_*, NLOHMANN_DEFINE_DERIVED_TYPE_* and
NLOHMANN_JSON_SERIALIZE_ENUM(_STRICT) macros accept basic_json values and
views; a derived type whose base reads only json reads the base part
through materialize(). The library's own from_json() overloads are not
called with views: from_json() of a view (and of the probe) only looks up
functions by ADL.

New public trait nlohmann::is_json_view<T>, and the member type
basic_json_type of basic_json_view. With duplicate keys, the members of
such a T are those the view's lookups find (the first ones).

bench_decode.cpp measures decoding into structs (4-6x faster than
materialize().get<T>() on twitter, citm_catalog, and synthetic records).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json_view.hpp

@@ -106,6 +106,8 @@ class basic_json_view
     using navigation = detail::view::navigation<is_editable>;
 
   public:
+    /// the basic_json type of the document (it also marks views for is_json_view)
+    using basic_json_type = BasicJsonType;
     using value_t = detail::value_t;
     using string_t = typename BasicJsonType::string_t;
     using number_integer_t = typename BasicJsonType::number_integer_t;
@@ -567,8 +569,10 @@ class basic_json_view
 
     /// the value converted to T, as BasicJsonType::get<T>(): arithmetic types,
     /// strings (string_view_t without a copy), std::nullptr_t, std::vector,
-    /// maps with string keys, and views are converted directly; other types
-    /// through materialize().get<T>()
+    /// maps with string keys, and views are converted directly; types whose
+    /// from_json() accepts views (those of the NLOHMANN_DEFINE_TYPE_* and
+    /// NLOHMANN_JSON_SERIALIZE_ENUM macros, say) read the view; other types
+    /// go through materialize().get<T>()
     template<typename T>
     NLOHMANN_VIEW_ALWAYS_INLINE T get() const
     {
@@ -576,11 +580,12 @@ class basic_json_view
     }
 
     /// as BasicJsonType::get_to(): the types get<T>() converts directly are
-    /// assigned, all others are passed to from_json with v as it is
+    /// assigned, all others are passed to from_json with v as it is (with
+    /// the view if from_json() accepts views, else with materialize())
     template<typename T>
     T& get_to(T& v) const
     {
-        get_to_impl(v, detail::view::is_direct_value<T, BasicJsonType, basic_json_view> {});
+        get_to_impl(v, get_to_kind<T> {});
         return v;
     }
 
@@ -923,6 +928,26 @@ class basic_json_view
         return detail::view::map_value<std::unordered_map<K, V, H, E, A>>(*this);
     }
 
+    /// whether the from_json() of T's serializer reads views (see
+    /// detail::has_view_from_json)
+    template<typename T>
+    using reads_view = detail::has_view_from_json<typename BasicJsonType::template json_serializer<T, void>, basic_json_view, T>;
+
+    /// how get_to() converts: 2 directly, 1 with a from_json() that reads
+    /// views, 0 through materialize()
+    template<typename T>
+    using get_to_kind = std::integral_constant < int, detail::view::is_direct_value<T, BasicJsonType, basic_json_view>::value
+                        ? 2 : static_cast<int>(reads_view<T>::value) >;
+
+    /// types whose from_json() reads views, without materialize()
+    template < typename T, typename std::enable_if < reads_view<T>::value && std::is_default_constructible<T>::value, int >::type = 0 >
+    T get_impl(detail::view::value_tag<T> /*unused*/, detail::priority_tag<1> /*unused*/) const
+    {
+        auto v = T();
+        BasicJsonType::template json_serializer<T, void>::from_json(*this, v);
+        return v;
+    }
+
     /// everything else through the BasicJsonType value (from_json included)
     template<typename T>
     T get_impl(detail::view::value_tag<T> /*unused*/, detail::priority_tag<0> /*unused*/) const
@@ -933,14 +958,20 @@ class basic_json_view
     // --- get_to() dispatch ---
 
     template<typename T>
-    void get_to_impl(T& v, std::true_type /*direct*/) const
+    void get_to_impl(T& v, std::integral_constant<int, 2> /*direct*/) const
     {
         v = get<T>();
     }
 
     /// from_json may keep parts of v (members a JSON object lacks, say)
     template<typename T>
-    void get_to_impl(T& v, std::false_type /*direct*/) const
+    void get_to_impl(T& v, std::integral_constant<int, 1> /*from_json reads views*/) const
+    {
+        BasicJsonType::template json_serializer<T, void>::from_json(*this, v);
+    }
+
+    template<typename T>
+    void get_to_impl(T& v, std::integral_constant<int, 0> /*through materialize()*/) const
     {
         materialize().get_to(v);
     }
@@ -1645,6 +1676,15 @@ class basic_json_document
     std::unique_ptr<document_data, document_data::deleter> m_data{}; // NOLINT(readability-redundant-member-init)
 };
 
+/*!
+@brief whether T is a basic_json_view (of any BasicJsonType and view_mode)
+
+For constraining templates that accept views, e.g. a from_json() written for
+both basic_json values and views.
+*/
+template<typename T>
+struct is_json_view : detail::is_json_view<T> {};
+
 /// a parsed JSON text for json
 using json_document = basic_json_document<json>;
 /// a value of a json_document