json_view: from_json() of the macros reads views directly (#5922)

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>

docs/mkdocs/docs/examples/is_json_view.cpp

@@ -0,0 +1,34 @@
+#include <iostream>
+#include <type_traits>
+#include <nlohmann/json_view.hpp>
+
+using nlohmann::json;
+
+namespace ns
+{
+struct person
+{
+    std::string name;
+    int age = 0;
+};
+
+// reads json values and views (of any document type), but nothing else
+template < typename J, typename std::enable_if < nlohmann::is_json_view<J>::value || std::is_same<J, json>::value, int >::type = 0 >
+void from_json(const J& j, person& p)
+{
+    p.name = j.at("name").template get<std::string>();
+    p.age = j.at("age").template get<int>();
+}
+} // namespace ns
+
+int main()
+{
+    std::cout << std::boolalpha
+              << nlohmann::is_json_view<nlohmann::json_view>::value << ' '
+              << nlohmann::is_json_view<nlohmann::ordered_json_editable_view>::value << ' '
+              << nlohmann::is_json_view<json>::value << '\n';
+
+    const nlohmann::json_document doc = nlohmann::json_document::parse(R"({"name": "Ada", "age": 36})");
+    const auto p = doc.root().get<ns::person>(); // reads the view, no json value is built
+    std::cout << p.name << ' ' << p.age << '\n';
+}

docs/mkdocs/docs/examples/is_json_view.output

@@ -0,0 +1,2 @@
+true true false
+Ada 36