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>

tests/benchmarks/json_view/README.md

@@ -59,6 +59,12 @@ An editable `json_document` edits in place; yyjson copies its immutable document
 (`yyjson_doc_mut_copy`); Boost.JSON and `json::parse` build mutable DOMs; simdjson cannot edit a document. All
 outputs are checked to describe the same value.
 
+`bench_decode.cpp` measures decoding into structs whose `from_json()` comes from the `NLOHMANN_DEFINE_TYPE_*` macros
+(twitter statuses, citm_catalog performances, and 20,000 synthetic records): `get<T>()` on a view, which reads the
+view directly, against `materialize().get<T>()`, `json::parse` plus `get<T>()`, and `get<T>()` on an already parsed
+`json` value. Only nlohmann/json takes part, as the other libraries have no comparable mapping; all engines are
+checked to decode the same values.
+
 Before anything is timed, all engines must accept each document and agree on the traversal: the number of values, the
 bytes of all strings and keys, and the sum of all numbers. All engines run interleaved in every round, and the best
 round is reported, as time and as a factor of the `json_view` time (below 1 means faster than `json_view`). Each timed

tests/benchmarks/json_view/bench_decode.cpp

@@ -0,0 +1,312 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+// Decoding into structs: the same types, with from_json() generated by the
+// NLOHMANN_DEFINE_TYPE_* macros, read three ways. Only nlohmann/json takes
+// part; the other libraries have no comparable mapping.
+//
+//   view           json_document::parse, then get<T>() on the view (from_json reads the view)
+//   materialize    json_document::parse, then materialize().get<T>() (a json value first)
+//   json::parse    json::parse, then get<T>()
+//   json::get      get<T>() on an already parsed json value (the decoding alone)
+//
+// Documents: twitter (the statuses with their users and hashtags), citm_catalog
+// (the performances with their seat categories), and a synthetic document of
+// 20,000 nested records.
+//
+// Build: see README.md (same flags as bench_view.cpp).
+#include <nlohmann/json_view.hpp>
+
+#include <algorithm>
+#include <chrono>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <fstream>
+#include <functional>
+#include <sstream>
+#include <string>
+#include <utility>
+#include <vector>
+
+using nlohmann::json;
+using nlohmann::json_document;
+
+static volatile std::size_t g_sink;
+
+namespace twitter
+{
+struct hashtag
+{
+    std::string text;
+    std::vector<int> indices;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(hashtag, text, indices)
+
+struct entities
+{
+    std::vector<hashtag> hashtags;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(entities, hashtags)
+
+struct user
+{
+    std::uint64_t id = 0;
+    std::string screen_name;
+    std::string description;
+    std::int64_t followers_count = 0;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(user, id, screen_name, description, followers_count)
+
+struct status
+{
+    std::uint64_t id = 0;
+    std::string text;
+    user user;
+    entities entities;
+    std::int64_t retweet_count = 0;
+    std::int64_t favorite_count = 0;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(status, id, text, user, entities, retweet_count, favorite_count)
+
+struct document
+{
+    std::vector<status> statuses;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(document, statuses)
+
+std::size_t digest(const document& d)
+{
+    std::size_t n = 0;
+    for (const auto& s : d.statuses)
+    {
+        n += s.text.size() + s.user.screen_name.size() + s.entities.hashtags.size() + static_cast<std::size_t>(s.retweet_count);
+    }
+    return n;
+}
+}  // namespace twitter
+
+namespace citm
+{
+struct area
+{
+    std::int64_t areaId = 0;
+    std::vector<std::int64_t> blockIds;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(area, areaId, blockIds)
+
+struct seat_category
+{
+    std::vector<area> areas;
+    std::int64_t seatCategoryId = 0;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(seat_category, areas, seatCategoryId)
+
+struct performance
+{
+    std::int64_t id = 0;
+    std::int64_t eventId = 0;
+    std::vector<seat_category> seatCategories;
+    std::int64_t start = 0;
+    std::string venueCode;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(performance, id, eventId, seatCategories, start, venueCode)
+
+struct document
+{
+    std::vector<performance> performances;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(document, performances)
+
+std::size_t digest(const document& d)
+{
+    std::size_t n = 0;
+    for (const auto& p : d.performances)
+    {
+        n += p.seatCategories.size() + p.venueCode.size() + static_cast<std::size_t>(p.start % 1000);
+        for (const auto& c : p.seatCategories)
+        {
+            n += c.areas.size();
+        }
+    }
+    return n;
+}
+}  // namespace citm
+
+namespace synthetic
+{
+struct position
+{
+    double x = 0;
+    double y = 0;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(position, x, y)
+
+struct child
+{
+    std::int64_t id = 0;
+    std::string label;
+    bool active = false;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(child, id, label, active)
+
+struct record
+{
+    std::int64_t id = 0;
+    std::string name;
+    std::vector<std::string> tags;
+    position pos;
+    std::vector<child> children;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(record, id, name, tags, pos, children)
+
+struct document
+{
+    std::vector<record> records;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(document, records)
+
+std::string make_text(std::size_t n)
+{
+    std::string s = "{\"records\":[";
+    for (std::size_t i = 0; i < n; ++i)
+    {
+        const std::string id = std::to_string(i);
+        s += (i == 0 ? "" : ",");
+        s += "{\"id\":" + id + ",\"name\":\"record number " + id + "\",\"tags\":[\"alpha\",\"beta\",\"gamma\"],";
+        s += "\"pos\":{\"x\":" + id + ".25,\"y\":-" + id + ".5},\"children\":[";
+        for (std::size_t k = 0; k < 3; ++k)
+        {
+            s += (k == 0 ? "" : ",");
+            s += "{\"id\":" + std::to_string(i * 3 + k) + ",\"label\":\"child " + std::to_string(k) + "\",\"active\":" + (k % 2 == 0 ? "true" : "false") + "}";
+        }
+        s += "]}";
+    }
+    s += "]}";
+    return s;
+}
+
+std::size_t digest(const document& d)
+{
+    std::size_t n = 0;
+    for (const auto& r : d.records)
+    {
+        n += r.name.size() + r.tags.size() + r.children.size() + static_cast<std::size_t>(r.pos.x);
+    }
+    return n;
+}
+}  // namespace synthetic
+
+namespace
+{
+std::string read_file(const std::string& p)
+{
+    std::ifstream f(p, std::ios::binary);
+    if (!f)
+    {
+        std::fprintf(stderr, "cannot open %s\n", p.c_str());
+        std::exit(1);
+    }
+    std::stringstream ss;
+    ss << f.rdbuf();
+    return ss.str();
+}
+
+double time_ns(const std::function<void()>& f)
+{
+    const auto t0 = std::chrono::steady_clock::now();
+    f();
+    const auto t1 = std::chrono::steady_clock::now();
+    return static_cast<double>(std::chrono::duration_cast<std::chrono::nanoseconds>(t1 - t0).count());
+}
+
+struct engine
+{
+    std::string name;
+    std::function<std::size_t()> run;
+};
+
+template<typename Document>
+std::vector<engine> engines(const std::string& text, const json& parsed)
+{
+    return
+    {
+        {"view", [&text] { const json_document d = json_document::parse(text); return digest(d.root().get<Document>()); }},
+        {"materialize", [&text] { const json_document d = json_document::parse(text); return digest(d.root().materialize().get<Document>()); }},
+        {"json::parse", [&text] { return digest(json::parse(text).get<Document>()); }},
+        {"json::get", [&parsed] { return digest(parsed.get<Document>()); }},
+    };
+}
+}  // namespace
+
+int main(int argc, char** argv)
+{
+    if (argc < 2)
+    {
+        std::fprintf(stderr, "usage: %s <json_test_data directory> [rounds]\n", argv[0]);
+        return 1;
+    }
+    const std::string dir = argv[1];
+    const int rounds = argc > 2 ? std::max(1, std::atoi(argv[2])) : 30;
+
+    struct document_case
+    {
+        std::string name;
+        std::string text;
+        json parsed;
+        std::vector<engine> engines;
+    };
+    std::vector<document_case> cases;
+    cases.push_back({"twitter", read_file(dir + "/nativejson-benchmark/twitter.json"), json(), {}});
+    cases.push_back({"citm_catalog", read_file(dir + "/nativejson-benchmark/citm_catalog.json"), json(), {}});
+    cases.push_back({"synthetic", synthetic::make_text(20000), json(), {}});
+    for (auto& c : cases)
+    {
+        c.parsed = json::parse(c.text);
+    }
+    cases[0].engines = engines<twitter::document>(cases[0].text, cases[0].parsed);
+    cases[1].engines = engines<citm::document>(cases[1].text, cases[1].parsed);
+    cases[2].engines = engines<synthetic::document>(cases[2].text, cases[2].parsed);
+
+    std::FILE* csv = std::fopen("bench_decode.csv", "w");
+    if (csv == nullptr)
+    {
+        std::perror("bench_decode.csv");
+        return 1;
+    }
+    std::fprintf(csv, "doc,bytes,engine,ns\n");
+    for (const auto& c : cases)
+    {
+        // all engines must decode the same values
+        const std::size_t expected = c.engines[0].run();
+        bool ok = true;
+        for (const auto& e : c.engines)
+        {
+            ok = ok && e.run() == expected;
+        }
+        std::vector<double> best(c.engines.size(), 1e300);
+        for (int r = 0; r < rounds; ++r)
+        {
+            for (std::size_t k = 0; k < c.engines.size(); ++k)
+            {
+                g_sink = c.engines[k].run(); // untimed: see bench_view.cpp
+                best[k] = std::min(best[k], time_ns([&] { g_sink = c.engines[k].run(); }));
+            }
+        }
+        std::printf("%-13s %-7s %s", c.name.c_str(), "decode", ok ? "" : "[MISMATCH] ");
+        for (std::size_t k = 0; k < c.engines.size(); ++k)
+        {
+            const double us = best[k] / 1000.0;
+            std::printf("  %s %.*fus (%.2fx)", c.engines[k].name.c_str(), us < 10 ? 3 : (us < 1000 ? 1 : 0), us, best[k] / best[0]);
+            std::fprintf(csv, "%s,%zu,%s,%.1f\n", c.name.c_str(), c.text.size(), c.engines[k].name.c_str(), best[k]);
+        }
+        std::printf("\n");
+    }
+    std::fclose(csv);
+    return 0;
+}

tests/benchmarks/json_view/compare.py

@@ -9,7 +9,7 @@
 
 """Compare json_view with yyjson, simdjson, Boost.JSON, and json::parse.
 
-Builds bench_view.cpp, bench_corpus.cpp, and bench_edit.cpp against the include/ directory of
+Builds bench_view.cpp, bench_corpus.cpp, bench_edit.cpp, and bench_decode.cpp against the include/ directory of
 this checkout, runs them, and writes the results with everything needed to
 reproduce them (date, commit, CPU, OS, compiler, library versions, flags) to
 results/<date>-<host>.md and .csv next to this script.
@@ -283,7 +283,7 @@ def main():
             objects.append(obj)
 
     binaries = {}
-    for bench in ['bench_view', 'bench_corpus', 'bench_edit']:
+    for bench in ['bench_view', 'bench_corpus', 'bench_edit', 'bench_decode']:
         exe = os.path.join(args.build_dir, bench)
         run(cxx + flags + include + [os.path.join(HERE, bench + '.cpp')] + objects + link + ['-o', exe])
         binaries[bench] = exe
@@ -297,6 +297,8 @@ def main():
                                   capture_output=True, text=True).stdout
     outputs['bench_edit'] = run([binaries['bench_edit'], args.data, str(max(1, args.rounds // 2))], cwd=args.build_dir,
                                 capture_output=True, text=True).stdout
+    outputs['bench_decode'] = run([binaries['bench_decode'], args.data, str(max(1, args.rounds // 2))], cwd=args.build_dir,
+                                  capture_output=True, text=True).stdout
     for name, text in outputs.items():
         print(text)
 
@@ -330,7 +332,7 @@ def main():
             f.write(f'\n## {name}\n\n```\n{text.rstrip()}\n```\n')
     with open(stem + '.csv', 'w', encoding='utf-8') as out:
         out.write(''.join(f'# {key}: {value}\n' for key, value in meta))
-        for name in ['bench_view', 'bench_corpus', 'bench_edit']:
+        for name in ['bench_view', 'bench_corpus', 'bench_edit', 'bench_decode']:
             path = os.path.join(args.build_dir, name + '.csv')
             if os.path.isfile(path):
                 with open(path, encoding='utf-8') as f:

tests/src/unit-json_view_from_json.cpp

@@ -0,0 +1,357 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json_view.hpp>
+using nlohmann::json;
+using nlohmann::json_document;
+using nlohmann::json_view;
+using nlohmann::json_editable_document;
+using nlohmann::ordered_json_document;
+
+#include "json_view_test_helpers.hpp"
+#if !defined(JSON_NOEXCEPTION)
+    using json_view_test::exception_of_call;
+#endif
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <deque>
+#include <forward_list>
+#include <list>
+#include <map>
+#include <set>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <valarray>
+#include <vector>
+
+#ifdef JSON_HAS_CPP_17
+    #include <optional>
+#endif
+
+// The from_json() functions of the NLOHMANN_DEFINE_TYPE_* and
+// NLOHMANN_JSON_SERIALIZE_ENUM macros read views directly; other from_json()
+// functions get the value of materialize().
+
+namespace view_from_json
+{
+struct point
+{
+    int x = 0;
+    int y = 0;
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE(point, x, y)
+};
+
+struct shape
+{
+    std::string name;
+    std::vector<point> points;
+    point center;
+    std::map<std::string, point> named;
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE(shape, name, points, center, named)
+};
+
+struct with_defaults
+{
+    int a = 1;
+    std::string b = "default";
+    std::vector<int> c = {1, 2};
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(with_defaults, a, b, c)
+
+struct with_names
+{
+    int value = 0;
+    std::string text;
+};
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(with_names, "Value", value, "Text", text)
+
+struct empty
+{
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE(empty)
+};
+
+enum class color
+{
+    red,
+    green,
+    blue
+};
+NLOHMANN_JSON_SERIALIZE_ENUM(color,
+{
+    {color::red, "red"},
+    {color::green, "green"},
+    {color::blue, nullptr},
+})
+
+enum class strict_color
+{
+    red,
+    green
+};
+NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(strict_color,
+{
+    {strict_color::red, "red"},
+    {strict_color::green, 1},
+})
+
+struct base
+{
+    int id = 0;
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE(base, id)
+};
+
+struct derived : base
+{
+    color c = color::red;
+    NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(derived, base, c)
+};
+
+struct derived_with_defaults : base
+{
+    std::string label = "none";
+    NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT(derived_with_defaults, base, label)
+};
+
+// a base whose from_json() is written for json only: the derived type reads
+// its own members from the view and the base part from materialize()
+struct legacy_base
+{
+    int id = 0;
+};
+// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
+inline void from_json(const json& j, legacy_base& b)
+{
+    b.id = j.at("id").get<int>();
+}
+
+struct derived_from_legacy : legacy_base
+{
+    int extra = 0;
+};
+NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(derived_from_legacy, legacy_base, extra)
+
+// from_json() for json only: converted through materialize()
+struct legacy
+{
+    int v = 0;
+};
+// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
+inline void from_json(const json& j, legacy& l)
+{
+    l.v = j.at("v").get<int>();
+}
+
+// an unconstrained from_json() template, as in the documentation of
+// arbitrary type conversions: converted through materialize()
+struct unconstrained
+{
+    int v = 0;
+};
+template<typename BasicJsonType>
+// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
+void from_json(const BasicJsonType& j, unconstrained& u)
+{
+    u.v = j.at("v").template get<int>();
+}
+
+// a from_json() constrained by the user to views and json: reads the view
+struct constrained
+{
+    int v = 0;
+};
+template < typename J, typename std::enable_if < nlohmann::is_json_view<J>::value || std::is_same<J, json>::value, int >::type = 0 >
+// NOLINTNEXTLINE(misc-use-internal-linkage): found by ADL, like the from_json() functions of users
+void from_json(const J& j, constrained& c)
+{
+    c.v = j.at("v").template get<int>();
+}
+}  // namespace view_from_json
+
+namespace
+{
+template<typename T, typename View = json_view>
+struct reads_view : nlohmann::detail::has_view_from_json<nlohmann::adl_serializer<T, void>, View, T> {};
+}  // namespace
+
+TEST_CASE("json_view from_json")
+{
+    using namespace view_from_json; // NOLINT(google-build-using-namespace)
+
+    SECTION("which from_json() functions read views")
+    {
+        static_assert(reads_view<point>::value, "macro types read views");
+        static_assert(reads_view<shape>::value, "macro types read views");
+        static_assert(reads_view<with_defaults>::value, "macro types read views");
+        static_assert(reads_view<with_names>::value, "macro types read views");
+        static_assert(reads_view<empty>::value, "macro types read views");
+        static_assert(reads_view<derived>::value, "macro types read views");
+        static_assert(reads_view<derived_from_legacy>::value, "macro types read views");
+        static_assert(reads_view<color>::value, "enum macro types read views");
+        static_assert(reads_view<strict_color>::value, "enum macro types read views");
+        static_assert(reads_view<constrained>::value, "constrained from_json() reads views");
+        static_assert(reads_view<point, nlohmann::json_editable_view>::value, "editable views as well");
+        static_assert(reads_view<point, nlohmann::ordered_json_view>::value, "views of ordered_json as well");
+        static_assert(!reads_view<legacy>::value, "from_json() for json");
+        static_assert(!reads_view<legacy_base>::value, "from_json() for json");
+        static_assert(!reads_view<unconstrained>::value, "unconstrained from_json()");
+
+        static_assert(nlohmann::is_json_view<json_view>::value, "is_json_view");
+        static_assert(nlohmann::is_json_view<nlohmann::ordered_json_editable_view>::value, "is_json_view");
+        static_assert(!nlohmann::is_json_view<json>::value, "is_json_view");
+        static_assert(!nlohmann::is_json_view<json_document>::value, "is_json_view");
+        static_assert(!nlohmann::is_json_view<int>::value, "is_json_view");
+    }
+
+    SECTION("the library's own conversions are not called with a view (and the probe compiles for them)")
+    {
+        // these go through materialize() (vectors, maps with string keys,
+        // strings, and numbers are converted by the view itself)
+        static_assert(!reads_view<std::list<int>>::value, "std types");
+        static_assert(!reads_view<std::deque<point>>::value, "std types");
+        static_assert(!reads_view<std::set<std::string>>::value, "std types");
+        static_assert(!reads_view<std::unordered_set<int>>::value, "std types");
+        static_assert(!reads_view<std::forward_list<int>>::value, "std types");
+        static_assert(!reads_view<std::valarray<double>>::value, "std types");
+        static_assert(!reads_view<std::array<int, 3>>::value, "std types");
+        static_assert(!reads_view<std::pair<int, std::string>>::value, "std types");
+        static_assert(!reads_view<std::tuple<int, bool, point>>::value, "std types");
+        static_assert(!reads_view<std::map<int, int>>::value, "std types");
+        static_assert(!reads_view<std::unordered_map<int, point>>::value, "std types");
+        static_assert(!reads_view<std::map<color, int>>::value, "std types");
+        static_assert(!reads_view<std::map<std::string, int>>::value, "std types");
+        static_assert(!reads_view<std::vector<point>>::value, "std types");
+        static_assert(!reads_view<std::vector<bool>>::value, "std types");
+        static_assert(!reads_view<std::vector<std::uint8_t>>::value, "std types");
+        static_assert(!reads_view<std::string>::value, "std types");
+        static_assert(!reads_view<int>::value, "std types");
+        static_assert(!reads_view<bool>::value, "std types");
+        static_assert(!reads_view<double>::value, "std types");
+        static_assert(!reads_view<std::nullptr_t>::value, "std types");
+        static_assert(!reads_view<json>::value, "std types");
+        static_assert(!reads_view<json::binary_t>::value, "std types");
+#ifdef JSON_HAS_CPP_17
+        static_assert(!reads_view<std::optional<point>>::value, "std types");
+#endif
+        CHECK(true);
+    }
+
+    SECTION("macro types")
+    {
+        const json_document d = json_document::parse(R"({"name": "s", "points": [{"x": 1, "y": 2}, {"x": 3, "y": 4}],
+            "center": {"x": 5, "y": 6}, "named": {"a": {"x": 7, "y": 8}}})");
+        const shape s = d.root().get<shape>();
+        CHECK(s.name == "s");
+        CHECK(s.points.size() == 2);
+        CHECK(s.points[1].y == 4);
+        CHECK(s.center.x == 5);
+        CHECK(s.named.at("a").y == 8);
+
+        point p;
+        d.root()["center"].get_to(p);
+        CHECK(p.x == 5);
+        CHECK(p.y == 6);
+
+        static_cast<void>(d.root()["center"].get<empty>());
+    }
+
+    SECTION("defaults and names")
+    {
+        const json_document d = json_document::parse(R"({"a": 5, "Value": 3, "Text": "t"})");
+        const with_defaults w = d.root().get<with_defaults>();
+        CHECK(w.a == 5);
+        CHECK(w.b == "default");
+        CHECK(w.c == std::vector<int> {1, 2});
+        const json_document null_document = json_document::parse("null");
+        CHECK(null_document.root().get<with_defaults>().a == 1);
+        const with_names n = d.root().get<with_names>();
+        CHECK(n.value == 3);
+        CHECK(n.text == "t");
+    }
+
+    SECTION("enums")
+    {
+        const json_document d = json_document::parse(R"(["green", null, "purple", 1, "red"])");
+        CHECK(d.root()[0].get<color>() == color::green);
+        CHECK(d.root()[1].get<color>() == color::blue);
+        CHECK(d.root()[2].get<color>() == color::red); // unknown values map to the first entry
+        CHECK(d.root()[3].get<strict_color>() == strict_color::green);
+        CHECK(d.root()[4].get<strict_color>() == strict_color::red);
+#if !defined(JSON_NOEXCEPTION)
+        const json j = d.root().materialize();
+        CHECK(exception_of_call([&] { static_cast<void>(d.root()[2].get<strict_color>()); }) == "[json.exception.out_of_range.410] enum value out of range for strict_color: \"purple\"");
+#if !JSON_DIAGNOSTICS
+        CHECK(exception_of_call([&] { static_cast<void>(d.root()[2].get<strict_color>()); }) == exception_of_call([&] { static_cast<void>(j[2].get<strict_color>()); }));
+#endif
+#endif
+    }
+
+    SECTION("derived types")
+    {
+        const json_document d = json_document::parse(R"({"id": 7, "c": "green", "extra": 3})");
+        const derived x = d.root().get<derived>();
+        CHECK(x.id == 7);
+        CHECK(x.c == color::green);
+        const derived_with_defaults y = d.root().get<derived_with_defaults>();
+        CHECK(y.id == 7);
+        CHECK(y.label == "none");
+        const derived_from_legacy z = d.root().get<derived_from_legacy>();
+        CHECK(z.id == 7);
+        CHECK(z.extra == 3);
+    }
+
+    SECTION("other from_json() functions get the value of materialize()")
+    {
+        const json_document d = json_document::parse(R"({"v": 4})");
+        CHECK(d.root().get<legacy>().v == 4);
+        CHECK(d.root().get<unconstrained>().v == 4);
+        CHECK(d.root().get<constrained>().v == 4);
+        legacy l;
+        d.root().get_to(l);
+        CHECK(l.v == 4);
+    }
+
+    SECTION("duplicate keys: members are looked up in the view, where the first one wins")
+    {
+        const json_document d = json_document::parse(R"({"x": 1, "y": 2, "x": 3})");
+        CHECK(d.root().get<point>().x == 1);
+        // materialize() keeps the last value, like parse()
+        CHECK(d.root().materialize().get<point>().x == 3);
+    }
+
+    SECTION("editable documents and ordered_json")
+    {
+        json_editable_document e = json_editable_document::parse(R"({"x": 1, "y": 2})");
+        e.set(e.root(), "x", 10);
+        CHECK(e.root().get<point>().x == 10);
+        const ordered_json_document o = ordered_json_document::parse(R"({"name": "o", "points": [], "center": {"x": 1, "y": 2}, "named": {}})");
+        CHECK(o.root().get<shape>().center.y == 2);
+    }
+
+#if !defined(JSON_NOEXCEPTION) && !JSON_DIAGNOSTICS
+    SECTION("errors are those of the same conversion of a json value")
+    {
+        // (with JSON_DIAGNOSTICS, those of json name the path of the value)
+        const json_document d = json_document::parse(R"([{"x": 1}, {"x": "1", "y": 2}, [1, 2], {"id": "7", "c": "red"}])");
+        const json j = d.root().materialize();
+        for (std::size_t i = 0; i < 3; ++i)
+        {
+            CAPTURE(i)
+            CHECK(exception_of_call([&] { static_cast<void>(d.root()[i].get<point>()); }) == exception_of_call([&] { static_cast<void>(j[i].get<point>()); }));
+        }
+        CHECK(exception_of_call([&] { static_cast<void>(d.root()[3].get<derived>()); }) == exception_of_call([&] { static_cast<void>(j[3].get<derived>()); }));
+    }
+#endif
+}