update

This is the bare minimum implementation of this idea.

benchmark/kostya/simdjson_ondemand.h

@@ -24,6 +24,35 @@ struct simdjson_ondemand {
 
 BENCHMARK_TEMPLATE(kostya, simdjson_ondemand)->UseManualTime();
 
+
+#if SIMDJSON_SUPPORTS_EXTRACT
+using namespace simdjson::ondemand;
+
+struct simdjson_ondemand_extract {
+  static constexpr diff_flags DiffFlags = diff_flags::NONE;
+
+  ondemand::parser parser{};
+
+  bool run(simdjson::padded_string &json, std::vector<point> &result) {
+    auto doc = parser.iterate(json);
+    for (ondemand::object object_point : doc.find_field("coordinates")) {
+      point p;
+      auto error = object_point.extract(
+        to{"x", p.x},
+        to{"y", p.y},
+        to{"z", p.z}
+      );
+      if(error) { return false; }
+      result.push_back(p);
+    }
+    return true;
+  }
+};
+
+BENCHMARK_TEMPLATE(kostya, simdjson_ondemand_extract)->UseManualTime();
+
+#endif // SIMDJSON_SUPPORTS_EXTRACT
+
 } // namespace kostya
 
 #endif // SIMDJSON_EXCEPTIONS

benchmark/partial_tweets/simdjson_ondemand.h

@@ -9,7 +9,7 @@ namespace partial_tweets {
 using namespace simdjson;
 
 struct simdjson_ondemand {
-  using StringType=std::string_view;
+  using StringType = std::string_view;
 
   ondemand::parser parser{};
 
@@ -43,6 +43,48 @@ struct simdjson_ondemand {
 
 BENCHMARK_TEMPLATE(partial_tweets, simdjson_ondemand)->UseManualTime();
 
+
+#if SIMDJSON_SUPPORTS_EXTRACT
+
+using namespace simdjson::ondemand;
+
+struct simdjson_ondemand_extract {
+  using StringType = std::string_view;
+  ondemand::parser parser{};
+  bool run(simdjson::padded_string &json, std::vector<tweet<std::string_view>> &result) {
+    // Walk the document, parsing the tweets as we go
+    auto doc = parser.iterate(json);
+    for (ondemand::object tweet_object : doc.find_field("statuses")) {
+      tweet<std::string_view> t;
+      auto error = tweet_object.extract(
+        to{"created_at", t.created_at},
+        to{"id", t.id},
+        to{"text", t.result},
+        to{"in_reply_to_status_id", [&t](auto val) {
+          if(val.is_null()) {
+            t.in_reply_to_status_id = 0;
+          } else {
+            t.in_reply_to_status_id = val;
+          }
+         }},
+        to{"user", sub{
+          to{"id", t.user.id},
+          to{"screen_name", t.user.screen_name},
+        }},
+        to{"retweet_count", t.retweet_count},
+        to{"favorite_count", t.favorite_count});
+        if(error) { return false; }
+        result.push_back(t);
+    }
+    return true;
+  }
+};
+
+BENCHMARK_TEMPLATE(partial_tweets, simdjson_ondemand_extract)->UseManualTime();
+
+
+#endif
+
 } // namespace partial_tweets
 
 #endif // SIMDJSON_EXCEPTIONS

include/simdjson/generic/ondemand/object-inl.h

@@ -15,6 +15,134 @@ namespace simdjson {
 namespace SIMDJSON_IMPLEMENTATION {
 namespace ondemand {
 
+
+#ifdef SIMDJSON_SUPPORTS_EXTRACT
+
+template <endpoint ...Funcs>
+simdjson_inline error_code object::extract(Funcs&&... endpoints)
+#ifndef _MSC_VER // msvc thinks noexcept is not the same in definition
+ noexcept((nothrow_endpoint<Funcs> && ...))
+#endif
+{
+  raw_json_string field_key;
+  error_code error = SUCCESS;
+  for(auto pair : *this) {
+    if (error = pair.key().get(field_key); error) {
+      break;
+    }
+    std::ignore = ((field_key.unsafe_is_equal(endpoints.key()) ? (error = endpoints(pair.value())) == SUCCESS : true) && ...);
+    if (error) {
+      break;
+    }
+  }
+  return error;
+}
+
+template <typename T>
+struct to {
+private:
+  T *pointer;
+  std::string_view m_key;
+
+public:
+  constexpr explicit(false)
+      to(std::string_view const inp_key, T &obj_ref) noexcept
+      : pointer{std::addressof(obj_ref)}, m_key{inp_key} {}
+
+  constexpr to(to const &) = default;
+  constexpr to(to &&) noexcept = default;
+  constexpr to &operator=(to const &) = default;
+  constexpr to &operator=(to &&) noexcept = default;
+  constexpr ~to() = default;
+
+  [[nodiscard]] constexpr std::string_view key() const noexcept {
+    return m_key;
+  }
+
+  [[nodiscard]] constexpr error_code operator()(simdjson_result<value> val) noexcept(
+      std::is_nothrow_assignable_v<T, simdjson_result<value>>) {
+    return val.get<T>(*pointer);
+  }
+};
+
+template <typename Func>
+  requires(std::is_invocable_v<Func, simdjson_result<value>>)
+struct to<Func> {
+private:
+  Func func;
+  std::string_view m_key;
+
+public:
+  constexpr explicit(false)
+      to(std::string_view const inp_key,
+         Func &&inp_func) noexcept(std::is_nothrow_copy_assignable_v<Func>)
+      : func{std::forward<Func>(inp_func)}, m_key{inp_key} {}
+
+  constexpr to(to const &) = default;
+  constexpr to(to &&) noexcept = default;
+  constexpr to& operator=(to const &) = default;
+  constexpr to& operator=(to &&) noexcept = default;
+  constexpr ~to() = default;
+
+  [[nodiscard]] constexpr std::string_view key() const noexcept {
+    return m_key;
+  }
+
+  [[nodiscard]] constexpr error_code operator()(simdjson_result<value> val) noexcept(
+      std::is_nothrow_invocable_v<Func, simdjson_result<value>>) {
+    if constexpr (std::is_invocable_r_v<error_code, Func, simdjson_result<value>>) {
+      return func(val);
+    } else {
+      static_cast<void>(func(val));
+      return SUCCESS;
+    }
+  }
+};
+
+template <typename Func>
+  requires(std::is_invocable_v<Func, simdjson_result<value>>)
+to(std::string_view, Func &&) -> to<Func>;
+
+template <typename T>
+to(std::string_view, T&) -> to<T>;
+
+template <endpoint... Tos>
+struct sub {
+private:
+  using tuple_type = std::tuple<Tos...>;
+  tuple_type tos;
+
+public:
+
+  // double templating to make perfect forwarding work
+  template <typename ...T>
+    requires ((std::same_as<T, Tos> && ...))
+  explicit constexpr sub(T&&...inp_tos) noexcept(std::is_nothrow_constructible_v<tuple_type, T...>)
+      : tos{std::forward<T>(inp_tos)...} {}
+
+  constexpr sub(sub const &) = default;
+  constexpr sub(sub &&) noexcept = default;
+  constexpr sub &operator=(sub const &) = default;
+  constexpr sub &operator=(sub &&) = default;
+  constexpr ~sub() = default;
+
+  [[nodiscard]] constexpr error_code operator()(simdjson_result<value> val) noexcept((nothrow_endpoint<Tos> && ...)) {
+    object obj;
+    if (auto const err = val.get_object().get(obj); err) {
+      return err;
+    }
+    return std::apply([&obj]<typename... T>(T &&...app_tos) {
+      return obj.extract(std::forward<T>(app_tos)...);
+    }, tos);
+  }
+};
+
+template <endpoint... Tos>
+sub(Tos&&...) -> sub<Tos...>;
+
+#endif
+
+
 simdjson_inline simdjson_result<value> object::find_field_unordered(const std::string_view key) & noexcept {
   bool has_value;
   SIMDJSON_TRY( iter.find_field_unordered_raw(key).get(has_value) );

include/simdjson/generic/ondemand/object.h

@@ -11,6 +11,20 @@ namespace simdjson {
 namespace SIMDJSON_IMPLEMENTATION {
 namespace ondemand {
 
+#if defined(__cpp_concepts) && defined(__cpp_consteval)
+#define SIMDJSON_SUPPORTS_EXTRACT 1
+
+template <typename T>
+concept endpoint = std::is_invocable_r_v<error_code, T, simdjson_result<value>> &&
+                   std::is_copy_constructible_v<T> && requires(T to) {
+                     { to.key() } noexcept -> std::convertible_to<std::string_view>;
+                   };
+
+template <typename T>
+concept nothrow_endpoint = endpoint<T> && std::is_nothrow_invocable_r_v<error_code, T, simdjson_result<value>>;
+
+#endif
+
 /**
  * A forward-only JSON object field iterator.
  */
@@ -65,6 +79,21 @@ public:
   /** @overload simdjson_inline simdjson_result<value> find_field(std::string_view key) & noexcept; */
   simdjson_inline simdjson_result<value> find_field(std::string_view key) && noexcept;
 
+#ifdef SIMDJSON_SUPPORTS_EXTRACT
+  /**
+   * Extract all the fields in one go
+   * Funcs are invocables that take a simdjson_result<value> as input.
+   */
+  template <endpoint ...Funcs>
+  simdjson_inline error_code extract(Funcs&&... endpoints)
+#ifndef _MSC_VER // msvc thinks noexcept is not the same in definition
+ noexcept((nothrow_endpoint<Funcs> && ...))
+#endif
+ ;
+#endif
+
+
+
   /**
    * Look up a field by name on an object, without regard to key order.
    *

tests/ondemand/CMakeLists.txt

@@ -28,6 +28,7 @@ add_cpp_test(ondemand_to_string              LABELS ondemand acceptance per_impl
 add_cpp_test(ondemand_twitter_tests          LABELS ondemand acceptance per_implementation)
 add_cpp_test(ondemand_wrong_type_error_tests LABELS ondemand acceptance per_implementation)
 add_cpp_test(ondemand_iterate_many_csv       LABELS ondemand acceptance per_implementation)
+add_cpp_test(ondemand_extract_tests          LABELS ondemand acceptance per_implementation)
 if(NOT SIMDJSON_SANITIZE)
   add_cpp_test(ondemand_cacheline              LABELS ondemand acceptance per_implementation)
 endif()

tests/ondemand/ondemand_extract_tests.cpp

@@ -0,0 +1,116 @@
+#include "simdjson.h"
+#include "test_ondemand.h"
+
+using namespace simdjson;
+
+namespace multi_get_tests {
+#ifdef __cpp_concepts
+struct Car {
+  std::string_view make;
+  std::string_view model;
+  std::int64_t year = 0;
+  struct wheel_size_type {
+    std::int64_t front = 0;
+    std::int64_t back = 0;
+  } wheels;
+
+  static simdjson_result<Car> create(auto& value) {
+    using ondemand::to;
+    using ondemand::sub;
+
+    simdjson::ondemand::object obj;
+    auto error = value.get_object().get(obj);
+    if (error) {
+      return error;
+    }
+    Car car{};
+    // Instead of this:
+    //     for (auto field : obj) {
+    //       simdjson::ondemand::raw_json_string key;
+    //       error = field.key().get(key);
+    //       if (error) {
+    //         return error;
+    //       }
+    //       if (key == "make") {
+    //         error = field.value().get_string(car.make);
+    //         if (error) {
+    //           return error;
+    //         }
+    //       } else if (key == "model") {
+    //         error = field.value().get_string(car.model);
+    //         if (error) {
+    //           return error;
+    //         }
+    //       } else if (key == "year") {
+    //         error = field.value().get(car.year);
+    //         if (error) {
+    //           return error;
+    //         }
+    //     }
+    //
+    // we can do this now:
+    error = obj.extract(
+      to{"wheels", sub{
+        to{"front", car.wheels.front},
+        to{"back", car.wheels.back},
+      }},
+      to{"make", car.make},
+      to{"model", car.model},
+      to{"year", [&car](auto val) {
+        car.year = val;
+      }});
+    if (error) {
+      return error;
+    }
+    return car;
+  }
+};
+
+bool car_example() {
+  TEST_START();
+  simdjson::padded_string json =
+      R"( [ { "make": "Toyota", "model": "Camry",  "year": 2018,
+       "tire_pressure": [ 40.1, 39.9 ], "wheels": { "front": 10, "back": 10 } },
+  { "make": "Kia",    "model": "Soul",   "year": 2012,
+       "tire_pressure": [ 30.1, 31.0 ], "wheels": { "front": 10, "back": 10 } },
+  { "make": "Toyota", "model": "Tercel", "year": 1999,
+       "tire_pressure": [ 29.8, 30.0 ], "wheels": { "front": 10, "back": 10 } }
+])"_padded;
+
+  simdjson::ondemand::parser parser;
+  simdjson::ondemand::document doc = parser.iterate(json);
+  for (auto val : doc) {
+    auto car = Car::create(val);
+    ASSERT_EQUAL(car.error(), SUCCESS);
+    Car const c(car.value());
+    if (c.make != "Toyota" && c.make != "Kia") {
+      return false;
+    }
+    if (c.model != "Camry" && c.model != "Soul" && c.model != "Tercel") {
+      return false;
+    }
+    if (c.year != 2018 && c.year != 2012 && c.year != 1999) {
+      return false;
+    }
+    if (c.wheels.front != 10 || c.wheels.back != 10) {
+      return false;
+    }
+  }
+
+  TEST_SUCCEED();
+}
+
+#endif
+bool run() {
+  return
+#ifdef __cpp_concepts
+      car_example() &&
+#endif
+      true;
+}
+
+} // namespace multi_get_tests
+
+int main(int argc, char *argv[]) {
+  return test_main(argc, argv, multi_get_tests::run);
+}