Speed up array-to-container from_json

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

include/nlohmann/detail/conversions/from_json.hpp

@@ -266,6 +266,22 @@ template<typename ConstructibleArrayType>
 void from_json_array_reserve(ConstructibleArrayType& /*arr*/, std::size_t /*size*/, priority_tag<0> /*unused*/)
 {}
 
+// append through push_back() where available; containers without it (such as
+// std::set) fall back to insert(end(), ...), which is what std::inserter did
+template<typename ConstructibleArrayType, typename ValueType>
+auto from_json_array_append(ConstructibleArrayType& arr, ValueType&& value, priority_tag<1> /*unused*/)
+-> decltype(arr.push_back(std::forward<ValueType>(value)), void())
+{
+    arr.push_back(std::forward<ValueType>(value));
+}
+
+template<typename ConstructibleArrayType, typename ValueType>
+void from_json_array_append(ConstructibleArrayType& arr, ValueType&& value, priority_tag<0> /*unused*/)
+{
+    using std::end;
+    arr.insert(end(arr), std::forward<ValueType>(value));
+}
+
 template<typename BasicJsonType, typename ConstructibleArrayType,
          enable_if_t<
              std::is_assignable<ConstructibleArrayType&, ConstructibleArrayType>::value,
@@ -275,17 +291,15 @@ auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, p
     j.template get<typename ConstructibleArrayType::value_type>(),
     void())
 {
-    using std::end;
-
     ConstructibleArrayType ret;
-    from_json_array_reserve(ret, j.size(), priority_tag<1> {});
-    std::transform(j.begin(), j.end(),
-                   std::inserter(ret, end(ret)), [](const BasicJsonType & i)
+    const auto* inner_array = j.template get_ptr<const typename BasicJsonType::array_t*>();
+    from_json_array_reserve(ret, inner_array->size(), priority_tag<1> {});
+    for (const auto& elem : *inner_array)
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
-        return i.template get<typename ConstructibleArrayType::value_type>();
-    });
+        from_json_array_append(ret, elem.template get<typename ConstructibleArrayType::value_type>(), priority_tag<1> {});
+    }
     arr = std::move(ret);
 }
 

single_include/nlohmann/json.hpp

@@ -5970,6 +5970,22 @@ template<typename ConstructibleArrayType>
 void from_json_array_reserve(ConstructibleArrayType& /*arr*/, std::size_t /*size*/, priority_tag<0> /*unused*/)
 {}
 
+// append through push_back() where available; containers without it (such as
+// std::set) fall back to insert(end(), ...), which is what std::inserter did
+template<typename ConstructibleArrayType, typename ValueType>
+auto from_json_array_append(ConstructibleArrayType& arr, ValueType&& value, priority_tag<1> /*unused*/)
+-> decltype(arr.push_back(std::forward<ValueType>(value)), void())
+{
+    arr.push_back(std::forward<ValueType>(value));
+}
+
+template<typename ConstructibleArrayType, typename ValueType>
+void from_json_array_append(ConstructibleArrayType& arr, ValueType&& value, priority_tag<0> /*unused*/)
+{
+    using std::end;
+    arr.insert(end(arr), std::forward<ValueType>(value));
+}
+
 template<typename BasicJsonType, typename ConstructibleArrayType,
          enable_if_t<
              std::is_assignable<ConstructibleArrayType&, ConstructibleArrayType>::value,
@@ -5979,17 +5995,15 @@ auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, p
     j.template get<typename ConstructibleArrayType::value_type>(),
     void())
 {
-    using std::end;
-
     ConstructibleArrayType ret;
-    from_json_array_reserve(ret, j.size(), priority_tag<1> {});
-    std::transform(j.begin(), j.end(),
-                   std::inserter(ret, end(ret)), [](const BasicJsonType & i)
+    const auto* inner_array = j.template get_ptr<const typename BasicJsonType::array_t*>();
+    from_json_array_reserve(ret, inner_array->size(), priority_tag<1> {});
+    for (const auto& elem : *inner_array)
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
-        return i.template get<typename ConstructibleArrayType::value_type>();
-    });
+        from_json_array_append(ret, elem.template get<typename ConstructibleArrayType::value_type>(), priority_tag<1> {});
+    }
     arr = std::move(ret);
 }
 

tests/benchmarks/src/benchmarks.cpp

@@ -9,7 +9,10 @@
 #include <benchmark/benchmark.h>
 #include <nlohmann/json.hpp>
 #include <fstream>
+#include <array>
+#include <deque>
 #include <numeric>
+#include <set>
 #include <vector>
 #include <test_data.hpp>
 
@@ -621,4 +624,27 @@ static void FromCborChunkedString(benchmark::State& state, const std::size_t chu
 
 BENCHMARK_CAPTURE(FromCborChunkedString, 10000 chunks, 10000);
 
+// conversion of a JSON array into standard containers
+template<typename Container>
+static void GetArray(benchmark::State& state)
+{
+    json j = json::array();
+    for (int i = 0; i < 1000; ++i)
+    {
+        j.push_back(i);
+    }
+
+    for (auto _ : state)
+    {
+        auto c = j.get<Container>();
+        benchmark::DoNotOptimize(c);
+    }
+    state.SetItemsProcessed(state.iterations() * 1000);
+}
+
+BENCHMARK_TEMPLATE(GetArray, std::vector<int>);
+BENCHMARK_TEMPLATE(GetArray, std::deque<int>);
+BENCHMARK_TEMPLATE(GetArray, std::set<int>);
+BENCHMARK_TEMPLATE(GetArray, std::array<int, 1000>);
+
 BENCHMARK_MAIN();