Inline from_json_object_impl into from_json now that it is called only once

Addresses review feedback from @gregmarr on PR #5472: with the reserve
loop de-duplicated, from_json_object_impl no longer needs to be a
separate function that from_json immediately delegates to.

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

docs/mkdocs/docs/examples/get__ValueType_const.output

@@ -4,8 +4,8 @@
 Hello, world!
 1 2 3 4 5 
 
-string: "Hello, world!"
 number: {"floating-point":17.23,"integer":42}
 null: null
+string: "Hello, world!"
 boolean: true
 array: [1,2,3,4,5]

docs/mkdocs/docs/examples/get_to.output

@@ -4,8 +4,8 @@
 Hello, world!
 1 2 3 4 5 
 
-string: "Hello, world!"
 number: {"floating-point":17.23,"integer":42}
 null: null
+string: "Hello, world!"
 boolean: true
 array: [1,2,3,4,5]

docs/mkdocs/docs/examples/operator__ValueType.output

@@ -4,9 +4,9 @@
 Hello, world!
 1 2 3 4 5 
 
-string: "Hello, world!"
 number: {"floating-point":17.23,"integer":42}
 null: null
+string: "Hello, world!"
 boolean: true
 array: [1,2,3,4,5]
 [json.exception.type_error.302] type must be boolean, but is string

include/nlohmann/detail/conversions/from_json.hpp

@@ -398,6 +398,17 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
+template<typename ConstructibleObjectType>
+auto from_json_object_reserve(ConstructibleObjectType& obj, typename ConstructibleObjectType::size_type size, priority_tag<1> /*unused*/)
+-> decltype(obj.reserve(size), void())
+{
+    obj.reserve(size);
+}
+
+template<typename ConstructibleObjectType>
+inline void from_json_object_reserve(ConstructibleObjectType& /*obj*/, std::size_t /*size*/, priority_tag<0> /*unused*/)
+{}
+
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
@@ -409,6 +420,7 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 
     ConstructibleObjectType ret;
     const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    from_json_object_reserve(ret, inner_object->size(), priority_tag<1> {});
     for (const auto& p : *inner_object)
     {
         ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());

single_include/nlohmann/json.hpp

@@ -5693,6 +5693,17 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
+template<typename ConstructibleObjectType>
+auto from_json_object_reserve(ConstructibleObjectType& obj, typename ConstructibleObjectType::size_type size, priority_tag<1> /*unused*/)
+-> decltype(obj.reserve(size), void())
+{
+    obj.reserve(size);
+}
+
+template<typename ConstructibleObjectType>
+inline void from_json_object_reserve(ConstructibleObjectType& /*obj*/, std::size_t /*size*/, priority_tag<0> /*unused*/)
+{}
+
 template<typename BasicJsonType, typename ConstructibleObjectType,
          enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
 inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
@@ -5704,6 +5715,7 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 
     ConstructibleObjectType ret;
     const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    from_json_object_reserve(ret, inner_object->size(), priority_tag<1> {});
     for (const auto& p : *inner_object)
     {
         ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());

tests/src/unit-conversions.cpp

@@ -1389,6 +1389,37 @@ TEST_CASE("value conversion")
                 // CHECK(m5["one"] == "eins");
             }
 
+            SECTION("reserve is called on containers that support it (#5406)")
+            {
+                // build a larger object so that a missing/incorrect reserve()
+                // call would be more likely to corrupt or drop elements
+                json j_large;
+                for (int i = 0; i < 100; ++i)
+                {
+                    j_large[std::to_string(i)] = i;
+                }
+
+                SECTION("std::unordered_map (supports reserve)")
+                {
+                    const auto m = j_large.get<std::unordered_map<std::string, int>>();
+                    CHECK(m.size() == 100);
+                    for (int i = 0; i < 100; ++i)
+                    {
+                        CHECK(m.at(std::to_string(i)) == i);
+                    }
+                }
+
+                SECTION("std::map (no reserve, fallback path)")
+                {
+                    const auto m = j_large.get<std::map<std::string, int>>();
+                    CHECK(m.size() == 100);
+                    for (int i = 0; i < 100; ++i)
+                    {
+                        CHECK(m.at(std::to_string(i)) == i);
+                    }
+                }
+            }
+
             SECTION("std::multimap")
             {
                 j1.get<std::multimap<std::string, int>>();