Update doc example outputs for new object-conversion iteration order

Reserving capacity in from_json()'s object-conversion path before
inserting elements changes libstdc++'s std::unordered_map bucket
layout, which changes the iteration order used by
get__ValueType_const.cpp, get_to.cpp and operator__ValueType.cpp to
print the elements of a converted std::unordered_map<std::string,
json>. Verified against a clean develop checkout (built with the
same GCC/libstdc++ used in CI) that the old order was produced
without this PR's change and the new order is produced with it, and
that the three affected examples now match their updated expected
output byte-for-byte.

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 +398,34 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
+template<typename BasicJsonType, typename ConstructibleObjectType>
+auto from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<1> /*unused*/)
+-> decltype(
+    obj.reserve(std::declval<typename ConstructibleObjectType::size_type>()),
+    void())
+{
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    ret.reserve(inner_object->size());
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
+}
+
+template<typename BasicJsonType, typename ConstructibleObjectType>
+inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
+{
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
+}
+
 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)
@@ -407,13 +435,7 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, concat("type must be object, but is ", j.type_name()), &j));
     }
 
-    ConstructibleObjectType ret;
+    from_json_object_impl(j, obj, priority_tag<1> {});
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments

single_include/nlohmann/json.hpp

@@ -5693,6 +5693,34 @@ inline void from_json(const BasicJsonType& j, CompatibleArrayType& bin)
     }
 }
 
+template<typename BasicJsonType, typename ConstructibleObjectType>
+auto from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<1> /*unused*/)
+-> decltype(
+    obj.reserve(std::declval<typename ConstructibleObjectType::size_type>()),
+    void())
+{
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    ret.reserve(inner_object->size());
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
+}
+
+template<typename BasicJsonType, typename ConstructibleObjectType>
+inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
+{
+    ConstructibleObjectType ret;
+    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    for (const auto& p : *inner_object)
+    {
+        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
+    }
+    obj = std::move(ret);
+}
+
 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)
@@ -5702,13 +5730,7 @@ inline void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, concat("type must be object, but is ", j.type_name()), &j));
     }
 
-    ConstructibleObjectType ret;
+    from_json_object_impl(j, obj, priority_tag<1> {});
-    const auto* inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    for (const auto& p : *inner_object)
-    {
-        ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
-    }
-    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments

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>>();