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Reserve capacity in from_json() object conversion when the target container supports it (#5472)

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* Reserve capacity in from_json() object conversion when supported

The object-to-container from_json() overload filled the target
container one element at a time without reserving capacity, even
when the target type supports reserve() (e.g. std::unordered_map)
and the number of elements is already known. This caused unnecessary
rehashing while parsing large objects into such containers.

Add a reserve-detecting overload (from_json_object_impl), mirroring
the priority_tag-based SFINAE technique already used by the array
conversion path (from_json_array_impl), so that reserve(size()) is
called up front when available and the loop falls back unchanged
otherwise (e.g. for std::map).

Fixes #5406

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

* 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>

* Factor out a reserve-dispatch helper instead of duplicating the object from_json loop

Addresses review feedback from @gregmarr on PR #5472: the emplace loop no
longer needs to exist twice for the reserve/no-reserve cases. A small
from_json_object_reserve() overload pair (SFINAE-dispatched on whether
reserve() exists, mirroring the priority_tag technique used elsewhere)
either calls reserve() or is a no-op; from_json_object_impl() calls it once.

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

* 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>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-09 09:54:12 +02:00
committed by GitHub
parent 8e26b9d4b3
commit b17c272f46
6 changed files with 58 additions and 3 deletions
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2
docs/mkdocs/docs/examples/get__ValueType_const.output
View File

@@ -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]

2
docs/mkdocs/docs/examples/get_to.output
View File

@@ -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]

2
docs/mkdocs/docs/examples/operator__ValueType.output
View File

@@ -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

12
include/nlohmann/detail/conversions/from_json.hpp
View File

@@ -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>());

12
single_include/nlohmann/json.hpp
View File

@@ -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>());

31
tests/src/unit-conversions.cpp
View File

@@ -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>>();