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Reserve capacity in from_json() object conversion when supported

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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>
This commit is contained in:
Niels Lohmann
2026-09-05 20:43:23 +02:00
parent 09b6b6b5ba
commit e29ac4f043
3 changed files with 89 additions and 14 deletions
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include/nlohmann/detail/conversions/from_json.hpp
+29 -7
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@@ -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;
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);
from_json_object_impl(j, obj, priority_tag<1> {});
}
// overload for arithmetic types, not chosen for basic_json template arguments