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

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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>
This commit is contained in:
Niels Lohmann
2026-09-08 19:51:33 +02:00
parent e59b455616
commit 75226beb40
2 changed files with 22 additions and 30 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/conversions/from_json.hpp
+11 -15
View File
@@ -398,27 +398,23 @@ 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())
template<typename ConstructibleObjectType>
auto from_json_object_reserve(ConstructibleObjectType& obj, typename ConstructibleObjectType::size_type size, priority_tag<1> /*unused*/)
-> decltype(obj.reserve(size), 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);
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>
inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
inline void from_json_object_impl(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>());
@@ -435,7 +431,7 @@ 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));
}
from_json_object_impl(j, obj, priority_tag<1> {});
from_json_object_impl(j, obj);
}
// overload for arithmetic types, not chosen for basic_json template arguments
single_include/nlohmann/json.hpp
+11 -15
View File
@@ -5693,27 +5693,23 @@ 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())
template<typename ConstructibleObjectType>
auto from_json_object_reserve(ConstructibleObjectType& obj, typename ConstructibleObjectType::size_type size, priority_tag<1> /*unused*/)
-> decltype(obj.reserve(size), 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);
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>
inline void from_json_object_impl(const BasicJsonType& j, ConstructibleObjectType& obj, priority_tag<0> /*unused*/)
inline void from_json_object_impl(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>());
@@ -5730,7 +5726,7 @@ 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));
}
from_json_object_impl(j, obj, priority_tag<1> {});
from_json_object_impl(j, obj);
}
// overload for arithmetic types, not chosen for basic_json template arguments