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Hint the end when converting an object to a standard container

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from_json for map-like containers called emplace() without a hint, which
searches the tree for every key. The keys of the source object are sorted, so
inserting with the end as the hint is constant time, as it was in 3.12.0 via
std::inserter. get<std::map<...>>() of a 25,000 entry object: 4.2 ms -> 3.25 ms
(3.12.0: 3.7 ms).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Niels Lohmann committed 2026-10-15 11:46:01 +02:00
1 parent ad4c240e8b
commit 420e9218cd
2 files changed
+10 -2
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include/nlohmann/detail/conversions/from_json.hpp
+5 -1
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@@ -397,9 +397,13 @@ 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> {});
using value_type = typename ConstructibleObjectType::value_type;
for (const auto& p : *inner_object)
{
ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
// the keys of the source are sorted for the usual object types, so
// hinting the end makes every insertion constant-time (for containers
// that order their keys the same way)
ret.insert(ret.end(), value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>()));
}
obj = std::move(ret);
}
single_include/nlohmann/json.hpp
Generated
+5 -1
View File
@@ -6101,9 +6101,13 @@ 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> {});
using value_type = typename ConstructibleObjectType::value_type;
for (const auto& p : *inner_object)
{
ret.emplace(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
// the keys of the source are sorted for the usual object types, so
// hinting the end makes every insertion constant-time (for containers
// that order their keys the same way)
ret.insert(ret.end(), value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>()));
}
obj = std::move(ret);
}