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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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tests/src/unit-conversions.cpp
+31
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@@ -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>>();