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Niels Lohmann 4b2d2244c3 Move instead of deep-copy ordered_json values when an object grows

ordered_map keeps its elements in a std::vector<std::pair<const Key, T>>.
With a std::string key, that pair is not nothrow move constructible (the
const key has to be copied), so std::vector copies every element when it
reallocates. For ordered_json, this deep-copies every member value an
object already holds, including whole nested subtrees, on each growth
step.

Grow the storage in ordered_map instead, copying the keys and moving the
values. This happens in two phases, so the strong exception guarantee is
kept without try/catch. The first phase may throw, but only touches a
temporary buffer: it copies the keys, value-initializes the values, and
constructs the new element. The second phase moves the values (noexcept)
and swaps the buffers. Because the new element is constructed before any
value is moved, arguments that refer to elements of the container stay
valid, as with std::vector. Types that cannot take this path keep the
std::vector behavior.

Parsing into ordered_json (ParseStringOrdered, Apple M1 Max, clang -O3):
twitter 3.20 -> 1.70 ms, citm_catalog 7.73 -> 3.67 ms, jeopardy 219 ->
177 ms, canada unchanged. The number of allocations for twitter and
citm_catalog drops by two thirds.

Also add ParseStringOrdered rows to the benchmarks, and document the
growth behavior and the exception safety of ordered_map.

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

2026-09-28 18:48:05 +02:00

docset

Add BON8 support (#2998 )

2026-09-27 16:56:21 +02:00

mkdocs

Move instead of deep-copy ordered_json values when an object grows

2026-09-28 18:48:05 +02:00

avatars.png

🔖 set version to 3.12.0 (#4727 )

2025-04-11 10:41:14 +02:00

json.gif

🔖 set version to 3.12.0 (#4727 )

2025-04-11 10:41:14 +02:00

Makefile

Clean up and document project files (#4560 )

2025-01-17 06:53:35 +01:00

README.md

docs: improve clarity and formatting in documentation generation section (#5021 )

2025-12-03 00:15:27 +01:00

README.md

Documentation

Generate documentation

Note on documentation: The source files contain links to the online documentation at https://json.nlohmann.me.
This URL provides the most recent documentation and also applies to previous versions. Documentation for deprecated functions is not removed; instead, it is marked as deprecated.

If you want to view the documentation for a specific tag or commit hash, you can generate it locally as follows (example using tag v3.10.2):

git clone https://github.com/nlohmann/json.git
cd json
git checkout v3.10.2
make install_venv serve -C docs/mkdocs

Open http://127.0.0.1:8000/ in your browser. Replace any URL in the source code that points to https://json.nlohmann.me with http://127.0.0.1:8000 to view the documentation for the selected tag or commit hash.