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dump() writes doubles with the conversion of Zmij by Victor Zverovich (https://github.com/vitaut/zmij, MIT), ported to C++11 in detail/conversions/zmij.hpp: the shortest decimal in the rounding interval, the closest one if there are several. Grisu2 does not always find the shortest digits; about 0.14% of random doubles are now written differently (0.08% with fewer digits, 0.06% with the closest last digit); short decimals such as 0.1 or 2555.56 are not affected. float keeps Grisu2. The layout of doubles is unchanged, but written differently: the digits are converted eight at a time (the BCD conversion of Xiang JunBo, as in Zmij) and stored with one byte swap per eight digits; leading and trailing zeros are counted from those bytes; and the layouts of format_buffer() are written with fixed-size moves instead of per-digit loops and moves of the buffer (to_chars() uses a local buffer if the caller's is shorter than the 41 bytes this may write). The powers of ten come from the table for number parsing, adjusted where it holds them rounded up, and from the compressed tables of Zmij beyond 10^308. json::dump() gets faster on floats: canada -53%, numbers -46%, mesh -37%, marine_ik -30%. Tests: the powers of ten recomputed with a small big-integer; for random doubles, all powers of two and of ten and their neighbors, and boundary values: the output reads back as the same value, no decimal with one digit fewer does, the layout equals that of format_buffer() for the same digits, and (C++17) the digits equal those of std::to_chars. The size ratios of canada.json in unit-binary_formats.cpp and one expectation in unit-to_chars.cpp change with the shorter output. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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License
The class is licensed under the MIT License:
Copyright © 2013-2026 Niels Lohmann
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed under the MIT License (see above). Copyright © 2008-2009 Björn Hoehrmann bjoern@hoehrmann.de
The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the MIT License (see above). Copyright © 2009 Florian Loitsch
The class contains a port of the shortest double-to-decimal conversion of Żmij by Victor Zverovich, which is licensed under the MIT License (see above). Copyright © 2025 Victor Zverovich
The class contains a copy of Hedley from Evan Nemerson which is licensed as CC0-1.0.
The class contains an adapted version of the Eisel-Lemire algorithm and its table of powers of five from fast_float by Daniel Lemire and contributors, which is available under the MIT License (used here), the Apache 2.0 License, and the Boost Software License. Copyright © 2021 The fast_float authors
The view's parser (<nlohmann/json_view.hpp>) contains techniques and code adapted from yyjson by YaoYuan, which is licensed under the MIT License (see above): table-driven decoding of \u escapes and fixed-offset unrolled checks.
The view's parser (<nlohmann/json_view.hpp>) validates non-ASCII strings with the vector UTF-8 check of simdjson by Daniel Lemire, Geoff Langdale, John Keiser, and contributors (its "lookup4" algorithm and tables, after J. Keiser and D. Lemire, "Validating UTF-8 In Less Than One Instruction Per Byte", 2021), which is available under the MIT License (used here) and the Apache 2.0 License. Copyright © 2018-2025 The simdjson authors