v3.12.0¶

Released on 2025-04-11.

Fixes bugs found in 3.11.3 and adds several features. No public API was removed or renamed.

• Adds diagnostic byte positions via JSON_DIAGNOSTIC_POSITIONS, exposed through the new start_pos and end_pos member functions.
• Makes the conversion macros templated (so they also work with ordered_json) and adds NLOHMANN_DEFINE_DERIVED_TYPE for derived classes.
• Adds std::optional support (C++17) and lets patch, diff, and flatten work with arbitrary string types.
• Extends the binary formats: BJData draft 3 and unsigned 64-bit integers for BSON.
• Adds multidimensional C-array conversion and UTF-8 encoded std::filesystem::path conversions, and raises the minimum CMake version to 3.5 and supports CMake 4.0.

Full release notes.

API changes¶

Added (4 pages):

• basic_json: end_pos, start_pos
• macros: JSON_DIAGNOSTIC_POSITIONS, nlohmann_define_derived_type

Changed:

• basic_json::accept: Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
• basic_json::parse: Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
• basic_json::to_bjdata: BJData version parameter (for draft3 binary encoding) added in version 3.12.0.
• JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, JSON_HAS_CPP_26: Added JSON_HAS_CPP_23 in version 3.12.0.
• NLOHMANN_JSON_NAMESPACE: Added the ABI tag _dp in version 3.12.0, and the ABI tags _bics, _psp, _snul, _sbu8, _ekmo, _dsbs, _dufio, _ns, _vns, and _vssse3 in version 3.13.0 unreleased; see nlohmann Namespace.