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Niels Lohmann c0ad2ba72f docs: correct and complete the BJData documentation

Verify the BJData chapter against the library and the Draft 2/3/4
specifications and fix what did not match:

- bjdata.md: correct the list of unused markers (N, not Z), the ND-array
  example (3x2, not 2x3), the conditions under which an object is packed
  as an ND-array (exactly three keys, row vectors, float-only elements for
  single/double, char 0..127, byte only in Draft 3), the binary table row
  and notes (which flag/draft combinations write [$B), the "$" type
  restriction (Draft 3, Z is excluded too), the H marker in the
  deserialization table, and the claim of a complete deserialization
  mapping. Document the supported drafts, that Draft 4 (SoA containers,
  extension type E) and column-major ND-arrays are rejected, ND-arrays of
  half floats, duplicate object keys, and link the Draft 3 specifications
  instead of master.
- Add an API page for basic_json::bjdata_version_t with an example, and
  link it from the nav, the API index and to_bjdata.
- Update binary_formats/index.md and binary_values.md, which still claimed
  that BJData has no binary type, and fix a wrong byte comment there.
- Document parse_error.115 and the out_of_range.408 cases for from_bjdata,
  the precise other_error.502 condition for to_bjdata, and reword the note
  on truncated unsigned integers in exceptions.md (only ND-array elements
  were truncated before 3.13.0).

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

2026-10-26 15:52:54 +01:00

3.9 KiB

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Binary Formats

Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance, over a network. Hence, the library supports

BJData (Binary JData),
BON8 (Binary Object Notation 8),
BSON (Binary JSON),
CBOR (Concise Binary Object Representation),
MessagePack, and
UBJSON (Universal Binary JSON)

to efficiently encode JSON values to byte vectors and to decode such vectors.

Comparison

Completeness

Format	Serialization	Deserialization
BJData	complete	incomplete: no column-major ND-arrays, no Draft 4 SoA/extension types
BON8	incomplete: no unsigned integers above int64	complete
BSON	incomplete: top-level value must be an object	incomplete, but all JSON types are supported
CBOR	complete	incomplete, but all JSON types are supported
MessagePack	complete	incomplete, but all JSON types are supported
UBJSON	complete	complete

Binary values

Format	Binary values	Binary subtypes
BJData	supported (Draft 3 only)	not supported
BON8	not supported	not supported
BSON	supported	supported
CBOR	supported	supported
MessagePack	supported	supported
UBJSON	not supported	not supported

See binary values for more information.

Sizes

Format	canada.json	twitter.json	citm_catalog.json	jeopardy.json
BJData	53.2 %	91.1 %	78.1 %	96.6 %
BJData (size)	58.6 %	92.1 %	86.7 %	97.4 %
BJData (size+type)	58.6 %	92.1 %	86.5 %	97.4 %
BON8	50.5 %	83.8 %	63.5 %	87.5 %
BSON	85.8 %	95.2 %	95.8 %	106.7 %
CBOR	50.5 %	86.3 %	68.4 %	88.0 %
MessagePack	50.5 %	86.0 %	68.5 %	87.9 %
UBJSON	53.2 %	91.3 %	78.2 %	96.6 %
UBJSON (size)	58.6 %	92.3 %	86.8 %	97.4 %
UBJSON (size+type)	55.9 %	92.3 %	85.0 %	95.0 %

Sizes compared to minified JSON value; the smallest size per file is printed in bold.

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