json/docs/mkdocs/docs/api/basic_json/to_ubjson.md

nlohmann::basic_json::to_ubjson

// (1)
static std::vector<std::uint8_t> to_ubjson(const basic_json& j,
                                           const bool use_size = false,
                                           const bool use_type = false,
                                           const error_handler_t error_handler = error_handler_t::keep);

// (2)
static void to_ubjson(const basic_json& j, detail::output_adapter<std::uint8_t> o,
                      const bool use_size = false, const bool use_type = false,
                      const error_handler_t error_handler = error_handler_t::keep);
static void to_ubjson(const basic_json& j, detail::output_adapter<char> o,
                      const bool use_size = false, const bool use_type = false,
                      const error_handler_t error_handler = error_handler_t::keep);

Serializes a given JSON value j to a byte vector using the UBJSON (Universal Binary JSON) serialization format. UBJSON aims to be more compact than JSON itself, yet more efficient to parse.

1. Returns a byte vector containing the UBJSON serialization.
2. Writes the UBJSON serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)

JSON value to serialize

o (in)

output adapter to write serialization to

use_size (in)

whether to add size annotations to container types; optional, #!cpp false by default.

use_type (in)

whether to add type annotations to container types (must be combined with #!cpp use_size = true); optional, #!cpp false by default.

error_handler (in)

how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_ubjson did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead.

Return value

1. UBJSON serialization as a byte vector
2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

• Throws other_error.502 if use_type is true and use_size is false, and j contains a non-empty array, object, or binary value.
• Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
• Throws type_error.321 if j or a value nested in it is discarded; example: "cannot serialize discarded value to UBJSON"

Complexity

Linear in the size of the JSON value j.

Examples

??? example "Example: serialize a JSON value to UBJSON"

The example shows the serialization of a JSON value to a byte vector in UBJSON format.
 
```cpp
--8<-- "examples/to_ubjson.cpp"
```

Output:

```json
--8<-- "examples/to_ubjson.output"
```

??? example "Example: other_error.502 exception"

The example shows how requesting type annotations (`use_type`) without size annotations (`use_size`) throws an
exception, because type-optimized containers can only be read back with a preceding size.

```cpp
--8<-- "examples/to_ubjson__exception.cpp"
```

Output:

```json
--8<-- "examples/to_ubjson__exception.output"
```

See also

• UBJSON - the format, its byte layout and how values are mapped
• Binary formats - overview and comparison of the supported binary formats
• Binary values - how binary values and subtypes are handled
• from_ubjson - create a JSON value from an input in UBJSON format
• JSON_UBJSON_VALUELESS_ELEMENTS_PER_BYTE - set how many elements of arrays of null, true, or false a reader accepts per byte of input
• to_cbor - create a CBOR serialization of a JSON value
• to_msgpack - create a MessagePack serialization of a JSON value
• to_bson - create a BSON serialization of a JSON value
• to_bjdata - create a BJData serialization of a JSON value
• to_bon8 - create a BON8 serialization of a JSON value

Version history

• Added in version 3.1.0.
• Added error_handler parameter in version 3.13.0. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316.
• Writes arrays of Z, T, or F without the $ type annotation ([#n Z Z ...) once their elements in one call add up to more than 1,048,576, in version 3.13.0, so that from_ubjson can read the output back. Such output is larger: 270,000 rows of 100 null values take 26.8 MB instead of 1.35 MB. The limit does not depend on JSON_UBJSON_VALUELESS_ELEMENTS_PER_BYTE: a reader accepts at least 1,048,576 elements whatever it is set to, so the writer follows that base only.
• Throws type_error.321 for a discarded value since version 3.13.0; previously, a discarded value nested in an array or object was silently skipped, producing invalid UBJSON.