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

nlohmann::basic_json::to_bson

// (1)
static std::vector<std::uint8_t> to_bson(const basic_json& j,
                                         const error_handler_t error_handler = error_handler_t::keep);

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

BSON (Binary JSON) is a binary format in which zero or more ordered key/value pairs are stored as a single entity (a so-called document). The members of an object are written in the order in which iteration visits them.

1. Returns a byte vector containing the BSON serialization.
2. Writes the BSON 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

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_bson 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. BSON 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 type_error.317 if the top-level type of the JSON value is not an object; example: "to serialize to BSON, top-level type must be object, but is string"
• Throws out_of_range.407 if j contains a signed integer outside the range of int64 or an unsigned integer outside the range of uint64, which is only possible with a number type wider than 64 bits; example: "integer number 9223372036854775808 cannot be represented by BSON as it does not fit int64"
• Throws out_of_range.409 if a key in the JSON object contains a null byte (code point U+0000); example: "BSON key cannot contain code point U+0000 (at byte 2)"
• Throws out_of_range.412 if the length of a document, array, string, or binary value exceeds the range of the 32-bit BSON length field; example: "BSON length 2147483661 exceeds maximum of 2147483647"
• Throws out_of_range.415 if the subtype of a binary value exceeds 255, the maximum of the BSON binary subtype; example: "subtype 70000 is too large for the BSON binary subtype (max 255)"
• Throws type_error.316 if a string or object key 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 a value nested in j is discarded (the top-level value itself is covered by type_error.317 above, since it must be an object); example: "cannot serialize discarded value to BSON"

Complexity

Linear in the size of the JSON value j. The length prefixes of all nested documents and arrays are computed in one pass before anything is written.

Examples

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

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

Output:

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

??? example "Example: out_of_range.409 exception"

The example shows how serializing a JSON object whose key contains a null byte (U+0000) throws an exception, because
BSON keys are null-terminated C strings and cannot contain U+0000 themselves.

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

Output:

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

See also

• BSON - 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_bson - create a JSON value from an input in BSON format
• to_cbor - create a CBOR serialization of a JSON value
• to_msgpack - create a MessagePack serialization of a JSON value
• to_ubjson - create a UBJSON 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 3.4.0.
• Throws out_of_range.412 and out_of_range.415 since version 3.13.0.
• Linear in the size of j, and no longer limited by the call stack for deeply nested values, since version 3.13.0.
• out_of_range.415 is now detected before anything is written, like the other exceptions above, since version 3.13.0.
• Throws type_error.321 for a discarded value nested in j since version 3.13.0; previously, it was silently skipped, producing a document whose declared size did not match what was actually written.
• 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 before anything is written.
• Throws out_of_range.407 for integers that do not fit 64 bits since version 3.13.0; previously, integers of a number type wider than 64 bits were silently truncated.