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

nlohmann::basic_json::to_msgpack

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

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

Serializes a given JSON value j to a byte vector using the MessagePack serialization format. MessagePack is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

1. Returns a byte vector containing the MessagePack serialization.
2. Writes the MessagePack 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_msgpack did before this parameter was added and as the MessagePack specification allows; strict throws; replace/ignore sanitize it the same way dump would. Unlike the other binary writers, the default stays keep even if JSON_STRICT_BINARY_UTF8 is enabled.

Return value

1. MessagePack 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 out_of_range.407 if j contains an integer outside [-2^63, 2^64-1], which is only possible with a number type wider than 64 bits; example: "integer number 18446744073709551616 cannot be represented by MessagePack as it does not fit [-2^63, 2^64-1]"
• Throws out_of_range.412 if the length of a string, binary value, array, or object exceeds 4294967295, the maximum MessagePack can store; example: "MessagePack length 4294967296 exceeds maximum of 4294967295"
• Throws out_of_range.415 if the subtype of a binary value exceeds 255, the largest value the library can store in the one-byte MessagePack ext type; example: "subtype 70000 is too large for the MessagePack ext type (max 255)"
• Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict
• Throws type_error.321 if j or a value nested in it is discarded; example: "cannot serialize discarded value to MessagePack"

Complexity

Linear in the size of the JSON value j.

Examples

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

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

Output:

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

??? example "Example: out_of_range.415 exception"

The example shows how serializing a binary value whose subtype exceeds 255 throws an exception, because the
MessagePack ext type stores the subtype in a single byte.

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

Output:

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

See also

• MessagePack - 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_msgpack - create a JSON value from an input in MessagePack format
• to_cbor - create a CBOR serialization of a JSON value
• to_bson - create a BSON 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 2.0.9.
• Throws out_of_range.412 and out_of_range.415 since version 3.13.0.
• 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.
• Fixed in version 3.13.0 to serialize number_integer_t/number_unsigned_t pairs of different width correctly; before, integers could be serialized with the wrong value if number_integer_t was narrower than number_unsigned_t.
• 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 MessagePack.
• 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.