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Niels Lohmann
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doc/mkdocs/docs/features/binary_formats/messagepack.md
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# MessagePack
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MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON. But it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one extra byte in addition to the strings themselves.
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!!! abstract "References"
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- [MessagePack website](https://msgpack.org)
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- [MessagePack specification](https://github.com/msgpack/msgpack/blob/master/spec.md)
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## Serialization
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The library uses the following mapping from JSON values types to MessagePack types according to the MessagePack specification:
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JSON value type | value/range | MessagePack type | first byte
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--------------- | --------------------------------- | ---------------- | ----------
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null | `null` | nil | 0xC0
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boolean | `true` | true | 0xC3
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boolean | `false` | false | 0xC2
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number_integer | -9223372036854775808..-2147483649 | int64 | 0xD3
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number_integer | -2147483648..-32769 | int32 | 0xD2
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number_integer | -32768..-129 | int16 | 0xD1
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number_integer | -128..-33 | int8 | 0xD0
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number_integer | -32..-1 | negative fixint | 0xE0..0xFF
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number_integer | 0..127 | positive fixint | 0x00..0x7F
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number_integer | 128..255 | uint 8 | 0xCC
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number_integer | 256..65535 | uint 16 | 0xCD
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number_integer | 65536..4294967295 | uint 32 | 0xCE
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number_integer | 4294967296..18446744073709551615 | uint 64 | 0xCF
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number_unsigned | 0..127 | positive fixint | 0x00..0x7F
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number_unsigned | 128..255 | uint 8 | 0xCC
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number_unsigned | 256..65535 | uint 16 | 0xCD
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number_unsigned | 65536..4294967295 | uint 32 | 0xCE
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number_unsigned | 4294967296..18446744073709551615 | uint 64 | 0xCF
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number_float | *any value* | float 64 | 0xCB
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string | *length*: 0..31 | fixstr | 0xA0..0xBF
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string | *length*: 32..255 | str 8 | 0xD9
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string | *length*: 256..65535 | str 16 | 0xDA
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string | *length*: 65536..4294967295 | str 32 | 0xDB
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array | *size*: 0..15 | fixarray | 0x90..0x9F
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array | *size*: 16..65535 | array 16 | 0xDC
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array | *size*: 65536..4294967295 | array 32 | 0xDD
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object | *size*: 0..15 | fix map | 0x80..0x8F
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object | *size*: 16..65535 | map 16 | 0xDE
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object | *size*: 65536..4294967295 | map 32 | 0xDF
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binary | *size*: 0..255 | bin 8 | 0xC4
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binary | *size*: 256..65535 | bin 16 | 0xC5
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binary | *size*: 65536..4294967295 | bin 32 | 0xC6
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!!! success "Complete mapping"
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The mapping is **complete** in the sense that any JSON value type can be converted to a MessagePack value.
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Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
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!!! warning "Size constraints"
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The following values can **not** be converted to a MessagePack value:
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- strings with more than 4294967295 bytes
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- byte strings with more than 4294967295 bytes
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- arrays with more than 4294967295 elements
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- objects with more than 4294967295 elements
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!!! info "Unused MessagePack types"
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The following MessagePack types are not used in the conversion: float 32 (0xCA)
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!!! info "NaN/infinity handling"
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If NaN or Infinity are stored inside a JSON number, they are serialized properly. function which serializes NaN or Infinity to `null`.
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!!! example
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```cpp
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--8<-- "examples/to_msgpack.cpp"
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```
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Output:
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```c
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--8<-- "examples/to_msgpack.output"
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```
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## Deserialization
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The library maps MessagePack types to JSON value types as follows:
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MessagePack type | JSON value type | first byte
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---------------- | --------------- | ----------
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positive fixint | number_unsigned | 0x00..0x7F
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fixmap | object | 0x80..0x8F
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fixarray | array | 0x90..0x9F
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fixstr | string | 0xA0..0xBF
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nil | `null` | 0xC0
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false | `false` | 0xC2
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true | `true` | 0xC3
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float 32 | number_float | 0xCA
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float 64 | number_float | 0xCB
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uint 8 | number_unsigned | 0xCC
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uint 16 | number_unsigned | 0xCD
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uint 32 | number_unsigned | 0xCE
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uint 64 | number_unsigned | 0xCF
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int 8 | number_integer | 0xD0
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int 16 | number_integer | 0xD1
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int 32 | number_integer | 0xD2
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int 64 | number_integer | 0xD3
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str 8 | string | 0xD9
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str 16 | string | 0xDA
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str 32 | string | 0xDB
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array 16 | array | 0xDC
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array 32 | array | 0xDD
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map 16 | object | 0xDE
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map 32 | object | 0xDF
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bin 8 | binary | 0xC4
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bin 16 | binary | 0xC5
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bin 32 | binary | 0xC6
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ext 8 | binary | 0xC7
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ext 16 | binary | 0xC8
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ext 32 | binary | 0xC9
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fixext 1 | binary | 0xD4
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fixext 2 | binary | 0xD5
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fixext 4 | binary | 0xD6
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fixext 8 | binary | 0xD7
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fixext 16 | binary | 0xD8
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negative fixint | number_integer | 0xE0-0xFF
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!!! info
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Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
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!!! example
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```cpp
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--8<-- "examples/from_msgpack.cpp"
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```
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Output:
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```json
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--8<-- "examples/from_msgpack.output"
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```
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