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