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.

References

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

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.

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

NaN/infinity handling

If NaN or Infinity are stored inside a JSON number, they are serialized properly in contrast to the dump function which serializes NaN or Infinity to null.

Example

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // create a JSON value
    json j = R"({"compact": true, "schema": 0})"_json;

    // serialize it to MessagePack
    std::vector<std::uint8_t> v = json::to_msgpack(j);

    // print the vector content
    for (auto& byte : v)
    {
        std::cout << "0x" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << " ";
    }
    std::cout << std::endl;
}

Output:

0x82 0xa7 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xc3 0xa6 0x73 0x63 0x68 0x65 0x6d 0x61 0x00

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

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create byte vector
    std::vector<std::uint8_t> v = {0x82, 0xa7, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,
                                   0x74, 0xc3, 0xa6, 0x73, 0x63, 0x68, 0x65, 0x6d,
                                   0x61, 0x00
                                  };

    // deserialize it with MessagePack
    json j = json::from_msgpack(v);

    // print the deserialized JSON value
    std::cout << std::setw(2) << j << std::endl;
}

Output:

{
  "compact": true,
  "schema": 0
}

Last update: August 5, 2022