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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.


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.

#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;


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


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


Any MessagePack output created by to_msgpack can be successfully parsed by from_msgpack.

#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;


  "compact": true,
  "schema": 0

Last update: March 8, 2023