Skip to content


Universal Binary JSON (UBJSON) is a binary form directly imitating JSON, but requiring fewer bytes of data. It aims to achieve the generality of JSON, combined with being much easier to process than JSON.



The library uses the following mapping from JSON values types to UBJSON types according to the UBJSON specification:

JSON value type value/range UBJSON type marker
null null null Z
boolean true true T
boolean false false F
number_integer -9223372036854775808..-2147483649 int64 L
number_integer -2147483648..-32769 int32 l
number_integer -32768..-129 int16 I
number_integer -128..127 int8 i
number_integer 128..255 uint8 U
number_integer 256..32767 int16 I
number_integer 32768..2147483647 int32 l
number_integer 2147483648..9223372036854775807 int64 L
number_unsigned 0..127 int8 i
number_unsigned 128..255 uint8 U
number_unsigned 256..32767 int16 I
number_unsigned 32768..2147483647 int32 l
number_unsigned 2147483648..9223372036854775807 int64 L
number_unsigned 2147483649..18446744073709551615 high-precision H
number_float any value float64 D
string with shortest length indicator string S
array see notes on optimized format array [
object see notes on optimized format map {

Complete mapping

The mapping is complete in the sense that any JSON value type can be converted to a UBJSON value.

Any UBJSON output created by to_ubjson can be successfully parsed by from_ubjson.

Size constraints

The following values can not be converted to a UBJSON value:

  • strings with more than 9223372036854775807 bytes (theoretical)

Unused UBJSON markers

The following markers are not used in the conversion:

  • Z: no-op values are not created.
  • C: single-byte strings are serialized with S markers.

NaN/infinity handling

If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the dump() function which serializes NaN or Infinity to null.

Optimized formats

The optimized formats for containers are supported: Parameter use_size adds size information to the beginning of a container and removes the closing marker. Parameter use_type further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The use_type parameter must only be used together with use_size = true.

Note that use_size = true alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed on the number of elements of the container.

Binary values

If the JSON data contains the binary type, the value stored is a list of integers, as suggested by the UBJSON documentation. In particular, this means that serialization and the deserialization of a JSON containing binary values into UBJSON and back will result in a different JSON object.

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

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

// function to print UBJSON's diagnostic format
void print_byte(uint8_t byte)
    if (32 < byte and byte < 128)
        std::cout << (char)byte;
        std::cout << (int)byte;

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

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

    // print the vector content
    for (auto& byte : v)
    std::cout << std::endl;

    // create an array of numbers
    json array = {1, 2, 3, 4, 5, 6, 7, 8};

    // serialize it to UBJSON using default representation
    std::vector<std::uint8_t> v_array = json::to_ubjson(array);
    // serialize it to UBJSON using size optimization
    std::vector<std::uint8_t> v_array_size = json::to_ubjson(array, true);
    // serialize it to UBJSON using type optimization
    std::vector<std::uint8_t> v_array_size_and_type = json::to_ubjson(array, true, true);

    // print the vector contents
    for (auto& byte : v_array)
    std::cout << std::endl;

    for (auto& byte : v_array_size)
    std::cout << std::endl;

    for (auto& byte : v_array_size_and_type)
    std::cout << std::endl;




The library maps UBJSON types to JSON value types as follows:

UBJSON type JSON value type marker
no-op no value, next value is read N
null null Z
false false F
true true T
float32 number_float d
float64 number_float D
uint8 number_unsigned U
int8 number_integer i
int16 number_integer I
int32 number_integer l
int64 number_integer L
string string S
char string C
array array (optimized values are supported) [
object object (optimized values are supported) {

Complete mapping

The mapping is complete in the sense that any UBJSON value can be converted to a JSON value.

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

using json = nlohmann::json;

int main()
    // create byte vector
    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,
                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,
                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D

    // deserialize it with UBJSON
    json j = json::from_ubjson(v);

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


  "compact": true,
  "schema": 0

Last update: August 5, 2022