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Niels Lohmann
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# CBOR
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The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely
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small code size, fairly small message size, and extensibility without the need for version negotiation.
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!!! abstract "References"
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- [CBOR Website](http://cbor.io) - the main source on CBOR
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- [CBOR Playground](http://cbor.me) - an interactive webpage to translate between JSON and CBOR
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- [RFC 7049](https://tools.ietf.org/html/rfc7049) - the CBOR specification
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## Serialization
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The library uses the following mapping from JSON values types to CBOR types according to the CBOR specification (RFC 7049):
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| JSON value type | value/range | CBOR type | first byte |
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|-----------------|--------------------------------------------|-----------------------------------|------------|
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| null | `null` | Null | 0xF6 |
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| boolean | `true` | True | 0xF5 |
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| boolean | `false` | False | 0xF4 |
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| number_integer | -9223372036854775808..-2147483649 | Negative integer (8 bytes follow) | 0x3B |
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| number_integer | -2147483648..-32769 | Negative integer (4 bytes follow) | 0x3A |
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| number_integer | -32768..-129 | Negative integer (2 bytes follow) | 0x39 |
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| number_integer | -128..-25 | Negative integer (1 byte follow) | 0x38 |
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| number_integer | -24..-1 | Negative integer | 0x20..0x37 |
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| number_integer | 0..23 | Integer | 0x00..0x17 |
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| number_integer | 24..255 | Unsigned integer (1 byte follow) | 0x18 |
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| number_integer | 256..65535 | Unsigned integer (2 bytes follow) | 0x19 |
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| number_integer | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A |
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| number_integer | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B |
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| number_unsigned | 0..23 | Integer | 0x00..0x17 |
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| number_unsigned | 24..255 | Unsigned integer (1 byte follow) | 0x18 |
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| number_unsigned | 256..65535 | Unsigned integer (2 bytes follow) | 0x19 |
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| number_unsigned | 65536..4294967295 | Unsigned integer (4 bytes follow) | 0x1A |
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| number_unsigned | 4294967296..18446744073709551615 | Unsigned integer (8 bytes follow) | 0x1B |
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| number_float | *any value representable by a float* | Single-Precision Float | 0xFA |
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| number_float | *any value NOT representable by a float* | Double-Precision Float | 0xFB |
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| string | *length*: 0..23 | UTF-8 string | 0x60..0x77 |
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| string | *length*: 23..255 | UTF-8 string (1 byte follow) | 0x78 |
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| string | *length*: 256..65535 | UTF-8 string (2 bytes follow) | 0x79 |
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| string | *length*: 65536..4294967295 | UTF-8 string (4 bytes follow) | 0x7A |
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| string | *length*: 4294967296..18446744073709551615 | UTF-8 string (8 bytes follow) | 0x7B |
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| array | *size*: 0..23 | array | 0x80..0x97 |
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| array | *size*: 23..255 | array (1 byte follow) | 0x98 |
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| array | *size*: 256..65535 | array (2 bytes follow) | 0x99 |
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| array | *size*: 65536..4294967295 | array (4 bytes follow) | 0x9A |
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| array | *size*: 4294967296..18446744073709551615 | array (8 bytes follow) | 0x9B |
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| object | *size*: 0..23 | map | 0xA0..0xB7 |
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| object | *size*: 23..255 | map (1 byte follow) | 0xB8 |
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| object | *size*: 256..65535 | map (2 bytes follow) | 0xB9 |
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| object | *size*: 65536..4294967295 | map (4 bytes follow) | 0xBA |
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| object | *size*: 4294967296..18446744073709551615 | map (8 bytes follow) | 0xBB |
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| binary | *size*: 0..23 | byte string | 0x40..0x57 |
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| binary | *size*: 23..255 | byte string (1 byte follow) | 0x58 |
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| binary | *size*: 256..65535 | byte string (2 bytes follow) | 0x59 |
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| binary | *size*: 65536..4294967295 | byte string (4 bytes follow) | 0x5A |
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| binary | *size*: 4294967296..18446744073709551615 | byte string (8 bytes follow) | 0x5B |
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Binary values with subtype are mapped to tagged values (0xD8..0xDB) depending on the subtype, followed by a byte string,
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see "binary" cells in the table above.
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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 CBOR value.
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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. This behavior differs from the normal JSON serialization which serializes NaN or Infinity to `null`.
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!!! info "Unused CBOR types"
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The following CBOR types are not used in the conversion:
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- UTF-8 strings terminated by "break" (0x7F)
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- arrays terminated by "break" (0x9F)
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- maps terminated by "break" (0xBF)
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- byte strings terminated by "break" (0x5F)
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- date/time (0xC0..0xC1)
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- bignum (0xC2..0xC3)
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- decimal fraction (0xC4)
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- bigfloat (0xC5)
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- expected conversions (0xD5..0xD7)
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- simple values (0xE0..0xF3, 0xF8)
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- undefined (0xF7)
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- half-precision floats (0xF9)
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- break (0xFF)
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!!! info "Tagged items"
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Binary subtypes will be serialized as tagged items. See [binary values](../binary_values.md#cbor) for an example.
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??? example
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```cpp
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--8<-- "examples/to_cbor.cpp"
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```
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Output:
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```c
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--8<-- "examples/to_cbor.output"
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```
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## Deserialization
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The library maps CBOR types to JSON value types as follows:
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| CBOR type | JSON value type | first byte |
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|------------------------|-----------------|------------|
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| Integer | number_unsigned | 0x00..0x17 |
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| Unsigned integer | number_unsigned | 0x18 |
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| Unsigned integer | number_unsigned | 0x19 |
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| Unsigned integer | number_unsigned | 0x1A |
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| Unsigned integer | number_unsigned | 0x1B |
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| Negative integer | number_integer | 0x20..0x37 |
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| Negative integer | number_integer | 0x38 |
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| Negative integer | number_integer | 0x39 |
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| Negative integer | number_integer | 0x3A |
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| Negative integer | number_integer | 0x3B |
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| Byte string | binary | 0x40..0x57 |
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| Byte string | binary | 0x58 |
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| Byte string | binary | 0x59 |
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| Byte string | binary | 0x5A |
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| Byte string | binary | 0x5B |
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| UTF-8 string | string | 0x60..0x77 |
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| UTF-8 string | string | 0x78 |
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| UTF-8 string | string | 0x79 |
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| UTF-8 string | string | 0x7A |
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| UTF-8 string | string | 0x7B |
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| UTF-8 string | string | 0x7F |
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| array | array | 0x80..0x97 |
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| array | array | 0x98 |
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| array | array | 0x99 |
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| array | array | 0x9A |
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| array | array | 0x9B |
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| array | array | 0x9F |
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| map | object | 0xA0..0xB7 |
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| map | object | 0xB8 |
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| map | object | 0xB9 |
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| map | object | 0xBA |
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| map | object | 0xBB |
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| map | object | 0xBF |
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| False | `false` | 0xF4 |
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| True | `true` | 0xF5 |
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| Null | `null` | 0xF6 |
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| Half-Precision Float | number_float | 0xF9 |
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| Single-Precision Float | number_float | 0xFA |
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| Double-Precision Float | number_float | 0xFB |
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!!! warning "Incomplete mapping"
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The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:
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- date/time (0xC0..0xC1)
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- bignum (0xC2..0xC3)
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- decimal fraction (0xC4)
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- bigfloat (0xC5)
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- expected conversions (0xD5..0xD7)
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- simple values (0xE0..0xF3, 0xF8)
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- undefined (0xF7)
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!!! warning "Object keys"
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CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected.
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!!! warning "Tagged items"
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Tagged items will throw a parse error by default. They can be ignored by passing `cbor_tag_handler_t::ignore` to function `from_cbor`. They can be stored by passing `cbor_tag_handler_t::store` to function `from_cbor`.
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??? example
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```cpp
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--8<-- "examples/from_cbor.cpp"
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```
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Output:
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```json
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--8<-- "examples/from_cbor.output"
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```
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