features/binary_formats/bson.md

# BSON

BSON, short for Binary JSON, is a binary-encoded serialization of JSON-like documents. Like JSON, BSON supports the
embedding of documents and arrays within other documents and arrays. BSON also contains extensions that allow
representation of data types that are not part of the JSON spec. For example, BSON has a Date type and a BinData type.

!!! abstract "References"

    - [BSON Website](http://bsonspec.org) - the main source on BSON
    - [BSON Specification](http://bsonspec.org/spec.html) - the specification
   

## Serialization

The library uses the following mapping from JSON values types to BSON types:

| JSON value type | value/range                               | BSON type | marker |
|-----------------|-------------------------------------------|-----------|--------|
| null            | `null`                                    | null      | 0x0A   |
| boolean         | `true`, `false`                           | boolean   | 0x08   |
| number_integer  | -9223372036854775808..-2147483649         | int64     | 0x12   |
| number_integer  | -2147483648..2147483647                   | int32     | 0x10   |
| number_integer  | 2147483648..9223372036854775807           | int64     | 0x12   |
| number_unsigned | 0..2147483647                             | int32     | 0x10   |
| number_unsigned | 2147483648..9223372036854775807           | int64     | 0x12   |
| number_unsigned | 9223372036854775808..18446744073709551615 | uint64    | 0x11   |
| number_float    | *any value*                               | double    | 0x01   |
| string          | *any value*                               | string    | 0x02   |
| array           | *any value*                               | document  | 0x04   |
| object          | *any value*                               | document  | 0x03   |
| binary          | *any value*                               | binary    | 0x05   |

!!! warning "Incomplete mapping"

    The mapping is **incomplete**, since only JSON-objects (and things contained therein) can be serialized to BSON.
    Also, keys may not contain U+0000, since they are serialized a zero-terminated c-strings.

!!! warning "BSON type 0x11 interoperability"

    The BSON specification defines type `0x11` as a Timestamp. This library uses marker `0x11` when serializing
    `number_unsigned` values in the range `9223372036854775808..18446744073709551615`. Other BSON implementations may
    therefore interpret these values as Timestamps instead of unsigned integers.

!!! info "Binary values without a subtype"

    BSON requires every binary value to have a subtype. If a binary value has no subtype, this library serializes it
    with the generic subtype `0x00`. After deserialization, `has_subtype()` returns `true` and `subtype()` returns `0`.
    As a result, serializing and deserializing a JSON object containing such a value produces a different JSON object,
    even though the binary data is unchanged.

??? example

    ```cpp
    --8<-- "examples/to_bson.cpp"
    ```
    
    Output:

    ```c
    --8<-- "examples/to_bson.output"
    ```


## Deserialization

The library maps BSON record types to JSON value types as follows:

| BSON type                | BSON marker byte | JSON value type |
|--------------------------|------------------|-----------------|
| double                   | 0x01             | number_float    |
| string                   | 0x02             | string          |
| document                 | 0x03             | object          |
| array                    | 0x04             | array           |
| binary                   | 0x05             | binary          |
| undefined                | 0x06             | *unsupported*   |
| ObjectId                 | 0x07             | *unsupported*   |
| boolean                  | 0x08             | boolean         |
| UTC Date-Time            | 0x09             | *unsupported*   |
| null                     | 0x0A             | null            |
| Regular Expr.            | 0x0B             | *unsupported*   |
| DB Pointer               | 0x0C             | *unsupported*   |
| JavaScript Code          | 0x0D             | *unsupported*   |
| Symbol                   | 0x0E             | *unsupported*   |
| JavaScript Code w/ scope | 0x0F             | *unsupported*   |
| int32                    | 0x10             | number_integer  |
| uint64(Timestamp)        | 0x11             | number_unsigned |
| int64                    | 0x12             | number_integer  |
| 128-bit decimal float    | 0x13             | *unsupported*   |
| Max Key                  | 0x7F             | *unsupported*   |
| Min Key                  | 0xFF             | *unsupported*   |

!!! warning "Incomplete mapping"

    The mapping is **incomplete**. The unsupported mappings are indicated in the table above.

!!! note "Handling of BSON type 0x11"

    This library deserializes BSON type `0x11` (Timestamp) as a `number_unsigned` value. The 64-bit value is preserved,
    but the Timestamp type information is not.

!!! warning "Lenient BSON input handling"

    The BSON reader is lenient in a few areas where the BSON specification is more restrictive:

    - array element keys are not checked against the required decimal sequence (`0`, `1`, `2`, ...),
    - any non-zero byte is accepted as `true` for the boolean type, and
    - the payload for binary subtype `0x02` is returned as-is, including its inner length prefix.

    If BSON input must be validated for strict specification compliance, validate it separately before passing it to
    `from_bson()`.

??? example

    ```cpp
    --8<-- "examples/from_bson.cpp"
    ```

    Output:

    ```json
    --8<-- "examples/from_bson.output"
    ```