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nlohmann
137 lines
5.3 KiB
Markdown
137 lines
5.3 KiB
Markdown
# BSON
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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.
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References
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- [BSON Website](http://bsonspec.org) - the main source on BSON
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- [BSON Specification](http://bsonspec.org/spec.html) - the specification
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## Serialization
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The library uses the following mapping from JSON values types to BSON types:
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| JSON value type | value/range | BSON type | marker |
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| --------------- | ----------------------------------------- | --------- | ------ |
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| null | `null` | null | 0x0A |
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| boolean | `true`, `false` | boolean | 0x08 |
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| number_integer | -9223372036854775808..-2147483649 | int64 | 0x12 |
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| number_integer | -2147483648..2147483647 | int32 | 0x10 |
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| number_integer | 2147483648..9223372036854775807 | int64 | 0x12 |
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| number_unsigned | 0..2147483647 | int32 | 0x10 |
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| number_unsigned | 2147483648..9223372036854775807 | int64 | 0x12 |
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| number_unsigned | 9223372036854775808..18446744073709551615 | uint64 | 0x11 |
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| number_float | *any value* | double | 0x01 |
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| string | *any value* | string | 0x02 |
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| array | *any value* | document | 0x04 |
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| object | *any value* | document | 0x03 |
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| binary | *any value* | binary | 0x05 |
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Incomplete mapping
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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.
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Example
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```
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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using namespace nlohmann::literals;
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int main()
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{
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// create a JSON value
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json j = R"({"compact": true, "schema": 0})"_json;
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// serialize it to BSON
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std::vector<std::uint8_t> v = json::to_bson(j);
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// print the vector content
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for (auto& byte : v)
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{
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std::cout << "0x" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << " ";
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}
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std::cout << std::endl;
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}
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```
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Output:
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```
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0x1b 0x00 0x00 0x00 0x08 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0x00 0x01 0x10 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 0x00 0x00 0x00 0x00 0x00
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```
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## Deserialization
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The library maps BSON record types to JSON value types as follows:
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| BSON type | BSON marker byte | JSON value type |
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| ------------------------ | ---------------- | --------------- |
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| double | 0x01 | number_float |
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| string | 0x02 | string |
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| document | 0x03 | object |
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| array | 0x04 | array |
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| binary | 0x05 | binary |
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| undefined | 0x06 | *unsupported* |
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| ObjectId | 0x07 | *unsupported* |
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| boolean | 0x08 | boolean |
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| UTC Date-Time | 0x09 | *unsupported* |
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| null | 0x0A | null |
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| Regular Expr. | 0x0B | *unsupported* |
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| DB Pointer | 0x0C | *unsupported* |
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| JavaScript Code | 0x0D | *unsupported* |
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| Symbol | 0x0E | *unsupported* |
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| JavaScript Code w/ scope | 0x0F | *unsupported* |
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| int32 | 0x10 | number_integer |
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| uint64(Timestamp) | 0x11 | number_unsigned |
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| int64 | 0x12 | number_integer |
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| 128-bit decimal float | 0x13 | *unsupported* |
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| Max Key | 0x7F | *unsupported* |
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| Min Key | 0xFF | *unsupported* |
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Incomplete mapping
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The mapping is **incomplete**. The unsupported mappings are indicated in the table above.
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Handling of BSON type 0x11
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BSON type 0x11 is used to represent uint64 numbers. This library treats these values purely as uint64 numbers and does not parse them into date-related formats.
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Example
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```
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create byte vector
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std::vector<std::uint8_t> v = {0x1b, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6f, 0x6d,
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0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73,
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0x63, 0x68, 0x65, 0x6d, 0x61, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00
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};
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// deserialize it with BSON
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json j = json::from_bson(v);
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// print the deserialized JSON value
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std::cout << std::setw(2) << j << std::endl;
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}
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```
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Output:
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```
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{
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"compact": true,
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"schema": 0
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}
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```
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