445 lines
15 KiB
Markdown
445 lines
15 KiB
Markdown
# Binary Values
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The library implements several [binary formats](binary_formats/index.md) that encode JSON in an efficient way. Most of
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these formats support binary values; that is, values that have semantics defined outside the library and only define a
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sequence of bytes to be stored.
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JSON itself does not have a binary value. As such, binary values are an extension that this library implements to store
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values received by a binary format. Binary values are never created by the JSON parser and are only part of a
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serialized JSON text if they have been created manually or via a binary format.
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## API for binary values
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```mermaid
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classDiagram
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class binary_t ["json::binary_t"] {
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+void set_subtype(std::uint64_t subtype)
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+void clear_subtype()
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+std::uint64_t subtype() const
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+bool has_subtype() const
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}
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class vector ["std::vector<uint8_t>"]
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vector <|-- binary_t
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```
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<p class="nj-caption" markdown>Class diagram of `json::binary_t`: it derives from `std::vector<uint8_t>` and adds functions to set, clear, query, and test an optional subtype.</p>
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By default, binary values are stored as `std::vector<std::uint8_t>`. This type can be changed by providing a template
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parameter to the `basic_json` type. To store binary subtypes, the storage type is extended and exposed as
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[`json::binary_t`](../api/basic_json/binary_t.md):
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```cpp
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auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});
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auto binary_with_subtype = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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```
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There are several convenience functions to check and set the subtype:
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```cpp
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binary.has_subtype(); // returns false
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binary_with_subtype.has_subtype(); // returns true
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binary_with_subtype.clear_subtype();
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binary_with_subtype.has_subtype(); // returns false
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binary_with_subtype.set_subtype(42);
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binary.set_subtype(23);
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binary.subtype(); // returns 23
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```
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As `json::binary_t` is subclassing `std::vector<std::uint8_t>`, all member functions are available:
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```cpp
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binary.size(); // returns 4
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binary[1]; // returns 0xFE
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```
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JSON values can be constructed from `json::binary_t`:
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```cpp
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json j = binary;
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```
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Binary values are primitive values just like numbers or strings, as reflected by
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[`is_binary()`](../api/basic_json/is_binary.md) and [`is_primitive()`](../api/basic_json/is_primitive.md):
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```cpp
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j.is_binary(); // returns true
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j.is_primitive(); // returns true
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```
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Given a binary JSON value, the `binary_t` can be accessed by reference via
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[`get_binary()`](../api/basic_json/get_binary.md):
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```cpp
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j.get_binary().has_subtype(); // returns true
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j.get_binary().size(); // returns 4
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```
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For convenience, binary JSON values can be constructed via [`json::binary`](../api/basic_json/binary.md):
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```cpp
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auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);
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auto j3 = json::binary({0xCA, 0xFE, 0xBA, 0xBE});
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j2 == j; // returns true
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j3.get_binary().has_subtype(); // returns false
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j3.get_binary().subtype(); // returns std::uint64_t(-1) as j3 has no subtype
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```
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## Serialization
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Binary values are serialized differently according to the formats.
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### JSON
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JSON does not have a binary type, and this library does not introduce a new type as this would break conformance.
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Instead, binary values are serialized as an object with two keys: `bytes` holds an array of integers, and `subtype`
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is an integer or `null`.
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??? example "Example: serialize a binary value to JSON"
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Code:
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```cpp
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// create a binary value of subtype 42
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json j;
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j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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// serialize to standard output
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std::cout << j.dump(2) << std::endl;
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```
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Output:
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```json
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{
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"binary": {
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"bytes": [202, 254, 186, 190],
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"subtype": 42
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}
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}
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```
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!!! warning "No roundtrip for binary values"
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The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to
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remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.
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### BJData
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[BJData](binary_formats/bjdata.md) Draft 3 supports binary values as optimized arrays of `B` (byte) elements, but no
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subtypes. With `version = bjdata_version_t::draft3` and both `use_size` and `use_type` set, `to_bjdata` writes a binary
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value as `[$B#...`, and `from_bjdata` reads it back as a binary value (without subtype). In all other cases, including
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the default Draft 2 mode, the library follows the Draft 2 proposal and writes an array of integers (`U` in Draft 2, `B`
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in Draft 3), which is read back as an array.
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??? example "Example: serialize a binary value to BJData"
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Code:
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```cpp
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// create a binary value of subtype 42 (will be ignored in BJData)
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json j;
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j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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// convert to BJData
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auto v = json::to_bjdata(j);
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```
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`v` is a `std::vector<std::uint8_t>` with the following 20 elements:
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```c
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0x7B // '{'
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0x69 0x06 // i 6 (length of the key)
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0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
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0x5B // '['
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0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
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0x5D // ']'
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0x7D // '}'
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```
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The following code uses the type and size optimization for BJData:
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```cpp
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// convert to BJData using the size and type optimization
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auto v = json::to_bjdata(j, true, true);
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```
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The resulting vector has 22 elements; the optimization is not effective for examples with few values:
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```c
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0x7B // '{'
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0x23 0x69 0x01 // '#' 'i' 1 number of object members
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0x69 0x06 // i 6 (length of the key)
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0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
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0x5B // '[' array
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0x24 0x55 // '$' 'U' type of the array elements: unsigned integers
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0x23 0x69 0x04 // '#' i 4 number of array elements
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0xCA 0xFE 0xBA 0xBE // content
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```
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Note that subtype (42) is **not** serialized and that, in Draft 2 mode, BJData has **no binary type**, and deserializing `v` would
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yield the following value:
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```json
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{
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"binary": [202, 254, 186, 190]
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}
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```
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### BON8
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[BON8](binary_formats/bon8.md) neither supports binary values nor subtypes. The library serializes binary values as an
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array of integers.
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??? example "Example: serialize a binary value to BON8"
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Code:
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```cpp
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// create a binary value of subtype 42 (will be ignored in BON8)
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json j;
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j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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// convert to BON8
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auto v = json::to_bon8(j);
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```
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`v` is a `std::vector<std::uint8_t>` with the following 16 elements:
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```c
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0x87 // object with 1 member
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0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
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0x84 // array with 4 elements
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0xC3 0x22 0xC3 0x56 0xC3 0x12 0xC3 0x16 // content (each byte, here all above 39, as a 2-byte integer)
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```
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Note that the subtype is lost, and deserializing `v` would yield the following value:
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```json
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{
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"binary": [202, 254, 186, 190]
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}
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```
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### BSON
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[BSON](binary_formats/bson.md) supports binary values and subtypes. If a subtype is given, it is used and added as an
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unsigned 8-bit integer; a subtype above 255 throws
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[`out_of_range.415`](../home/exceptions.md#jsonexceptionout_of_range415). If no subtype is given, the generic binary
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subtype 0x00 is used. The payload of the "old binary" subtype 0x02 is preceded by a second length, which the library
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writes and removes again when reading; it is not part of the payload.
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??? example "Example: serialize a binary value to BSON"
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Code:
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```cpp
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// create a binary value of subtype 42
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json j;
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j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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// convert to BSON
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auto v = json::to_bson(j);
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```
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`v` is a `std::vector<std::uint8_t>` with the following 22 elements:
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```c
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0x16 0x00 0x00 0x00 // number of bytes in the document
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0x05 // binary value
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0x62 0x69 0x6E 0x61 0x72 0x79 0x00 // key "binary" + null byte
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0x04 0x00 0x00 0x00 // number of bytes
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0x2a // subtype
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0xCA 0xFE 0xBA 0xBE // content
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0x00 // end of the document
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```
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Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
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```json
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{
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"binary": {
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"bytes": [202, 254, 186, 190],
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"subtype": 42
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}
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}
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```
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### CBOR
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[CBOR](binary_formats/cbor.md) supports binary values and subtypes. Any binary value will be serialized as a byte
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string; a subtype is serialized as a tag preceding it. When parsing, tags are only read back as subtypes with
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`cbor_tag_handler_t::store` (see [`from_cbor`](../api/basic_json/from_cbor.md)). The library will choose the smallest
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representation using the length of the byte array.
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??? example "Example: serialize a binary value to CBOR"
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Code:
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```cpp
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// create a binary value of subtype 42
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json j;
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j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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// convert to CBOR
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auto v = json::to_cbor(j);
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```
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`v` is a `std::vector<std::uint8_t>` with the following 15 elements:
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```c
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0xA1 // map(1)
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0x66 // text(6)
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0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
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0xD8 0x2A // tag(42)
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0x44 // bytes(4)
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0xCA 0xFE 0xBA 0xBE // content
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```
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Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless
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`json::cbor_tag_handler_t::ignore` or `json::cbor_tag_handler_t::store` is passed to
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[`json::from_cbor`](../api/basic_json/from_cbor.md) (see
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[`cbor_tag_handler_t`](../api/basic_json/cbor_tag_handler_t.md)).
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With `ignore`, the subtype is dropped (`"subtype": null`); with `store`, the tag is kept (`"subtype": 42`):
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```json
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{
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"binary": {
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"bytes": [202, 254, 186, 190],
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"subtype": null
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}
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}
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```
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```json
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{
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"binary": {
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"bytes": [202, 254, 186, 190],
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"subtype": 42
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}
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}
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```
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### MessagePack
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[MessagePack](binary_formats/messagepack.md) supports binary values and subtypes. If a subtype is given, the ext family
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is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, fixext16, ext8,
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ext16, and ext32. The subtype is then written as the signed 8-bit ext type: subtypes 128 to 255 map to the negative
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(reserved) ext types, and on reading the ext type is exposed as an unsigned subtype from 0 to 255.
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If no subtype is given, the bin family (bin8, bin16, bin32) is used.
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??? example "Example: serialize a binary value to MessagePack"
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Code:
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```cpp
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// create a binary value of subtype 42
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json j;
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j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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// convert to MessagePack
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auto v = json::to_msgpack(j);
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```
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`v` is a `std::vector<std::uint8_t>` with the following 14 elements:
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```c
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0x81 // fixmap1
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0xA6 // fixstr6
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0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
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0xD6 // fixext4
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0x2A // subtype
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0xCA 0xFE 0xBA 0xBE // content
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```
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Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
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```json
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{
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"binary": {
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"bytes": [202, 254, 186, 190],
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"subtype": 42
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}
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}
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```
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### UBJSON
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[UBJSON](binary_formats/ubjson.md) neither supports binary values nor subtypes and proposes to serialize binary values
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as an array of uint8 values. The library implements this translation.
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??? example "Example: serialize a binary value to UBJSON"
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Code:
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```cpp
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// create a binary value of subtype 42 (will be ignored in UBJSON)
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json j;
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j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
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// convert to UBJSON
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auto v = json::to_ubjson(j);
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```
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`v` is a `std::vector<std::uint8_t>` with the following 20 elements:
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```c
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0x7B // '{'
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0x69 0x06 // i 6 (length of the key)
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0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
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0x5B // '['
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0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
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0x5D // ']'
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0x7D // '}'
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```
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The following code uses the type and size optimization for UBJSON:
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```cpp
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// convert to UBJSON using the size and type optimization
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auto v = json::to_ubjson(j, true, true);
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```
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The resulting vector has 23 elements; the optimization is not effective for examples with few values:
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```c
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0x7B // '{'
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0x24 // '$' type of the object elements
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0x5B // '[' array
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0x23 0x69 0x01 // '#' i 1 number of object elements
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0x69 0x06 // i 6 (length of the key)
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0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
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0x24 0x55 // '$' 'U' type of the array elements: unsigned integers
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0x23 0x69 0x04 // '#' i 4 number of array elements
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0xCA 0xFE 0xBA 0xBE // content
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```
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Note that subtype (42) is **not** serialized and that UBJSON has **no binary type**, and deserializing `v` would
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yield the following value:
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```json
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{
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"binary": [202, 254, 186, 190]
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}
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```
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## See also
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- [Binary formats](binary_formats/index.md) - the formats that can store binary values
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- [`binary_t`](../api/basic_json/binary_t.md) - type for binary values
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- [`byte_container_with_subtype`](../api/byte_container_with_subtype/index.md) - container for binary values with subtype
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- [`binary`](../api/basic_json/binary.md) - create a binary value
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- [`get_binary`](../api/basic_json/get_binary.md) - access the stored binary value
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- [`is_binary`](../api/basic_json/is_binary.md) - check whether a value is binary
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- [`cbor_tag_handler_t`](../api/basic_json/cbor_tag_handler_t.md) - store or ignore CBOR tags when parsing
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