9.3 KiB
Binary Values
The library implements several binary formats that encode JSON in an efficient way. Most of these formats support binary values; that is, values that have semantics define outside the library and only define a sequence of bytes to be stored.
JSON itself does not have a binary value. As such, binary values are an extension that this library implements to store values received by a binary format. Binary values are never created by the JSON parser, and are only part of a serialized JSON text if they have been created manually or via a binary format.
API for binary values
class json::binary_t {
-- setters --
+void set_subtype(std::uint8_t subtype)
+void clear_subtype()
-- getters --
+std::uint8_t subtype() const
+bool has_subtype() const
}
"std::vector<uint8_t>" <|-- json::binary_t
By default, binary values are stored as std::vector<std::uint8_t>. This type can be changed by providing a template parameter to the basic_json type. To store binary subtypes, the storage type is extended and exposed as json::binary_t:
auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});
auto binary_with_subtype = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE}, 42);
There are several convenience functions to check and set the subtype:
binary.has_subtype(); // returns false
binary_with_subtype.has_subtype(); // returns true
binary_with_subtype.clear_subtype();
binary_with_subtype.has_subtype(); // returns true
binary_with_subtype.set_subtype(42);
binary.set_subtype(23);
binary.subtype(); // returns 23
As json::binary_t is subclassing std::vector<std::uint8_t>, all member functions are available:
binary.size(); // returns 4
binary[1]; // returns 0xFE
JSON values can be constructed from json::binary_t:
json j = binary;
Binary values are primitive values just like numbers or strings:
j.is_binary(); // returns true
j.is_primitive(); // returns true
Given a binary JSON value, the binary_t can be accessed by reference as via get_binary():
j.get_binary().has_subtype(); // returns true
j.get_binary().size(); // returns 4
For convencience, binary JSON values can be constructed via json::binary:
auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);
auto j3 = json::binary({0xCA, 0xFE, 0xBA, 0xBE});
j2 == j; // returns true
j3.get_binary().has_subtype(); // returns false
Serialization
Binary values are serialized differently according to the formats.
JSON
JSON does not have a binary type, and this library does not introduce a new type as this would break conformance. Instead, binary values are serialized as an object with two keys: bytes holds an array of integers, and subtype is an integer or null.
??? example
Code:
```cpp
// create a binary value of subtype 42
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// serialize to standard output
std::cout << j.dump(2) << std::endl;
```
Output:
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": 42
}
}
```
!!! warning "No roundtrip for binary values"
The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.
BSON
BSON supports binary values and subtypes. If a subtype is given, it is used and added as unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.
??? example
Code:
```cpp
// create a binary value of subtype 42
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to BSON
auto v = json::to_bson(j);
```
`v` is a `std::vector<std::uint8t>` with the following 22 elements:
```c
0x16 0x00 0x00 0x00 // number of bytes in the document
0x05 // binary value
0x62 0x69 0x6E 0x61 0x72 0x79 0x00 // key "binary" + null byte
0x04 0x00 0x00 0x00 // number of bytes
0x2a // subtype
0xCA 0xFE 0xBA 0xBE // content
0x00 // end of the document
```
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": 42
}
}
```
CBOR
CBOR supports binary values, but no subtypes. Subtypes will be serialized as tags. Any binary value will be serialized as byte strings. The library will choose the smallest representation using the length of the byte array.
??? example
Code:
```cpp
// create a binary value of subtype 42
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to CBOR
auto v = json::to_cbor(j);
```
`v` is a `std::vector<std::uint8t>` with the following 15 elements:
```c
0xA1 // map(1)
0x66 // text(6)
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0xD8 0x2A // tag(42)
0x44 // bytes(4)
0xCA 0xFE 0xBA 0xBE // content
```
Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless `json::cbor_tag_handler_t::ignore` is passed to `json::from_cbor`.
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": null
}
}
```
MessagePack
MessagePack supports binary values and subtypes. If a subtype is given, the ext family is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, ext8, ext16, and ext32. The subtype is then added as singed 8-bit integer.
If no subtype is given, the bin family (bin8, bin16, bin32) is used.
??? example
Code:
```cpp
// create a binary value of subtype 42
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to MessagePack
auto v = json::to_msgpack(j);
```
`v` is a `std::vector<std::uint8t>` with the following 14 elements:
```c
0x81 // fixmap1
0xA6 // fixstr6
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0xD6 // fixext4
0x2A // subtype
0xCA 0xFE 0xBA 0xBE // content
```
Note that the serialization preserves the subtype, and deserializing `v` would yield the following value:
```json
{
"binary": {
"bytes": [202, 254, 186, 190],
"subtype": 42
}
}
```
UBJSON
UBJSON neither supports binary values nor subtypes, and proposes to serialize binary values as array of uint8 values. This translation is implemented by the library.
??? example
Code:
```cpp
// create a binary value of subtype 42 (will be ignored in UBJSON)
json j;
j["binary"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);
// convert to UBJSON
auto v = json::to_msgpack(j);
```
`v` is a `std::vector<std::uint8t>` with the following 20 elements:
```c
0x7B // '{'
0x69 0x06 // i 6 (length of the key)
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0x5B // '['
0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE // content (each byte prefixed with 'U')
0x5D // ']'
0x7D // '}'
```
The following code uses the type and size optimization for UBJSON:
```cpp
// convert to UBJSON using the size and type optimization
auto v = json::to_ubjson(j, true, true);
```
The resulting vector has 23 elements; the optimization is not effective for examples with few values:
```c
0x7B // '{'
0x24 // '$' type of the object elements
0x5B // '[' array
0x23 0x69 0x01 // '#' i 1 number of object elements
0x69 0x06 // i 6 (length of the key)
0x62 0x69 0x6E 0x61 0x72 0x79 // "binary"
0x24 0x55 // '$' 'U' type of the array elements: unsinged integers
0x23 0x69 0x04 // '#' i 4 number of array elements
0xCA 0xFE 0xBA 0xBE // content
```
Note that subtype (42) is **not** serialized and that UBJSON has **no binary type**, and deserializing `v` would yield the following value:
```json
{
"binary": [202, 254, 186, 190]
}
```