6a7392058e
* 👥 update contributor and sponsor list * 🚧 document BJData format * 🚧 document BJData format * 📝 clarified documentation of [json.exception.parse_error.112] * ✏️ adjust titles * 📝 add more examples * 🚨 adjust warnings for index.md files * 📝 add more examples * 🔥 remove example for deprecated code * 📝 add missing enum entry * 📝 overwork table for binary formats * ✅ add test to create table for binary formats * 📝 fix wording in example * 📝 add more examples * Update iterators.md (#3481) * ✨ add check for overloads to linter #3455 * 👥 update contributor list * 📝 add more examples * 📝 fix documentation * 📝 add more examples * 🎨 fix indentation * 🔥 remove example for destructor * 📝 overwork documentation * Updated BJData documentation, #3464 (#3493) * update bjdata.md for #3464 * Minor edit * Fix URL typo * Add info on demoting ND array to a 1-D optimized array when singleton dimension Co-authored-by: Chaoqi Zhang <prncoprs@163.com> Co-authored-by: Qianqian Fang <fangqq@gmail.com>
373 lines
12 KiB
Markdown
373 lines
12 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 define 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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```plantuml
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class json::binary_t {
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-- setters --
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+void set_subtype(std::uint64_t subtype)
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+void clear_subtype()
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-- getters --
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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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"std::vector<uint8_t>" <|-- json::binary_t
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```
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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`:
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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 true
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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:
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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 as via `get_binary()`:
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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`:
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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
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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) neither supports binary values nor subtypes, and proposes to serialize binary values
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as array of uint8 values. This translation is implemented by the library.
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??? example
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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::uint8t>` 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_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' type of the array elements: unsigned integers
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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 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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### 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
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unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.
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??? example
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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::uint8t>` 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, but no subtypes. Subtypes will be serialized as tags. Any binary
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value will be serialized as byte strings. The library will choose the smallest representation using the length of the
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byte array.
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??? example
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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::uint8t>` 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 `json::from_cbor`.
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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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### 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, ext8, ext16, and
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ext32. The subtype is then added as signed 8-bit integer.
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If no subtype is given, the bin family (bin8, bin16, bin32) is used.
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??? example
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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::uint8t>` 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 array of uint8 values. This translation is implemented by the library.
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??? example
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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::uint8t>` 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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