nlohmann::basic_json::binary_t

using binary_t = byte_container_with_subtype<BinaryType>;

This type is a type designed to carry binary data that appears in various serialized formats, such as CBOR's Major Type 2, MessagePack's bin, and BSON's generic binary subtype. This type is NOT a part of standard JSON and exists solely for compatibility with these binary types. As such, it is simply defined as an ordered sequence of zero or more byte values.

Additionally, as an implementation detail, the subtype of the binary data is carried around as a std::uint64_t, which is compatible with both of the binary data formats that use binary subtyping, (though the specific numbering is incompatible with each other, and it is up to the user to translate between them). The subtype is added to BinaryType via the helper type byte_container_with_subtype.

CBOR's RFC 7049 describes this type as:

Major type 2: a byte string. The string's length in bytes is represented following the rules for positive integers (major type 0).

MessagePack's documentation on the bin type family describes this type as:

Bin format family stores a byte array in 2, 3, or 5 bytes of extra bytes in addition to the size of the byte array.

BSON's specifications describe several binary types; however, this type is intended to represent the generic binary type which has the description:

Generic binary subtype - This is the most commonly used binary subtype and should be the 'default' for drivers and tools.

None of these impose any limitations on the internal representation other than the basic unit of storage be some type of array whose parts are decomposable into bytes.

The default representation of this binary format is a #!cpp std::vector<std::uint8_t>, which is a very common way to represent a byte array in modern C++.

Template parameters

BinaryType: container type to store arrays
Although not formally expressed as a C++ concept, BinaryType must be default-constructible, copy/move-constructible, and support push_back(), .data(), and .size(), because byte_container_with_subtype derives directly from it. Its value_type must additionally be exactly one byte wide (e.g., std::uint8_t/char/std::byte): the binary serializers (CBOR, MessagePack, BSON, UBJSON) read and write the container's raw bytes via reinterpret_cast, which is only correct for byte-sized elements -- a container like #!cpp std::vector<std::intptr_t> will not work as BinaryType.

Notes

Default type

The default values for BinaryType is #!cpp std::vector<std::uint8_t>.

Custom BinaryType behavior

When a custom BinaryType is configured (other than the default #!cpp std::vector<std::uint8_t>), you can assign values of that type directly to a basic_json instance, and they will automatically be recognized as binary values rather than arrays:

using custom_json = nlohmann::basic_json<
    nlohmann::ordered_map,  // ObjectType
    std::vector,            // ArrayType
    std::string,            // StringType
    bool,                   // BooleanType
    std::int64_t,           // NumberIntegerType
    std::uint64_t,          // NumberUnsignedType
    double,                 // NumberFloatType
    std::allocator,         // AllocatorType
    nlohmann::adl_serializer,
    std::vector<std::byte>  // Custom BinaryType
>;

std::vector<std::byte> data{std::byte{1}, std::byte{2}, std::byte{3}};
custom_json j = data;  // Creates a binary value, not an array
assert(j.is_binary());

// Round-tripping works seamlessly
auto extracted = j.get<std::vector<std::byte>>();
assert(extracted == data);

This automatic type detection is a convenience feature that only applies to custom (non-default) BinaryType configurations. The default nlohmann::json continues to treat #!cpp std::vector<std::uint8_t> as arrays for backward compatibility.

Storage

Binary Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of the type #!cpp binary_t* must be dereferenced.

Notes on subtypes

CBOR
- Binary values are represented as byte strings. Subtypes are written as tags.
MessagePack
- If a subtype is given and the binary array contains exactly 1, 2, 4, 8, or 16 elements, the fixext family (fixext1, fixext2, fixext4, fixext8) is used. For other sizes, the ext family (ext8, ext16, ext32) is used. The subtype is then added as a signed 8-bit integer.
- If no subtype is given, the bin family (bin8, bin16, bin32) is used.
BSON
- If a subtype is given, it is used and added as an unsigned 8-bit integer.
- If no subtype is given, the generic binary subtype 0x00 is used.

Examples

??? example

The following code shows that `binary_t` is by default, a typedef to
`#!cpp nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>`.
 
```cpp
--8<-- "examples/binary_t.cpp"
```

Output:

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

Version history

Added in version 3.8.0. Changed the type of subtype to std::uint64_t in version 3.10.0.

5.4 KiB Raw Blame History