86 lines
2.5 KiB
Markdown
86 lines
2.5 KiB
Markdown
# <small>nlohmann::basic_json::</small>bjdata_version_t
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```cpp
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enum class bjdata_version_t
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{
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draft2,
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draft3
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};
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```
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This enumeration is used in [`to_bjdata`](to_bjdata.md) to choose the version of the BJData specification to write:
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draft2
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: BJData Draft 2 (the default). Binary values are written as arrays of `U` integers, and ND-arrays with
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`"_ArrayType_": "byte"` are written as regular objects, since the `B` marker is not part of Draft 2.
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draft3
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: BJData Draft 3. Binary values are written as `B` elements: as an optimized binary array (`[$B#`) if both `use_size`
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and `use_type` are set, and as an array of `B`-prefixed elements otherwise. ND-arrays with
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`"_ArrayType_": "byte"` are written as packed arrays (`[$B#[...]`).
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Only the writer is affected by the version. [`from_bjdata`](from_bjdata.md) reads the markers of both drafts.
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## Examples
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??? example
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The example below shows how the different values of the `bjdata_version_t` influence the serialization of a binary
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value with [`to_bjdata`](to_bjdata.md).
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```cpp
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#include <iomanip>
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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void print_hex(const char* label, const std::vector<std::uint8_t>& v)
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{
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std::cout << label << ':';
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for (auto b : v)
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{
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std::cout << ' ' << std::hex << std::setw(2) << std::setfill('0') << static_cast<int>(b);
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}
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std::cout << std::dec << std::setfill(' ') << '\n';
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}
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int main()
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{
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// a binary value
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json j = json::binary({0xCA, 0xFE, 0xBA, 0xBE});
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// Draft 2 (the default) writes an array of integers
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auto v2 = json::to_bjdata(j, true, true, json::bjdata_version_t::draft2);
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print_hex("draft2", v2);
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// Draft 3 writes an optimized binary array
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auto v3 = json::to_bjdata(j, true, true, json::bjdata_version_t::draft3);
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print_hex("draft3", v3);
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// reading yields an array for Draft 2 and a binary value for Draft 3
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std::cout << json::from_bjdata(v2) << '\n';
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std::cout << json::from_bjdata(v3) << '\n';
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}
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```
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Output:
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```json
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draft2: 5b 24 55 23 69 04 ca fe ba be
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draft3: 5b 24 42 23 69 04 ca fe ba be
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[202,254,186,190]
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{"bytes":[202,254,186,190],"subtype":null}
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```
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## See also
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- [to_bjdata](to_bjdata.md) serializes a JSON value to BJData
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- [BJData](../../features/binary_formats/bjdata.md) - the article on the BJData format
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## Version history
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- Added in version 3.12.0.
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