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api/basic_json/bjdata_version_t/index.md

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