to_bson member function ignores BinaryType template parameter

[danielalves]

Description

Currrently to_bson is implemented like this:

static std::vector<std::uint8_t> to_bson(const basic_json& j)
{
    std::vector<std::uint8_t> result;
    to_bson(j, result);
    return result;
}

which ignores the BinaryType template parameter received by basic_json.

It should use the BinaryType instead:

static BinaryType to_bson(const basic_json& j)
{
    BinaryType result;
    to_bson(j, result);
    return result;
}

Reproduction steps

#include <type_traits>

typedef nlohmann::basic_json<
    std::map,
    std::vector,
    std::string,
    bool,
    std::int64_t,
    std::uint64_t,
    double,
    std::allocator,
    nlohmann::adl_serializer,
    std::vector<std::byte>,
    void
> CustomJson;

void test() {
    CustomJson oauthDataJson = {
        {"key", "value"}
    };
    auto result = CustomJson::to_bson(oauthDataJson);

    // This won't compile because static_assert will fail 
    static_assert(std::is_same_v<CustomJson::binary_t, decltype(result)>);
}

Expected vs. actual results

• Expected:

  • static_assert(std::is_same_v<CustomJson::binary_t, decltype(result)>) shouldn't fail

• Actual

  • static_assert(std::is_same_v<CustomJson::binary_t, decltype(result)>) fails

Minimal code example

// Same as `Reproduction Steps`

#include <type_traits>

typedef nlohmann::basic_json<
    std::map,
    std::vector,
    std::string,
    bool,
    std::int64_t,
    std::uint64_t,
    double,
    std::allocator,
    nlohmann::adl_serializer,
    std::vector<std::byte>,
    void
> CustomJson;

void test() {
    CustomJson oauthDataJson = {
        {"key", "value"}
    };
    auto result = CustomJson::to_bson(oauthDataJson);

    // This won't compile because static_assert will fail 
    static_assert(std::is_same_v<CustomJson::binary_t, decltype(result)>);
}

Error messages

No response

Compiler and operating system

all

Library version

3.11.2

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[nlohmann]

The BinaryType is not meant to control the return type of the to_xyz functions, but is used for the binary types which are part of the binary formats, such as fixext1 in MessagePack, see https://json.nlohmann.me/api/basic_json/binary_t.

To write to a container other than std::vector<std::uint8_t>, use overload (2) (see https://json.nlohmann.me/api/basic_json/to_bson/).