Extract key attributes mongocxx

问题

I am trying to extract only key attributes through mongocxx but using find() and find_one() function .I am not able to pull only key values.

int main(int, char**)
{
    mongocxx::instance inst{};
    mongocxx::client conn{mongocxx::uri{}};
    auto collection = conn["test"]["restaurants"];


    bsoncxx::stdx::optional<bsoncxx::document::value> maybe_result =collection.find_one(document{} << finalize);
    if(maybe_result)
    {
       std::cout <<bsoncxx::to_json(*maybe_result)<< "\n";
    }
} 

Above code pull only the one document with keys and their values and I want only keys. Can anyone help me on this?

来源：https://stackoverflow.com/questions/46680578/extract-key-attributes-mongocxx