I try to understand how the iterators of mongo-cxx-driver work.

I create a document using bsoncxx::builder::basic::make_document and bsoncxx::builder::basic::kvp like this:

mongocxx::instance inst{};

std::tm tm;

std::stringstream ss("1922-03-28");

ss >> std::get_time(&tm, "%Y-%m-%d");

auto tp = std::chrono::system_clock::from_time_t(std::mktime(&tm));

auto doc = make_document(kvp("name", "John doe"),

                                                 kvp("dob", bsoncxx::types::b_date(tp)),

                                                 kvp("id", "abcde"),

                                                 kvp("sex", "M"));

auto client = mongocxx::client(mongocxx::uri());

auto collection = client["patientdb_test"]["patient"];

collection.insert_one(doc.view());

delay(2000); // Wait during two seconds.

After that, I search all documents:

auto cursor = collection.find({});

Finally, I test the cursor to see if it's empty and the number of element found using std::distance function:

if(cursor.begin() != cursor.end())

{

      auto it = cursor.begin();

     if(std::distance(it, cursor.end()) == 1)

      {            

             if (cursor.begin() != cursor.end()) // <--- Segmentation fault

             

Unknown macro: {              }

      }

}

I don't understand why the begin iterator change after the call of std::distance method.

Is this a bug or do I use the cursor wrongly?