How to read data from rosbag2 in ros2 - c++

Question

I am writing a program to read data from rosbag directly without playing it in ros2. Sample code snippet is below. The intention of the code is that it checks for a ros2 topic and fetches only message in that topic. I am not able to fetch the data from the bag. When printed the console is printing hexadecimal values.
auto read_only_storage = factory.open_read_only(bag_file_path, storage_id);
while(read_only_storage->has_next())
{
auto msg = read_only_storage->read_next();
if(msg->topic_name == topic)
{
cout << msg->serialized_data<<endl;
}
}
Any help in this regard would be appreciable.

Answer 1

You have to deserialize "msg->serialized_data" data. If you are using data serialized "cdr" format, please look below code.
// deserialization and conversion to ros message
my_pkg::msg::Msg msg;
auto ros_message = std::make_shared<rosbag2_introspection_message_t>();
ros_message->time_stamp = 0;
ros_message->message = nullptr;
ros_message->allocator = rcutils_get_default_allocator();
ros_message->message = &msg;
auto type_support = rosbag2::get_typesupport("my_pkg/msg/Msg", "rosidl_typesupport_cpp");
rosbag2::SerializationFormatConverterFactory factory;
std::unique_ptr<rosbag2::converter_interfaces::SerializationFormatDeserializer> cdr_deserializer_;
cdr_deserializer_ = factory.load_deserializer("cdr");
cdr_deserializer_->deserialize(msg, type_support, ros_message);
Full code: https://github.com/Kyungpyo-Kim/ROS2BagFileParsing