ROS2 reading bag files - c++

Question

I have got a bag file in db3 format and I was trying to read its messages, deserialise them and access their fields. I couldn't find any appropriate document or working example anywhere. I only managed to load the file and display all its message types using the rosbag2_cpp API as follows:
#include <rclcpp/rclcpp.hpp>
#include <tf2_msgs/msg/tf_message.hpp>
#include <ament_index_cpp/get_package_share_directory.hpp>
#include <rosbag2_cpp/readers/sequential_reader.hpp>
#include <rosbag2_cpp/converter_interfaces/serialization_format_converter.hpp>
#include <rosbag2_storage/storage_options.hpp>
int main(int argc, char** argv)
{
(void) argc;
(void) argv;
rclcpp::init(argc, argv);
rclcpp::Node node("test");
rosbag2_storage::StorageOptions storage_options{};
auto file_path = ament_index_cpp::get_package_share_directory("test")
+ "/data/rosbag_autoware_receiver_0.db3";
storage_options.uri = file_path;
storage_options.storage_id = "sqlite3";
rosbag2_cpp::ConverterOptions converter_options{};
converter_options.input_serialization_format = "cdr";
converter_options.output_serialization_format = "cdr";
rosbag2_cpp::readers::SequentialReader reader;
reader.open(storage_options, converter_options);
const auto topics = reader.get_all_topics_and_types();
for (const auto topic : topics)
RCLCPP_INFO(node.get_logger(), topic.name.c_str());
return 0;
}
Any hint, help or guide on reading the actual messages and deserialising them is much appreciated.
Regards

Answer 1

What you're looking for is the has_next() property.
Declare a msg variable of the type(s) you're looking for (such as sensor_msgs::msg::Image msg), and deserialize as following:
while (reader.has_next())
{
// serialized data
auto serialized_message = reader.read_next();
rclcpp::SerializedMessage extracted_serialized_msg(*serialized_message->serialized_data);
auto topic = serialized_message->topic_name;
if (topic.find("whatever...") != std::string::npos)
{
serialization_info.deserialize_message(&extracted_serialized_msg, &msg);
}
}