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Reading from a bag file (C++)

Goal: Read data from a bag without using the CLI.

Tutorial level: Advanced

Time: 10 minutes

Background

rosbag2 doesn’t just provide the ros2 bag command line tool. It also provides a C++ API for reading from and writing to a bag from your own source code. This allows you to read the contents from a bag without having to play the bag, which can sometimes be useful.

Prerequisites

You should have the rosbag2 packages installed as part of your regular ROS 2 setup.

If you need to install ROS 2, see the Installation instructions.

You should have already completed the basic ROS 2 bag tutorial, and we will be using the subset bag you created there.

Tasks

1 Create a Package

Open a new terminal and source your ROS 2 installation so that ros2 commands will work.

In a new or existing workspace, navigate to the src directory and create a new package:

ros2 pkg create --build-type ament_cmake --license Apache-2.0 bag_reading_cpp --dependencies rclcpp rosbag2_cpp turtlesim

Your terminal will return a message verifying the creation of your package bag_reading_cpp and all its necessary files and folders. The --dependencies argument will automatically add the necessary dependency lines to package.xml and CMakeLists.txt. In this case, the package will use the rosbag2_cpp package as well as the rclcpp package. A dependency on the turtlesim package is also required for working with the custom turtlesim messages.

1.1 Update package.xml

Because you used the --dependencies option during package creation, you don’t have to manually add dependencies to package.xml or CMakeLists.txt. As always, though, make sure to add the description, maintainer email and name, and license information to package.xml.

<description>C++ bag reading tutorial</description>
<maintainer email="you@email.com">Your Name</maintainer>
<license>Apache-2.0</license>

2 Write the C++ Reader

Inside your package’s src directory, create a new file called simple_bag_reader.cpp and paste the following code into it.

#include <chrono>
#include <functional>
#include <iostream>
#include <memory>
#include <string>

#include "rclcpp/rclcpp.hpp"
#include "rclcpp/serialization.hpp"
#include "rosbag2_cpp/reader.hpp"
#include "turtlesim/msg/pose.hpp"

using namespace std::chrono_literals;

class PlaybackNode : public rclcpp::Node
{
  public:
    PlaybackNode(const std::string & bag_filename)
    : Node("playback_node")
    {
      publisher_ = this->create_publisher<turtlesim::msg::Pose>("/turtle1/pose", 10);

      timer_ = this->create_wall_timer(100ms,
          [this](){return this->timer_callback();}
      );

      reader_.open(bag_filename);
    }

  private:
    void timer_callback()
    {
      while (reader_.has_next()) {
        rosbag2_storage::SerializedBagMessageSharedPtr msg = reader_.read_next();

        if (msg->topic_name != "/turtle1/pose") {
          continue;
        }

        rclcpp::SerializedMessage serialized_msg(*msg->serialized_data);
        turtlesim::msg::Pose::SharedPtr ros_msg = std::make_shared<turtlesim::msg::Pose>();

        serialization_.deserialize_message(&serialized_msg, ros_msg.get());

        publisher_->publish(*ros_msg);
        std::cout << '(' << ros_msg->x << ", " << ros_msg->y << ")\n";

        break;
      }
    }

    rclcpp::TimerBase::SharedPtr timer_;
    rclcpp::Publisher<turtlesim::msg::Pose>::SharedPtr publisher_;

    rclcpp::Serialization<turtlesim::msg::Pose> serialization_;
    rosbag2_cpp::Reader reader_;
};

int main(int argc, char ** argv)
{
  if (argc != 2) {
    std::cerr << "Usage: " << argv[0] << " <bag>" << std::endl;
    return 1;
  }

  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<PlaybackNode>(argv[1]));
  rclcpp::shutdown();

  return 0;
}

2.1 Examine the code

The #include statements at the top are the package dependencies. Note the inclusion of headers from the rosbag2_cpp package for the functions and structures necessary to work with bag files.

The next line creates the node which will read from the bag file and play back the data.

class PlaybackNode : public rclcpp::Node

Now, we can create a timer callback which will run at 10 hz. Our goal is to replay one message to the /turtle1/pose topic each time the callback is run. Note the constructor takes a path to the bag file as a parameter.

public:
  PlaybackNode(const std::string & bag_filename)
  : Node("playback_node")
  {
    publisher_ = this->create_publisher<turtlesim::msg::Pose>("/turtle1/pose", 10);

    timer_ = this->create_wall_timer(100ms,
      [this](){return this->timer_callback();}
    );

We also open the bag in the constructor.

reader_.open(bag_filename);

Now, inside our timer callback, we loop through messages in the bag until we read a message recorded from our desired topic. Note that the serialized message has timestamp metadata in addition to the topic name.

void timer_callback()
{
  while (reader_.has_next()) {
    rosbag2_storage::SerializedBagMessageSharedPtr msg = reader_.read_next();

    if (msg->topic_name != "/turtle1/pose") {
      continue;
    }

We then construct an rclcpp::SerializedMessage object from the serialized data we just read. Additionally, we need to create a ROS 2 deserialized message which will hold the result of our deserialization. Then, we can pass both these objects to the rclcpp::Serialization::deserialize_message method.

rclcpp::SerializedMessage serialized_msg(*msg->serialized_data);
turtlesim::msg::Pose::SharedPtr ros_msg = std::make_shared<turtlesim::msg::Pose>();

serialization_.deserialize_message(&serialized_msg, ros_msg.get());

Finally, we publish the deserialized message and print out the xy coordinate to the terminal. We also break out of the loop so that we publish the next message during the next timer calback.

  publisher_->publish(*ros_msg);
  std::cout << '(' << ros_msg->x << ", " << ros_msg->y << ")\n";

  break;
}

We must also declare the private variables used throughout the node.

  rclcpp::TimerBase::SharedPtr timer_;
  rclcpp::Publisher<turtlesim::msg::Pose>::SharedPtr publisher_;

  rclcpp::Serialization<turtlesim::msg::Pose> serialization_;
  rosbag2_cpp::Reader reader_;
};

Lastly, we create the main function which will check that the user passes an argument for the bag file path and spins our node.

int main(int argc, char ** argv)
{
  if (argc != 2) {
    std::cerr << "Usage: " << argv[0] << " <bag>" << std::endl;
    return 1;
  }

  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<PlaybackNode>(argv[1]));
  rclcpp::shutdown();

  return 0;
}

2.2 Add executable

Now open the CMakeLists.txt file.

Below the dependencies block, which contains find_package(rosbag2_cpp REQUIRED), add the following lines of code.

add_executable(simple_bag_reader src/simple_bag_reader.cpp)
ament_target_dependencies(simple_bag_reader rclcpp rosbag2_cpp turtlesim)

install(TARGETS
  simple_bag_reader
  DESTINATION lib/${PROJECT_NAME}
)

3 Build and run

Navigate back to the root of your workspace and build your new package.

LinuxmacOSWindows

colcon build --packages-select bag_reading_cpp

Next, source the setup files.

LinuxmacOSWindows

source install/setup.bash

Now, run the script. Make sure to replace /path/to/subset with the path to your subset bag.

ros2 run bag_reading_cpp simple_bag_reader /path/to/subset

You should see the (x, y) coordinates of the turtle printed to the console.

Summary

You created a C++ executable that reads data from a bag. You then compiled and ran the executable which printed some information from the bag to the console.