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Using C++ message filters in ROS2

asked 2020-09-15 16:03:55 -0500

srubs_r gravatar image

I am trying to subscribe to two topics. If their header stamps are equal they should be combined into a single message and published.

I believe that an exact_time message_filter is the way to go here.

In my case the messages will be high resolution images so would like the node to be written in C++ to reduce the CPU cost.

For testing I have wrote up a Python node to to publish two messages (just sensor_msgs::msg::Temperature for this testing case) with the same header stamp. This node works with no issue.

import rclpy
from rclpy.node import Node

from sensor_msgs.msg import Temperature

class DoublePublisher(Node):

    def __init__(self):

        # create publishers
        self.publisher_temp_1_ = self.create_publisher(Temperature, 'temp_1', 10)
        self.publisher_temp_2_ = self.create_publisher(Temperature, 'temp_2', 10)

        # create timer
        timer_period = 1 
        self.timer_ = self.create_timer(timer_period, self.timer_callback)

        # initialize counter
        self.i = 0

    def timer_callback(self):
        self.get_logger().info("Publishing messages")

        # get current time for headers
        msg_stamp =

        # create and publish Temperature messages
        msg_1 = Temperature()
        msg_1.header.stamp = msg_stamp
        msg_1.temperature = float(self.i)

        msg_2 = Temperature()
        msg_2.header.stamp = msg_stamp
        msg_2.temperature = float(self.i + 2)

        self.i += 1

def main(args=None):

    minimal_publisher = DoublePublisher()



if __name__ == '__main__':

I then tried to make a C++ node that would use message filters to subscribe to the two topics and call a callback if two messages have the same timestamp. I can't really find any solid examples on how to do this in ROS2. I tried to figure it out from the comments in the code from this repo. Below is my code and the compile errors that I get.

The code:

#include <memory>
#include <string>
#include <cstring>

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
#include "sensor_msgs/msg/temperature.hpp"
#include "message_filters/subscriber.h"
#include "message_filters/time_synchronizer.h"

using std::placeholders::_1;
using std::placeholders::_2;

class ExactTimeSubscriber : public rclcpp::Node
      : Node("exact_time_subscriber")
    subscription_temp_1_.subscribe(this, "temp_1");
    subscription_temp_2_.subscribe(this, "temp_2");

    message_filters::TimeSynchronizer<sensor_msgs::msg::Temperature, sensor_msgs::msg::Temperature> sync_(subscription_temp_1_, subscription_temp_2_, 3);
    sync_.registerCallback(std::bind(&ExactTimeSubscriber::topic_callback, this, _1, _2));

  void topic_callback(const sensor_msgs::msg::Temperature::SharedPtr tmp_1, const sensor_msgs::msg::Temperature::SharedPtr tmp_2) const
    const char *temp_1 = std::to_string(tmp_1->temperature).c_str();
    const char *temp_2 = std::to_string(tmp_2->temperature).c_str();
    RCLCPP_INFO(this->get_logger(), "I heard: '%s' and '%s'", temp_1, temp_2);
  message_filters::Subscriber<sensor_msgs::msg::Temperature> subscription_temp_1_;
  message_filters::Subscriber<sensor_msgs::msg::Temperature> subscription_temp_2_;

int main(int argc, char *argv[])
  rclcpp::init(argc, argv);

  return 0;

Does anyone know how to use the message_filters::TimeSynchronizer (or similar) to synchronize messages in ROS2?

My system details are as follows:

  • X86 64 CPU architecture
  • Ubuntu 18.04.5
  • Eloquent

My compile error follows:

--- stderr: synchronizer                             
In file included from /opt/ros/eloquent/include/message_filters/synchronizer.h:47:0,
                 from /opt/ros/eloquent/include/message_filters/time_synchronizer.h:41,
                 from /workspaces/hfss-v3-ros2 ...
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1 Answer

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answered 2020-09-15 19:18:42 -0500

Geoff gravatar image

Your callback has the wrong signature. The message filter callback must take in a reference to a const std::shared_ptr<const...>&, whereas you have just a const std::shared_ptr<> Change your callback declaration line to this:

void topic_callback(
  const sensor_msgs::msg::Temperature::ConstSharedPtr& tmp_1,
  const sensor_msgs::msg::Temperature::ConstSharedPtr& tmp_2) const

Then it will compile.

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Asked: 2020-09-15 16:03:55 -0500

Seen: 55 times

Last updated: Sep 15