Compile error creating message_filter with std_msgs/String

Question

Hi,

Based on https://wiki.ros.org/message_filters#Time_Synchronizer I am trying to subscribe a node on two topics. I made two of the talker nodes like in https://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber%28c%2B%2B%29. One talker publishes on chatter and other on chatter1. So below is the code for listenerMultiple.cpp :

#include "ros/ros.h"
#include <message_filters/subscriber.h>
#include <message_filters/synchronizer.h>
#include <message_filters/sync_policies/approximate_time.h>
#include <std_msgs/String.h>

using namespace message_filters;
using namespace std_msgs;
using namespace std;


void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
        ROS_INFO("I heard: [%s]", msg->data.c_str());
}

int main(int argc, char **argv)
{

        ros::init(argc, argv, "listenerMultipleTopics");
        ros::NodeHandle n;

        message_filters::Subscriber<std_msgs::String> camera_sub(n, "chatter",1);
        message_filters::Subscriber<std_msgs::String> imu_sub(n, "chatter2",1);

        typedef sync_policies::ApproximateTime<String, String> MySyncPolicy;
        Synchronizer<MySyncPolicy> sync(MySyncPolicy(10), camera_sub, imu_sub); // adding these lines create problem!!!
        sync.registerCallback(boost::bind(&chatterCallback, _1, _2)); // adding these lines create problem!!!

        ros::spin();
        return 0;
}

edit gvdhorn explained this won't work because there is no header for the String messages. So how can I take the String from the two topics and concatenate them?

This creates a super huge compilation error (how could I solve this?) :

This code block was moved to the following github gist: https://gist.github.com/answers-se-migration-openrobotics/e9b7cb1f53c1e51e53e2361ea474e535

Originally posted by edamondo on ROS Answers with karma: 5 on 2018-02-08

Post score: 0

Answer 1

Just about all of the message_filter filters only work with messages with a Header field, as the filters need timestamps to work with.

std_msgs/String doesn't have a header, so that's most likely why you get the compilation error.

Search for header on the wiki/message_filters - Time Synchronizer page you linked.

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Originally posted by gvdhoorn with karma: 86574 on 2018-02-08

This answer was ACCEPTED on the original site

Post score: 3

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Original comments

Comment by edamondo on 2018-02-08:
Hello gvdhoorn, So how can I use the messages received from the two talkers and stack them into a vector?

Comment by gvdhoorn on 2018-02-09:
If you have no requirements for a particular order, I suggest you write a callback that does something like:

void callback(... msg)
{
    my_vector_.push_back(msg->data);
}

provided my_vector_ is a std::vector<std::string> and is in scope, that should work.

Comment by edamondo on 2018-02-09:
But my_vector_ would be defined in the main function, so how can it be in the scope of the callback? Sould I pass the my_vector_ by reference to the callback? But then should I bind the callback or something?

Comment by gvdhoorn on 2018-02-09:
Either place variables in the global scope, or use a class.

But this is more a C++ question, not a ROS issue.

Comment by edamondo on 2018-02-09:
okay, I am going for the class way. Thanks!