publisher and subscriber in single node

edit

After publishing on topic you need to let the ROS spin, to make sure data is actually send on the topic. Calling "publish" doesn't put the data on the topic, so if you destroy publisher before the data is send, you lose the information stored in the buffer.

The best solution is to make your callback a class member and access data as a private variable of the class from function that is publishing this information. Quick code example:

int main (int argc, char **argv)
{
    ros::init (..);
    myClass myObject;

    while(ros::ok())
    {
        myObject.publishDataIfRecieved();
        ros::spinOnce();
    }

}

I will skip the class, but constructor should create both subscriber and publisher, callback should be part of a class, as well as the temporary data storage variable/structure and some boolean to indicate new data presence. For high frequency data you should provide some security for overwriting by callback not yet published data.

Also if you are not doing any computation an the data, you can just publish it inside the callback, using publisher that is already initialized as a class member, but you should avoid too much operation in your callback

Easier solution is global variable, as mentioned before, but it is not so elegant one.

EDIT:

ros::spinOnce() callend once in a while (as often as possible) will do the trick, but if you reallllly want to use ros::spin() instead, other solution is:

class myClass
{
    (...)
    public:
    myClass() { pub = (...); sub = (...) }

    private:
    ros::Publisher pub;
    ros::Subscriber sub;

    void chattercallback(...) { pub.publish(...); }
}

int main (int argc, char **argv)
{
    ros::init (..);
    myClass myObject;
    ros::spin();
}

You should not declare your publiser in your callback function : this callback function is called only when data is received on topic 'a', that is why topic 'c' depends on topic 'a'. Store the data you got from topic 'a' into a "global" variable and declare your published topic 'c' into the main. For example :

float64 aux;

void chatterCallback(const& msg)
{
   aux=msg->data;
}

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

ros::init (..);

ros::Subscriber sub = .....(.. , .. , chattercallback);

ros::Publisher d =......;

d.publish(..) ;

}

Update : To your question "how to access data from a topic" (see comments)

For example, if you want to access the x position of your robot spawned in gazebo empty world. You can get that information from the topic /model_state that you can get in gazebo your callback function would look like this :

void chatterCallback(const& msg)
{
   aux=msg->pose[1].position.x;
}

Look at the links above. If you need better understanding, launch a robot in gazebo and run the following

$ rostopic echo /gazebo/model_states

hello, I have done it as per the solutions give here.

I am subscribing to a sensor_msgs::Image, and I want to publish my_package/RectifiedImage.msg over a topic "mycustomtopic".

When I do "rostopic list", it is showing /mycustomtopic

When I do "rostopic echo mycustomtopic", it is showing "the rosdep view is empty: call 'sudo rosdep init' and 'rosdep update' ERROR: Cannot load message class for [stereo_msgs/RectifiedImage]. Are your messages built?"

this is what I was doing:

void callback(const ..){

..

my_msg_pub_.publish(myimage);

..

return;

}

and

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

subscribe to sensor_msgs::Image

callback

my_msg_pub_ = nh.advertise<stereo_msgs::rectifiedimage>("mycustomtopic", 1);

ros::spin();

return 0;

}

where could be my mistake?