What to put ROS_INFO parameter for float data type in a subscriber? [closed]

asked 2018-08-31 02:29:10 -0500

J. J. gravatar image

Hi I'm trying to make a simple subscriber class. However I'm having an error :

error: ‘Float32’ in namespace ‘std_msgs’ does not name a type

void chatterCallback(const std_msgs::Float32::ConstPtr& msg)

Here is my code:

#include <ros/ros.h>
#include <std_msgs/Float32.h>

class FloatSub

    ros::Publisher sub;
    std_msgs::Float32 abc;

    void Callback(const std_msgs::Float32::ConstPtr& msg);

void FloatSub::Callback(const std_msgs::Float32::ConstPtr& msg)
    ROS_INFO("I heard: [%f]", msg->data);

int main(int argc, char **argv)
  ros::init(argc, argv, "listener_class");

  ros::NodeHandle nh;

  FloatSub floatsub;
  ros::Subscriber sub = nh.subscribe("chatter", 1000, &FloatSub::Callback, &floatsub);
  return 0;

I think my problem comes from the line :

ROS_INFO("I heard: [%f]", msg->data);

From my understanding, since the topic is publishing a float, I also need to take in a float however I do not know how the 'msg->data' works and how it can be modified to suite different data types. If anyone could link me or any help would be welcome. Thanks.

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Closed for the following reason the question is answered, right answer was accepted by J. J.
close date 2018-09-10 00:27:45.121325


Did you add std_msgs to the dependencies of your package in the CMakeLists.txt and package.xml?

Chaos gravatar image Chaos  ( 2018-08-31 02:46:01 -0500 )edit

I just fixed! apparently I was subscribing to the wrong topic But I still do not know how the 'msg-> data' works. I just copied it from another code. I mean what would I put in the msg -> X if I have a float64 or a sensor_msg::Range data type?

J. J. gravatar image J. J.  ( 2018-08-31 02:55:23 -0500 )edit

In your case, msg is a pointer to the class std_msgs::Float32ConstPtr, that is a ROS's standard message type that has only one field: data. For other types' reference you can search on the ROS wiki (i.e.: sensor_msgs::Range)

Chaos gravatar image Chaos  ( 2018-08-31 03:00:10 -0500 )edit

As I am using rosmsg show std_msgs/Float32 and it says float32 data I am assuming the 'data' from msg->'data' comes from this? Sorry if this is a noob question.

J. J. gravatar image J. J.  ( 2018-08-31 03:12:05 -0500 )edit

It's actually a pointer to a std_msgs::Float32, being passed by reference. You access its fields using the pointer operator ->.

pcoenen gravatar image pcoenen  ( 2018-08-31 03:15:02 -0500 )edit

Yes, you're right. Every message in ROS is to be considered as a class (or as a struct). By using rosmsg show you can see how the message is defined. Don't worry: everybody was a noob!

Chaos gravatar image Chaos  ( 2018-08-31 03:16:02 -0500 )edit