std_msgs::string compare in a callback cpp node

Question

Hi all,

I'm trying to create a node with rosserial on Arduino. It should publish some sensor information and acts when I publish a command through a command topic. I wrote a cpp node with a callback function:

void commandsCB (const std_msgs::String& commands)
{
  nh.loginfo("Reciving new message... \n");
  nh.loginfo(commands.data);
  nh.loginfo(scan.data);
  if (commands.data == scan.data)
  {
    scanning(pos, flag, minAngle, maxAngle, myservo, servoPos, pub_pos, scansNum, range_msg, pub_range);
    nh.loginfo("Scanning...");
  }
  if (commands.data == forward.data)
  {
    cart.forwardDrive();
    getRange(range_msg, pub_range);
    nh.loginfo("Driving forward...");
  }
  if (commands.data == backward.data)
  {
    cart.backwardDrive();
    getRange(range_msg, pub_range);
    nh.loginfo("Driving backward...");
  }
  if (commands.data == rightTurn.data)
  {
    cart.rightTurn();
    getRange(range_msg, pub_range);
    nh.loginfo("Turning right...");
  }
  if (commands.data == leftTurn.data)
  {
    cart.leftTurn();
    getRange(range_msg, pub_range);
    nh.loginfo("Turning left...");
  }
}

ros::Subscriber<std_msgs::String> sub_commands ("/commands", &commandsCB);

Where scan.data, forward.data etc all std_msgs::String types.

When I try this node and publish command, for example,

scan

, I get from both

nh.loginfo(commands.data)

and

nh.loginfo(scan.data)

"scan", but it doesn't go into the if condition in the following lines in the callback.

I wonder if that's the right way to write this callback.

I'll appreciate any help, Thanks in advance, Iftach.

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Originally posted by Iftahnaf on ROS Answers with karma: 23 on 2021-04-17

Post score: 0

Answer 1

your code compares the pointers of the strings. These are different, because those are two unique strings. If you want to compare the content of the string, use https://www.cplusplus.com/reference/string/string/compare/ This would be in your code ...

if (commands.data.compare(scan.data) == 0)
{
   scanning(pos, flag, minAngle, maxAngle, myservo, servoPos, pub_pos, scansNum, range_msg, pub_range);
   nh.loginfo("Scanning...");
}

etc

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Originally posted by ct2034 with karma: 862 on 2021-04-17

This answer was ACCEPTED on the original site

Post score: 1

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Original comments

Comment by gvdhoorn on 2021-04-17:
And it's probably obvious, but this is not ROS-specific. It's C++.

Comment by Iftahnaf on 2021-04-17:
Thanks for the answer.

I used std_msgs::String and not std::String, can I still access the compare method?

Comment by ct2034 on 2021-04-17:
yeah, that is probably a little confusing: An object of std_msgs::String has one member, thats called data and is of type std::String. This is the same for all std_msgs types http://wiki.ros.org/std_msgs . Thats why you have two additional .data in the answer above.

Comment by Iftahnaf on 2021-04-17:
Great, It works after a bit of modification.

Thanks again for your help.