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How is “const std_msgs::String::ConstPtr& msg” transmitted into callback function since there is no parameter in the main function?

melodic

asked 2022-03-11 12:29:37 -0600

jealor
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Hey everybody I have just started to learn CPP and ROS,and I've some confusion about the subscribe function.

When we create a subscriber,we use : ros::Subscriber sub=node.subscribe(turtle_name+"pose",10,&Callback); And here,there is no parameter transmitted into Callback, just the name of Callbackfunction is declared

But in the Callbackfunction,we use: void Callback(const turtlesim::PoseConstPtr& msg) Here the Callback function requires a parameter : & msg

Here it confuses me :Sincere there is no parameter transmitted in the main function when we call the Callback function How is &msg transmitted into Callback when it actuarally execuated?

Many thanks!

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answered 2022-03-12 09:43:51 -0600

Mike Scheutzow
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The ROS devs are being tricky.

When you call the subscribe() method you also specify the ROS topic name. At run-time, the subscribe() method looks up the message type for that topic and then checks to see if the argument type of the callback function you provided is a match. If it doesn't match, the function won't be called when a message arrives.

The roscpp code is smart enough to check for various forms of the callback argument: const, non-const, non-ptr, regular ptr and ::ConstPtr. For big messages, some of these are much more efficient than others.

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So it means that the internal mechanism of the subscribe() and Callback function it's well designed by ROS devs. It's really hard for me,the very first beginner, to understand by myself. Thanks a lot bro,best wishes

jealor ( 2022-03-12 10:01:15 -0600 )edit

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Asked: 2022-03-11 12:29:37 -0600

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Last updated: Mar 12 '22

How is “const std_msgs::String::ConstPtr& msg” transmitted into callback function since there is no parameter in the main function? edit

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How is “const std_msgs::String::ConstPtr& msg” transmitted into callback function since there is no parameter in the main function?