Getting Started with ROS on Embedded Systems/User Guide/C++/Names

Getting Started with ROS on Embedded Systems
ROS on Embedded Systems Basics
Getting Started
Installation Configuration Setting up environment Execution
C++ User Guide
Package set up Initialization Topics Messages Names Publishers and subscribers Launch files Parameters
Examples
Basic pipelines V4L2 capture node Turn detector node Proximity detector node
Performance
Xavier TX1 TX2 Nano
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Introduction

This wiki is based on the following page: http://wiki.ros.org/roscpp/Overview/Publishers%20and%20Subscribers

Publishers and subscribers are one of the main ways of communication when using ROS.

Publishers

In order to publish messages, you will need a node handle, and create a ros::Publisher using the advertise function. Like the following example:

ros::Publisher pub = nh.advertise<std_msgs::String>("topic_name", 5);
std_msgs::String str;
str.data = "hello world";
pub.publish(str);

This uses the function

template<class M>
ros::Publisher advertise(const std::string& topic, uint32_t queue_size, bool latch = false);

In which:

• M is the message type to publish on the topic
• topic is the name of the topic to publish to
• queue_size is the size of the message queue, if the publishing is faster than the message sending, it will start dropping older messages.
• latch: Save messages for future subscriber connections automatically.

Subscribers

Similar to publishers, a ROS subscriber needs a node handle and with the subscribe function it will create a new ros::Subscriber, by using the following:

template<class M>
ros::Subscriber subscribe(const std::string& topic, uint32_t queue_size, <callback, which may involve multiple arguments>, const ros::TransportHints& transport_hints = ros::TransportHints());

In which:

• M is the message type to publish on the topic, normally deduced by the compiler
• topic is the name of the topic to subscribe to
• queue_size is the size of the message queue, if messages are too fast arriving and the application can't keep up, they will start dropping.
• callback: Normally a pointer to a function with the function to execute on each message arrival
• transport_hints: Hints to roscpp's transport layer. Such ad preferring UDP, tcp nodelay.

Callbacks

The first callback you can use is with a function, for example:

void callback(const std_msgs::StringConstPtr& str)
{
...
}

...
ros::Subscriber sub = nh.subscribe("my_topic", 1, callback);

The other callback is used with class methods, for example:

void Foo::callback(const std_msgs::StringConstPtr& message)
{
}

...
Foo foo_object;
ros::Subscriber sub = nh.subscribe("my_topic", 1, &Foo::callback, &foo_object);