README

Minimal subscriber cookbook recipes

This package contains a few different strategies for creating nodes which receive messages:

  • lambda.cpp uses a C++11 lambda function

  • member_function.cpp uses a C++ member function callback

  • not_composable.cpp uses a global function callback without a Node subclass

  • wait_set_subscriber.cpp uses a rclcpp::WaitSet to wait and poll for data

  • static_wait_set_subscriber.cpp uses a rclcpp::StaticWaitSet to wait and poll for data

  • time_triggered_wait_set_subscriber.cpp uses a rclcpp::Waitset and a timer to poll for data periodically

  • content_filtering.cpp uses the content filtering feature for Subscriptions

Note that not_composable.cpp instantiates a rclcpp::Node without subclassing it. This was the typical usage model in ROS 1, but this style of coding is not compatible with composing multiple nodes into a single process. Thus, it is no longer the recommended style for ROS 2.

All of these nodes do the same thing: they create a node called minimal_subscriber and subscribe to a topic named topic which is of datatype std_msgs/String. When a message arrives on that topic, the node prints it to the screen. We provide multiple examples of different coding styles which achieve this behavior in order to demonstrate that there are many ways to do this in ROS 2.

The following examples wait_set_subscriber.cpp, static_wait_set_subscriber.cpp and time_triggered_wait_set_subscriber.cpp show how to use a subscription in a node using a rclcpp wait-set. This is not a common use case in ROS 2 so this is not the recommended strategy to use by-default. This strategy makes sense in some specific situations, for example when the developer needs to have more control over callback order execution, to create custom triggering conditions or to use the timeouts provided by the wait-sets.

The example content_filtering.cpp shows how to use the content filtering feature for Subscriptions.