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Publishing to a Topic

See also: ros::NodeHandle::advertise() API docs, ros::Publisher API docs, ros::NodeHandle API docs

Creating a handle to publish messages to a topic is done using the ros::NodeHandle class, which is covered in more detail in the NodeHandles overview.

The NodeHandle::advertise() methods are used to create a ros::Publisher which is used to publish on a topic, e.g.:

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

Note: it is possible (though rare) for NodeHandle::advertise() to return an empty ros::Publisher. In the future these cases will probably throw exceptions, but for now they simply print an error. You can check for this with:

   1 if (!pub)
   2 {
   3 ...
   4 }

Intraprocess Publishing

When a publisher and subscriber to the same topic both exist inside the same node, roscpp can skip the serialize/deserialize step (potentially saving a large amount of processing and latency). It can only do this though if the message is published as a shared_ptr:

   1 ros::Publisher pub = nh.advertise<std_msgs::String>("topic_name", 5);
   2 std_msgs::StringPtr str(new std_msgs::String);
   3 str->data = "hello world";
   4 pub.publish(str);

This form of publishing is what can make nodelets such a large win over nodes in separate processes.

Note that when publishing in this fashion, there is an implicit contract between you and roscpp: you may not modify the message you've sent after you send it, since that pointer will be passed directly to any intraprocess subscribers. If you want to send another message, you must allocate a new one and send that.

Publisher Options

The signature for the simple version of advertise() is:

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

Other Operations on the Publisher Handle

ros::Publisher is reference counted internally -- this means that copying them is very fast, and does not create a "new" version of the ros::Publisher. Once all copies of a ros::Publisher are destructed the topic will be shutdown. There are some exceptions to this:

  1. ros::shutdown() is called -- this shuts down all publishers (and everything else).

  2. ros::Publisher::shutdown() is called. This will shutdown this topic, unless there have been...

  3. Multiple calls to NodeHandle::advertise() for the same topic, with the same message type. In this case all the ros::Publishers on a specific topic are treated as copies of each other.

ros::Publisher implements the ==, != and < operators, and it is possible to use them in std::map, std::set, etc.

You can retrieve the topic of a publisher with the ros::Publisher::getTopic() method.

publish() behavior and queueing

publish() in roscpp is asynchronous, and only does work if there are subscribers connected on that topic. publish() itself is meant to be very fast, so it does as little work as possible:

  1. Serialize the message to a buffer
  2. Pushes that buffer onto a queue for later processing

The queue it's pushed onto is then serviced as soon as possible by one of roscpp's internal threads, where it gets put onto a queue for each connected subscriber -- this second set of queues are the ones whose size is set with the queue_size parameter in advertise(). If one of these queues fills up the oldest message will be dropped before adding the next message to the queue.

Note that there may also be an OS-level queue at the transport level, such as the TCP/UDP send buffer.

Subscribing to a Topic

See also: ros::NodeHandle::subscribe() API docs, ros::Subscriber API docs, ros::NodeHandle API docs, ros::TransportHints API docs

Subscribing to a topic is also done using the ros::NodeHandle class (covered in more detail in the NodeHandles overview). A simple subscription using a global function would look like:

   1 void callback(const std_msgs::StringConstPtr& str)
   2 {
   3 ...
   4 }
   5 
   6 ...
   7 ros::Subscriber sub = nh.subscribe("my_topic", 1, callback);

Subscriber Options

There are many different versions of ros::NodeHandle::subscribe(), but the simple versions boil down to:

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

Callback Signature

The signature for the callback is:

   1 void callback(const boost::shared_ptr<Message const>&);

Every generated message provides typedefs for the shared pointer type, so you can also use, for example:

   1 void callback(const std_msgs::StringConstPtr&);

or

   1 void callback(const std_msgs::String::ConstPtr&);

Other Valid Signatures [ROS 1.1+]

As of ROS 1.1 we also support variations on the above callbacks:

   1 void callback(boost::shared_ptr<std_msgs::String const>);
   2 void callback(std_msgs::StringConstPtr);
   3 void callback(std_msgs::String::ConstPtr);
   4 void callback(const std_msgs::String&);
   5 void callback(std_msgs::String);
   6 void callback(const ros::MessageEvent<std_msgs::String const>&);

You can also request a non-const message, in which case a copy will be made if necessary (i.e. there are multiple subscriptions to the same topic in a single node):

   1 void callback(const boost::shared_ptr<std_msgs::String>&);
   2 void callback(boost::shared_ptr<std_msgs::String>);
   3 void callback(const std_msgs::StringPtr&);
   4 void callback(const std_msgs::String::Ptr&);
   5 void callback(std_msgs::StringPtr);
   6 void callback(std_msgs::String::Ptr);
   7 void callback(const ros::MessageEvent<std_msgs::String>&);

The MessageEvent versions are described below.

Callback Types

roscpp supports any callback supported by boost::function:

  1. functions
  2. class methods
  3. functor objects (including boost::bind)

Functions

Functions are the easiest to use:

   1 void callback(const std_msgs::StringConstPtr& str)
   2 {
   3 ...
   4 }
   5 
   6 ...
   7 ros::Subscriber sub = nh.subscribe("my_topic", 1, callback);

Class Methods

Class methods are also easy, though they require an extra parameter:

   1 void Foo::callback(const std_msgs::StringConstPtr& message)
   2 {
   3 }
   4 
   5 ...
   6 Foo foo_object;
   7 ros::Subscriber sub = nh.subscribe("my_topic", 1, &Foo::callback, &foo_object);

Functor Objects

A functor object is a class that declares operator(), e.g.:

   1 class Foo
   2 {
   3 public:
   4   void operator()(const std_msgs::StringConstPtr& message)
   5   {
   6   }
   7 };

A functor passed to subscribe() must be copyable. The Foo functor could be used with subscribe() like so:

   1 ros::Subscriber sub = nh.subscribe<std_msgs::String>("my_topic", 1, Foo());

Note: when using functor objects (like boost::bind, for example) you must explicitly specify the message type as a template argument, because the compiler cannot deduce it in this case.

MessageEvent [ROS 1.1+]

The MessageEvent class allows you to retrieve metadata about a message from within a subscription callback:

   1 void callback(const ros::MessageEvent<std_msgs::String const>& event)
   2 {
   3   const std::string& publisher_name = event.getPublisherName();
   4   const ros::M_string& header = event.getConnectionHeader();
   5   ros::Time receipt_time = event.getReceiptTime();
   6 
   7   const std_msgs::StringConstPtr& msg = event.getMessage();
   8 }

(see ROS/Connection Header for details on the fields in the connection header)

Queueing and Lazy Deserialization

When a message first arrives on a topic it gets put into a queue whose size is specified by the queue_size parameter in subscribe(). If the queue is full and a new message arrives, the oldest message will be thrown out. Additionally, the message is not actually deserialized until the first callback which needs it is about to be called.

Transport Hints

See also: ros::TransportHints API docs

ros::TransportHints are used to specify hints about how you want the transport layer to behave for this topic. For example, if you wanted to specify an "unreliable" connection (and not allow a "reliable" connection as a fall back):

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

Note that ros::TransportHints uses the Named Parameter Idiom, a form of method-chaining. This means you can do things like:

   1 ros::TransportHints()
   2         .unreliable()
   3         .reliable()
   4         .maxDatagramSize(1000)
   5         .tcpNoDelay();

As is this example you can specify multiple different transport options (unreliable as well as reliable). If the publisher on the topic you're subscribing to does not support the first connection (unreliable), the connection will be made with the second (reliable) if supported. In this case the order of the two methods is important since it determines the order of considered transports.

You cannot currently specify the transport hints on the Publisher side. This will likely be an option in the future.


2019-10-19 13:07