ROS: Use lambda as callback in nodehandle.subscribe

Question

I want to be able to define a callback through a lambda but I can't get any of the function signatures to match.

std::function<void(const sensor_msgs::PointCloud2)> callback = 
 [&mycapture](const sensor_msgs::PointCloud2 msg) { 
 // do something with msg and capture
 };

auto sub = ros_node.subscribe("/sometopic", 1, callback)

I can't get this to work as subscribe wants a function pointer. Is what I'm trying to do possible with ROS? That is, can I pass into the subscribe method a lambda with a capture?

score 11 · Answer 1 · answered Mar 18 '20 at 19:15

11

One-liner:

sensor_msgs::PointCloud2 pcl;
auto sub = nh.subscribe<sensor_msgs::PointCloud2>("scan", 1, [&](const sensor_msgs::PointCloud2ConstPtr& msg){pcl=*msg;});

Or explicit multi-liner utilizing the auto type which saves you lots of headaches with callbacks:

auto cb = [&](const sensor_msgs::PointCloud2ConstPtr& msg) {
  pcl=*msg;
};
auto sub = nh.subscribe<sensor_msgs::PointCloud2>("scan", 1, cb);

answered Mar 18 '20 at 19:15

Martin Pecka

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@PeterMitrano There's the new "say thanks" button under the up/down arrows, you can try it :) – Martin Pecka Jun 18 '20 at 21:02
I couldn't get your one liner method to work. I got Ross's method to work. (Noetic) – Geronimo Mar 29 '22 at 16:53
@Geronimo what was the error? – Martin Pecka Mar 29 '22 at 17:16
This answer was the most useful to my version of the error message. I had simply forgotten to explicitly pass the message type (in this example `sensor_msgs::PointCloud2`) as `subscribe()`'s template parameter, which is mostly due to the [simple subscriber example](http://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber%28c%2B%2B%29#roscpp_tutorials.2FTutorials.2FWritingPublisherSubscriber.The_Code-1)'s not writing it (which may be fine when using a standard callback but apparently isn't when dealing with lambda expressions). – Roland Sarrazin Oct 11 '22 at 13:54

score 8 · Answer 2 · answered Oct 05 '20 at 07:08

I have hit this issue as well in Melodic. While solution with explicit cast to boost function worked, it was very verbose. Under the hood, this solution boils to forcing compiler to choose the last overload of node_handle::subscribe that takes two template arguments:

   * \param M [template] the message type
   * \param C [template] the callback parameter type (e.g. const boost::shared_ptr<M const>& or const M&)

  template<class M, class C>
  Subscriber subscribe(const std::string& topic, uint32_t queue_size, const boost::function<void (C)>& callback

Because it is the only overload that has two template arguments, it can be enforced by specifying those parameters explicitly.

In specifics of the question asked, adding a subscriber will look like this (C++14):

ros::Subscriber subscriber = nh.subscribe<sensor_msgs::PointCloud2, const sensor_msgs::PointCloud2&>(topic, 1, [&mycapture](auto msg) {
    mycapture.workWith(msg.data);
  });

Ross · Answer 3 · 2018-03-29T15:24:48.407

6

I was able to get this working with capture.

T obj;
boost::function<void (const sensor_msgs::PointCloud2&)> callback = 
    [obj] (const sensor_msgs::PointCloud2& msg) {
    // Do something here 
    }
auto sub = ros_nh.subscribe<sensor_msgs::PointCloud>(topic, 1, callback);

ROS Version: Indigo
C++14

edited Mar 29 '18 at 15:24

answered Jan 29 '18 at 23:13

Ross

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score 3 · Answer 4 · answered Nov 21 '17 at 06:46

The partial solution in the question seemed a bit cleaner without having a static cast.

This code works fine (also with timers, services, etc.):

boost::function<void (const sensor_msgs::PointCloud2&, CustomObject&)> callback =
    [] (const sensor_msgs::PointCloud2& msg, CustomObject& obj) {
    // do something with msg and obj
    }

ros::Subscriber = ros_nh.subscribe(topic, 1, boost::bind(callback, _1, obj));

score 1 · Answer 5 · answered Jul 27 '17 at 05:11

Managed to get lambdas working. Couldn't find a way to capture though. Instead I passed the object in as a paramater.

auto *callback = static_cast<void (*)(const sensor_msgs::PointCloud2::ConstPtr &, CustomObject&)>([](const sensor_msgs::PointCloud2::ConstPtr &msg, CustomObject &o) {
                                                    ROS_INFO_STREAM("Received: \n"<<msg->header);
                                                });

ros_node.subscribe<sensor_msgs::PointCloud2>(topic, 1, boost::bind(callback, _1, custom_object))

score 1 · Answer 6 · answered Oct 24 '19 at 22:05

Define a lambda, capture any obj references you need, and wrap the message my_msg in argument list.

auto lambda = [&obj](const ros::MessageEvent< my_msg const >& msg) {
    auto const_ref_msg = *(msg.getConstMessage().get());
    obj.callback(const_ref_msg);
};

auto sub = nh.subscribe< my_msg >("/my_msg", 1, lambda);

Caveats:

The message must be wrapped in ros::MessageEvent.
- If lambda is declared to accept (my_msg msg) then compiler complains: "no known conversion for argument 1 from ‘const boost::shared_ptr<const my_msg_<std::allocator<void> > >’ to ‘my_msg {aka my_msg_<std::allocator<void> >}’"
Make sure you have auto subAck = so that the Subscriber remains on the stack frame. Otherwise compiler might optimize out the latter part mmNode.subscribe< my_msg >(my_msg::TOPIC, subscribeQueueSize, lambdaAck).

Can you explain the usefulness of this? I'm not accustomed to seeing lambda syntax used for a function that's not declared inline and frankly, it's hard to read. Besides capturing an object that could be passed as a parameter, what does the lambda do for you that a function in traditional notation (like they always use in the ROS tutorials) can't? OP is invited to answer this too. — user2084572, Jan 08 '20 at 21:39

ROS: Use lambda as callback in nodehandle.subscribe

6 Answers6