how can i register a callback for an action client

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Well, basically you just have to repeat the syntax given for the doneCB. As far as i understood, you need the binder because you cannot use the () operator to indicate if your function has inputs.

The first parameter references your function, the second one to which object the function belongs to. Afterwards, you just need parameters if you have inputs.

Here's n' example how i implemented this. ac.sendGoal(goal, boost::bind(&MyNode::doneCb, this, _1, _2), boost::bind(&MyNode::activeCB,this), boost::bind(&JacoClient::feedbackCb, this, _1));

If you want more details, you can check out the boost documentation ( it's quite good documented).