[ros-users] How to get copy of message using MessageEvent callback?

[Patrick Bouffard]

I am trying out using the MessageEvent style of subscriber callback
and having some difficulty keeping a copy of the message that I
receive. In the old style of callback I would do something like:

class MyClass
{
...
private:
some_pkg::a_message latest_msg;
...
void MyClass::subCallback(some_pkg::a_message msg)
{
latest_msg = msg;
}
...
}

and this seems to work just fine. But when I try to do something
similar with MessageEvent I can't get it to compile; the following
code is patterned exactly after what is at
http://www.ros.org/wiki/roscpp/Overview/Publishers%20and%20Subscribers#MessageEvent_.5BROS_1.1.2B-.5D:

  void controlModesStatusCallback(const
ros::MessageEvent<control_mode_status const>& event)
  {
    const std::string& publisher_name = event.getPublisherName();
    const ros::M_string& header = event.getConnectionHeader();
    ros::Time receipt_time = event.getReceiptTime();

    const control_mode_statusPtr& msg = event.getMessage(); // this is line 172

    ROS_INFO_STREAM("Got control mode status, from: " << publisher_name);
    latest_mode_status[node_mode_map[publisher_name]] = msg;
  }

The error message I get is:

/home/bouffard/dev/ros/ros_flyer/flyer_controller/nodes/controller.cpp:172:
error: invalid initialization of reference of type ‘const
flyer_controller::control_mode_statusPtr&’ from expression of type
‘boost::shared_ptr<const
flyer_controller::control_mode_status_<std::allocator<void> > >’
/home/bouffard/dev/ros/ros_flyer/flyer_controller/nodes/controller.cpp:175:
error: no match for ‘operator=’ in
‘((flyer_controller::Controller*)this)->flyer_controller::Controller::latest_mode_status.std::map<_Key,
_Tp, _Compare, _Alloc>::operator[] [with _Key =
std::basic_string<char, std::char_traits<char>, std::allocator<char>
>, _Tp = flyer_controller::control_mode_status_<std::allocator<void>
>, _Compare = std::less<std::basic_string<char,
std::char_traits<char>, std::allocator<char> > >, _Alloc =
std::allocator<std::pair<const std::basic_string<char,
std::char_traits<char>, std::allocator<char> >,
flyer_controller::control_mode_status_<std::allocator<void> > >
>](((const std::basic_string<char, std::char_traits<char>,
std::allocator<char> >&)((const std::basic_string<char,
std::char_traits<char>, std::allocator<char>
>*)((flyer_controller::Controller*)this)->flyer_controller::Controller::node_mode_map.std::map<_Key,
_Tp, _Compare, _Alloc>::operator[] [with _Key =
std::basic_string<char, std::char_traits<char>, std::allocator<char>
>, _Tp = std::basic_string<char, std::char_traits<char>,
std::allocator<char> >, _Compare = std::less<std::basic_string<char,
std::char_traits<char>, std::allocator<char> > >, _Alloc =
std::allocator<std::pair<const std::basic_string<char,
std::char_traits<char>, std::allocator<char> >,
std::basic_string<char, std::char_traits<char>, std::allocator<char> >
> >](((const std::basic_string<char, std::char_traits<char>,
std::allocator<char> >&)((const std::string*)publisher_name)))))) =
msg’

Also incidentally, I'm only using MessageEvent because I'm interested
in knowing the name of the publisher and based on the docs this seemed
to be the way one is meant do it. However it seems that the following
accomplishes what I need:

void controlModesStatusCallback(control_mode_status msg)
  {

    string publisher_name = (*(msg.__connection_header))["callerid"];
    ROS_INFO_STREAM("Got control mode status, from: " << publisher_name);
    latest_mode_status[node_mode_map[publisher_name]] = msg;
  }

.. is there any issue with using this?

Thanks,
Pat