How to check if a message has a header? (c++)

asked 2020-03-21 20:46:07 -0500

scottnothing
11 ●2 ●5 ●5

Is there any way program logic around whether a particular message contains a header (std_msgs/Header)? For example:

template <class MsgType>
ros::Time get_message_time(const boost::shared_ptr<MsgType>& msg)
{
    if (hasHeader(msg))
    {
        return msg->header.stamp;
    } else
    {
        return ros::Time::now();
    }
}

This code will not compile with messages that do not contain a header. Is there another way?

answered 2020-03-21 22:09:53 -0500

scottnothing
11 ●2 ●5 ●5

I figured out how to use message_traits. For the above code:

auto stamp = ros::message_traits::timeStamp(*msg);
if (stamp)
{
    return *stamp;
} else
{
    return ros::Time::now();
}

The message traits function will return a pointer which will be nullptr if it could not extract a timestamp (no header). Simply then test if the pointer is valid, then access it.

Similar traits exists for Header and FrameId, see http://docs.ros.org/indigo/api/roscpp...

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Wouldn't ros::message_traits::HasHeader<m> be a more direct way of checking whether a message has a header?

gvdhoorn ( 2020-03-22 02:56:00 -0500 )edit

Yes, I think the title question is due to my coming from a Python perspective where I would use hasattr(...) to determine if there was a header before accessing it. Due to c++ and type safety, this cant work. The real question, I guess, is how to get timestamp (or frame_id) from a header if one exists, and have this work for all possible message types.

scottnothing ( 2020-03-22 14:36:33 -0500 )edit

There seem to be two questions here:

how to determine whether a message has a header
if there is a header, how to retrieve the timestamp

For the first

at compile time: use ros::message_traits::HasHeader<m>
at runtime: afaik there is no support (except what you already found). You could take a look at libraries such as wiki/ros_type_introspection and/or StefanFabian/ros_babel_fish

For the second:

convention dictates that messages with headers store them in a field of type std_msgs/Header with the name header (this is also the location most/all code in roscpp will check
if convention is not followed, the libraries I linked to earlier could be used to find it, but it could be that a field with type std_msgs/Header is used without it actually being a header

gvdhoorn ( 2020-03-23 04:56:12 -0500 )edit

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