Extract Header from ShapeShifter

asked 2016-02-11 19:23:28 -0600

solb22
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updated 2018-07-30 09:08:34 -0600

8729 ●137 ●239 ●256 https://github.com/lucasw

I'm trying to implement something similar to rospy.anymessage via roscpp. I am using Shapeshifter to extract the data. I only need the Header.stamp (I want to compare received time from the time the topic was published).

I'm running into a wall trying to extract the header stamp. I know rosbag imeplement something like this, but I could only find they use SubscriptionCallbackHelperT, but not how.

How can I extract the Header.stamp from a generic ros topic message?

void PerformanceTracker::topicCallback(const ShapeShifter::ConstPtr topicType){
   ros::Time begin = ros::Time::now();
   unsigned char* data = ShapeShifterGetData(topicType);
}

unsigned char* ShapeShifterGetData(const ShapeShifter::ConstPtr message){
  unsigned char* data = new unsigned char[message->size()];
  std::OStream stream(data, message->size());
  message->write(stream);
  return data;
}

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answered 2016-02-12 01:29:37 -0600

Stefan Kohlbrecher

24361 ●173 ●299 ●444 https://www.energy-rob...

Here's easily understandable way that has to drawback of always serializing messages and interpreting the serialized result: topic_buffer_client.

It looks like a more efficient way using SFINAE seems to be implemented here: timeextractor.h With the template magic it's a bit harder to understand though ;)

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Thanks Stefan, I will try both and get back to you. What do you means the drawback of always serializing messages? Are you saying serializing the messages wastes computation time? I can set the received time to before the serialization starts, so it wont affect the measurement of published-received

solb22 ( 2016-02-12 01:49:54 -0600 )edit

1

Yes, serialization can take up some CPU time (and it seems the second approach can do it without serialization).

Stefan Kohlbrecher ( 2016-02-12 02:02:59 -0600 )edit

Ok thanks. Yes the SFINAE is much less clear. Would I just include that file and user timewarp.extractTime() ? Thank you very much for the help !

solb22 ( 2016-02-12 02:51:16 -0600 )edit

Might be possible, but you could also copy parts and make your own minimal version of it (You're not really using it for "timewarping" for instance).

Stefan Kohlbrecher ( 2016-02-12 03:20:14 -0600 )edit

Yeah, I'm going to port that to C++11

solb22 ( 2016-02-12 03:35:42 -0600 )edit

One last question. Why in the SFINAE, is it static yes& test(typename C::_header_type); ? What does C::_header_type mean? Type has a data member named _header_type? Does that match the current ros header data member name? And not just plain C::_header*?

solb22 ( 2016-02-12 04:08:26 -0600 )edit

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answered 2019-10-24 02:26:52 -0600

dllu
1

If you are sure that the shapeshifter contains a message with a std_msgs::Header, you can try temporarily morphing the shapeshifter into a Header and then instantiating it. This is essentially the same as always serializing it, but because you don't have to have raw uint8_t buffers floating around, it may be more understandable:

ros::Time readStamp(
    const topic_tools::ShapeShifter::ConstPtr& shapeshifter) {
    topic_tools::ShapeShifter* ss_tmp =
        const_cast<topic_tools::ShapeShifter*>(shapeshifter.get());
    auto md5sum = ss_tmp->getMD5Sum();
    auto datatype = ss_tmp->getDataType();
    auto msgdef = ss_tmp->getMessageDefinition();
    ss_tmp->morph(ros::message_traits::md5sum<std_msgs::Header>(),
                  ros::message_traits::datatype<std_msgs::Header>(), "",
                  "");
    auto tmp = ss_tmp->instantiate<std_msgs::Header>();
    ss_tmp->morph(md5sum, datatype, msgdef, "");

    return tmp->stamp;
}

Please note that if you make a copy of ShapeShifter then your code will crash. This is because the ShapeShifter class has a pointer free in its destructor but it does not have a deep copy constructor. So, copying it will result in a double pointer free bug when both instances go out of scope.

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