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where I can find contact_size() function

asked 2017-05-25 07:58:38 -0600

shawnysh gravatar image

In tutorial, there is a set of code shown below:

void ContactPlugin::OnUpdate()
{
  // Get all the contacts.
  msgs::Contacts contacts;
  contacts = this->parentSensor->GetContacts();
  for (unsigned int i = 0; i < contacts.contact_size(); ++i)
  {
    std::cout << "Collision between[" << contacts.contact(i).collision1()
              << "] and [" << contacts.contact(i).collision2() << "]\n";

    for (unsigned int j = 0; j < contacts.contact(i).position_size(); ++j)
    {
      std::cout << j << "  Position:"
                << contacts.contact(i).position(j).x() << " "
                << contacts.contact(i).position(j).y() << " "
                << contacts.contact(i).position(j).z() << "\n";
          std::cout << "   Normal:"
                << contacts.contact(i).normal(j).x() << " "
                << contacts.contact(i).normal(j).y() << " "
                << contacts.contact(i).normal(j).z() << "\n";
      std::cout << "   Depth:" << contacts.contact(i).depth(j) << "\n";
    }
  }
}

I wonder where I can find the API information about usage and definition of contact_size() function. I am not familiar with C++, is it because related to C++ library? Any hint or guide would be appreciated.

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answered 2017-05-25 10:41:02 -0600

Protobuf intro

Gazebo messages are generated using Protocol buffers (protobuf). The messages are described in .proto files, and Protobuf uses that file to generate a C++ class.

For example, this is the description for the contacts message:

import "contact.proto";
import "time.proto";

message Contacts
{
  repeated Contact contact   = 1;
  required Time time         = 2;
}

You can see that the contact field is a repeated field of the Contact (singular) message. For every repeated field, Protobuf generates a <field_name>_size() function, such as the contact_size() one.

This is all described in the Protocol Buffers C++ Generated Code documentation.

Header files

This is not ideal, but personally, what I do when I want to check a message's API, is to check its header files (who has time to read the protobuf documentation? :P).

Protobuf creates a <message_name>.pb.h header for each message and installs it in your system. You can look for the contacts.pb.h header like this:

find /usr | grep contacts.pb.h

Then you'll find something similar to this:

/usr/local/include/gazebo-7/gazebo/msgs/contacts.pb.h

So you can open that file and take a look at the API:

gedit /usr/local/include/gazebo-7/gazebo/msgs/contacts.pb.h

Digging through header files is definitely not ideal... We do have some form of message API documentation, but that's broken, as described on this issue.

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Thanks for your quick, helpful and considerate reply!

shawnysh gravatar imageshawnysh ( 2017-05-26 00:15:09 -0600 )edit
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Asked: 2017-05-25 07:58:38 -0600

Seen: 91 times

Last updated: May 25 '17