How to create custom topic?

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Create a gazebo node:

   gznode_ = gazebo::transport::NodePtr(new gazebo::transport::Node());

and advertise a new publisher:

   pub = gznode_->Advertise<gazebo::msgs::"message_type">("custom_topic_name");

change "message_type" and custom_topic_name to your own needs. Now you can publish a message by:

  pub->Publish(msg);

The types depend on the Gazebo Version you are using. For all versions < 9 you need to include:

  #include "gazebo/transport/transport.hh"

and the variable declaration look like this:

 gazebo::transport::NodePtr gznode_;
 gazebo::transport::PublisherPtr pub;