What is the topic ~/world_stats?

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I am following this tutorial . It teaches me to write a program to listen on the topic ~/worldstats. I did and it worked. However, when I run $ gz topic -l, I cannot find a topic named ~/worldstats. So what is ~/world_stats? and what does the tilde '~' mean here? And what is 'ConstWorldStatisticsPtr'?

The following is the listener.cpp files for your reference:

#include <gazebo/transport/transport.hh>
#include <gazebo/msgs/msgs.hh>
#include <gazebo/gazebo.hh>

#include <iostream>

/////////////////////////////////////////////////
// Function is called everytime a message is received.
void cb(ConstWorldStatisticsPtr &_msg)
{
  // Dump the message contents to stdout.
  std::cout << _msg->DebugString();
}

/////////////////////////////////////////////////
int main(int _argc, char **_argv)
{
  // Load gazebo
  gazebo::setupClient(_argc, _argv);

  // Create our node for communication
  gazebo::transport::NodePtr node(new gazebo::transport::Node());
  node->Init();

  // Listen to Gazebo world_stats topic
  gazebo::transport::SubscriberPtr sub = node->Subscribe("~/world_stats", cb);

  // Busy wait loop...replace with your own code as needed.
  while (true)
    gazebo::common::Time::MSleep(10);

  // Make sure to shut everything down.
  gazebo::shutdown();
}