using advertise function with SubscriberStatusCallback parameter [closed]

edit

I have a publisher node and I would like to call a callback function when a subscriber connects.

I've the same code of @Benoit Larochelle in this post (mine is the same problem) but instead of

ros::Time maxTime(ros::Time::now()+ros::Duration(1)); // Will wait at most 1000 ms
while(ros::Time::now()<maxTime)
{
    ros::spinOnce();
}

I use this piece of code to give the time to subscriber to connect (actually, i stuck in the while loop until there are no subscriber to my topic)

while(!pub.getNumSubscribers()) 
{  } //it doesn't work even if I put ros::spinOnce into the while loop

But in this way it doesn't work...And not even if I try with

ros::Rate loop_rate(1);
loop_rate.sleep();

Note: also if I use the above code with no ros::spinOnce in the loop:

ros::Time maxTime(ros::Time::now()+ros::Duration(1)); // Will wait at most 1000 ms
while(ros::Time::now()<maxTime)
{
    /* NO ros::spinOnce() */ 
}

I have the same situation.

What's the problem? Can you help me to understand?

Thank you all