Signature of callback function for creating a timer

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In my opinion, that "TF example" link is abusing the API in a very bad way is an advanced use that is very confusing to read. I suggest you ignore that example.

The Timer tutorial is the right way to code the callback for a Timer object.

Update: for your second link, if this is the only thing the node is going to do, I wouldn't use Timer object at all. This code is untested:

ros::init(argc, argv, "test");
ros::NodeHandle nh
tf::TransformListener listener(ros::Duration(10));;
ros::Rate rate(1); // Hz
while (ros::ok()) {
    transformPoint(listener);
    rate.sleep();
}

If you must use a Timer, then create a global ptr to listener and use it inside the callback. Just keep in mind that you have to create the listener after calling ros::init().