How can I use the data from the LaserScan.msg inside the callback function?

edit

The answer you linked is just an example where the person didn't modify his code. AutoExp::processLaserScan just means that this person created a method processLaserScan (exactly how you defined LaserCallback) within a class named AutoExp.

The relevant part of the answer is :

//scan->ranges[] are laser readings

You need to use the field ranges to get the laser scan data. For your question, you want to use the field range_max so you just have to do this :

void LaserCallback(const sensor_msgs::LaserScan::ConstPtr& scan)
{
   float x = scan->range_max;
}

You can see here all the fields from sensor_msgs/LaserScan.msg.

Quick answer:

float x = scan -> range_max;

The callback function receives a 'LaserScan' message as a parameter. In order to access its fields, you need to resolve/access it correctly.

Alternatively, were you to access, say the 'seq', int i = scan -> header.seq

In the answer linked, "scan->ranges[] are laser readings" refers to the data from the message that you would process.