How to extract and use laser data from /scan topic in ROS using C++?

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Here is an example:

ros::NodeHandle nh;
ros::Subscriber scanSub;

scanSub=nh.subscribe<sensor_msgs::LaserScan>("/scan",10,&AutoExp::processLaserScan,this);

You can implement the callback function however you like

void AutoExp::processLaserScan(const sensor_msgs::LaserScan::ConstPtr& scan){
     //scan->ranges[] are laser readings
     ...
}

Whenever a new laser reading is published to the "/scan" topic, your callback function is called, assuming that nothing goes wrong.

You can always use commands like

rostopic info /scan
rosmsg show sensor_msgs/LaserScan

to have a detailed information about the topic and the message type.

If you read the documentation of the message type, it helps a lot. Here is the link to sensor_msgs/LaserScan Message