How can i get infomation of the subscription of a node

edit

I have to write code to get the simulation of a robot that explores certains worlds with the stage simulator. The point is that i want to get information of the laser scanner and i don't know how to do it...

I have the subscription...

 // Create a subscriber object 
 ros::Subscriber sub = nh.subscribe("/base_scan", 1000, processScanCallback);

where processScanCallback is:

void processScanCallback(const sensor_msgs::LaserScan::ConstPtr& msg)
{
    int n_ranges = msg->ranges.size();
    double nearest = *(std::min_element(msg->ranges.begin(),msg->ranges.end()));
        ROS_INFO_STREAM("I've a total of " << n_ranges << " measurements to process! Are you ready? Nearest="  nearest );
}

and i call ros::SpinOnce(); inside a while-loop (where the robot is constantly doing measures and moving randomly), but those measurements are inside the function processScanCallback in the varible nearest. So, i have messages in the rosout node every time that the robot do a measure, but i can't do something with that value because is not returned...

How can i receive the value nearest from the processScanCallback function?