how to pass pointcloud from the callback function in subscriber

edit

Avoid global variables. it is bad practice. here is how you could do it:

class pass_cloud
{
public:
pass_cloud (ros::NodeHandle& nh) : nh_(nh), i(0)
{
  sub = nh_.subscribe ("/camera/depth_registered/points",     3,&pass_cloud::callback,this);
} // your constructor and store the passed node handle (nh) to your class attribute nh_ 

void callback (const pcl::PointCloud& cloud) 
{
 input= cloud; // copy the variable that the callback passes in to you class variable (attribute) input
 i=i+1;
}

process()
{
//You might want to add an if statement here to only print if the input is not empty, I am not sure how but something like this if(input is not empty){
std::cout << " " << input.points[i].x << " " << input.points[i].y <<  " " << input.points[i].z << std::endl; std::cout<<"reached here"<<std::endl; 

}

void spin ()
{
 ros::Rate rate (30);
 while (ros::ok ())
 {
 ros::spinOnce ();
 rate.sleep ();
 }
}

protected: // Your class attributes
ros::NodeHandle nh_;
pcl::PointCloud& input;
cloud ros::Subscriber sub;
int i;
}; // end of class

int main(int argc, char **argv)
{

ros::init(argc, argv, "pointcloud_node");
ros::NodeHandle nh;

pass_cloud* node = 0;

node = new pass_cloud(nh);

node->spin (); 
return 0;
}

Hope this helps!