callback function c++

edit

I have been trying to implement a call back function with no success.

Here is the problem. I have a subscriber that is supposed to listen to a topic. Let's call it A. A is of type std_msgs/Float32

How can I implement a callback function?

#include "ros/ros.h"
#include "geometry_msgs/Twist.h"
#include "std_msgs/Float32.h"
#include <sstream>

using namespace std;

ros::Publisher velocity_publisher;
ros::Subscriber pose_sub;
std_msgs::Float32 ball_pose;

void poseCallback(const std_msgs::Float32::ConstPtr & pose_message);
//void moveUp(std_msgs::Float32 distance_tolerance);

int main(int argc, char **argv)
{
    ros::init(argc, argv, "sphero_move");
    ros::NodeHandle n;

    velocity_publisher = n.advertise<geometry_msgs::twist>("/cmd_vel", 1000);
    pose_sub = n.subscribe("/ball_pose_x", 10, poseCallback);   
    ros::Rate loop_rate(0.5);

    //moveUp(30.00);
    loop_rate.sleep();

    ros::spin();

    return 0;
}

void poseCallback(const std_msgs::Float32::ConstPtr & pose_message)
{
    ball_pose = pose_message->data;
}

/**void moveUp(std_msgs::Float32 distance_tolerance)
{
    geometry_msgs::Twist vel_msg;
    ros::Rate loop_rate(10);

    do{
        vel_msg.linear.x = 25;
        vel_msg.linear.y = 0;
        vel_msg.linear.z = 0;

        velocity_publisher.publish(vel_msg);

        ros::spinOnce();
        loop_rate.sleep();

        }while((ball_pose-500)<distance_tolerance); }**="" <="" pre="">

I want to be able to update the position of the robot in every iteration to be able to act on the current position, since the robot is moving.
 
Here is the error I am receiving.
 
/home/sphero/catkin_ws/src/sphero_controller/src/sphero_move.cpp: In function ‘void poseCallback(const ConstPtr&)’:
/home/sphero/catkin_ws/src/sphero_controller/src/sphero_move.cpp:34:12: error: no match for ‘operator=’ (operand types are ‘std_msgs::Float32 {aka std_msgs::Float32_<std::allocator<void> >}’ and ‘const _data_type {aka const float}’)
  ball_pose = pose_message->data;
            ^
In file included from /home/sphero/catkin_ws/src/sphero_controller/src/sphero_move.cpp:3:0:
/opt/ros/kinetic/include/std_msgs/Float32.h:22:8: note: candidate: std_msgs::Float32_<std::allocator<void> >& std_msgs::Float32_<std::allocator<void> >::operator=(const std_msgs::Float32_<std::allocator<void> >&)
 struct Float32_
        ^
/opt/ros/kinetic/include/std_msgs/Float32.h:22:8: note:   no known conversion for argument 1 from ‘const _data_type {aka const float}’ to ‘const std_msgs::Float32_<std::allocator<void> >&’
sphero_controller/CMakeFiles/sphero_move_node.dir/build.make:62: recipe for target 'sphero_controller/CMakeFiles/sphero_move_node.dir/src/sphero_move.cpp.o' failed
make[2]: * [sphero_controller/CMakeFiles/sphero_move_node.dir/src/sphero_move.cpp.o] Error 1
CMakeFiles/Makefile2:808: recipe for target 'sphero_controller/CMakeFiles/sphero_move_node.dir/all' failed
make[1]:  [sphero_controller/CMakeFiles/sphero_move_node.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: ** [all] Error 2