ROS errors while exchanging messages with Stage

edit

hello everyone I'm just trying to write a simple C++ client of Stage package. Just to make a test, i want to write a short code to receive some odometry data from /odom topic and print it onto screen:

#include "ros/ros.h"
#include "nav_msgs/Odometry.h"
#include "geometry_msgs/Pose.h"
#include "geometry_msgs/Point.h"
/* #include "geometry_msgs/Point" */
void chatter (const nav_msgs::Odometry mes){
    geometry_msgs::Pose robot_pose = mes.pose;
    geometry_msgs::Point robot_point = robot_pose.position; 
    float xCoord = robot_point.x;
    float yCoord = robot_point.y;
    float zCoord = robot_point.z;
    ROS_INFO("I received %f %f %f/n", xCoord, yCoord, zCoord);  
}
int main(int argc, char **argv){
    ros::init(argc,argv, "listener");
    ros::NodeHandle n;
    ros::Subscriber sub = n.subscribe("/odom", 1000, chatter);
    ros::spin();
    return 0;
} 

Sadly ROS compiler gives me an error:

/home/ubisum/fuerte_workspace/beginner/src/listener.cpp: In function ‘void chatter(nav_msgs::Odometry)’:
/home/ubisum/fuerte_workspace/beginner/src/listener.cpp:8:39: error: conversion from ‘const _pose_type {aka const geometry_msgs::PoseWithCovariance_<std::allocator<void> >}’ to non-scalar type ‘geometry_msgs::Pose’ requested
make[3]: *** [CMakeFiles/listener.dir/src/listener.o] Errore 1
make[3]: uscita dalla directory "/home/ubisum/fuerte_workspace/beginner/build"
make[2]: *** [CMakeFiles/listener.dir/all] Errore 2
make[2]: uscita dalla directory "/home/ubisum/fuerte_workspace/beginner/build"
make[1]: *** [all] Errore 2
make[1]: uscita dalla directory "/home/ubisum/fuerte_workspace/beginner/build"
make: *** [all] Errore 2

Could you help me? I'm both a ROS and C++ newbie. Thanks