I'm trying to assign a value in a callback function and I get error for undendifined reference.

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I hava class UltrasonicSensors. I have my callback funtion as method and I try to assign the value of callback and then to use it in my method which has the subscriber. But I get this error:

/home/alex/catkin_ws/src/ros_ackerman_monster_truck/src/obstacle_avoidance/UltrasonicSensors.cpp:-1: error: undefined reference to `UltrasonicSensors::m_msg'
:-1: error: collect2: error: ld returned 1 exit status

Header file:

class UltrasonicSensors
        {

        public:
            UltrasonicSensors() {}
            ~UltrasonicSensors(){}

        static void u1_callback(const std_msgs::Int32& us_msg);
        static void u2_callback(const std_msgs::Int32& us_msg);
        static void u3_callback(const std_msgs::Int32& us_msg);
    int ultrasonic(const char* topic_name);
    static std_msgs::Int32 m_msg;

Cpp file::

    #include "UltrasonicSensors.h"
    #include <string.h>
    void UltrasonicSensors::u1_callback(const std_msgs::Int32& us_msg)
    {
     ROS_INFO("[%i]efefefe" , us_msg.data );
     m_msg = us_msg; // Here I want to assign the value of us_msg
    }
    void UltrasonicSensors::u2_callback(const std_msgs::Int32& us_msg)
    {ROS_INFO("[%i]" , us_msg.data );}


    void UltrasonicSensors::u3_callback(const std_msgs::Int32& us_msg)
    {ROS_INFO("[%i]" , us_msg.data );}

    int UltrasonicSensors::ultrasonic(const char* topic_name )
    {
        if(strcmp(topic_name,"ultrasonic_sensor_middle"))
            m_sub->subscribe(m_subscriber, "ultrasonic_sensor_middle", m_nh, u1_callback);
        else if(strcmp(topic_name,"ultrasonic_sensor_left"))
            m_sub->subscribe(m_subscriber, "ultrasonic_sensor_left", m_nh, u2_callback);
        else if(strcmp(topic_name,"ultrasonic_sensor_right"))
            m_sub->subscribe(m_subscriber, "ultrasonic_sensor_right", m_nh, u3_callback);
        else{ROS_ERROR("Invalid topic name for ultrasonic sensors!");}

      return m_msg.data;   //And return it here 
}