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Thanks to @Delb and @gvdhoom. Here is my working solution. I didn't implement the spin thread since it would consume a lot of cpu resource.

#include <ros/ros.h>
#include <std_msgs/Float32.h>
#include <sensor_msgs/Joy.h>
#include <std_msgs/Bool.h>


bool flag;


class Override
{
public:
    Override();
  void joyCallback(const sensor_msgs::Joy::ConstPtr& joy);
  void Publisher();
    std_msgs::Bool override_status;

private:
    ros::NodeHandle nh_;
    ros::Publisher override_pub;
  ros::Subscriber joy_sub_;
  bool currentReading;
    bool lastReading;
  std_msgs::Float32 button_status;

};

Override::Override()
{

  override_pub = nh_.advertise<std_msgs::Bool>("override_status", 1);
  joy_sub_ = nh_.subscribe<sensor_msgs::Joy>("joy", 10, &Override::joyCallback, this);


}

void Override::joyCallback(const sensor_msgs::Joy::ConstPtr& joy)
{
   button_status.data=joy->buttons[4];

  if (button_status.data == 0.0){
    currentReading = false;
  }else currentReading = true;

  if (currentReading && !lastReading) {
    flag=!flag;
    if (flag) {
      override_status.data = true;
    }
    else {
      override_status.data = false;
    }
  }
  lastReading = currentReading;

}

void Override::Publisher()
{
  override_pub.publish(override_status);
}




int main(int argc, char** argv)
{
  ros::init(argc, argv, "override_node");
  ROS_INFO("Node Started...");
  Override override;
  ros::Rate r(10);
  while (ros::ok()){
    override.Publisher();
    ros::spinOnce();    
  }
}