how to subscriber from /diagnastic

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Hi All, Please help me for how to subscriber from /diagnastic according levels(OK , WARN , ERROR). below my example code ...

Example :

DiagnosticSubscriber::DiagnosticSubscriber()
{

    cout<<"Constructor calling"<<endl;

}

void DiagnosticSubscriber::msgCallback(const diagnostic_msgs::DiagnosticStatus::ConstPtr& DiagnosticStatusmsg)
{
    cout<<"msgCallback calling"<<endl;
    ROS_INFO("CPP Subscriber  ");

}

void DiagnosticSubscriber::display()
{
    cout<<"display calling"<<endl;
    sub = nh.subscribe("/diagnostics",1,&DiagnosticSubscriber::msgCallback,this);

}


int main(int argc , char **argv)
{
    ros::init(argc,argv,"DiagnosticSubscriber");
    DiagnosticSubscriber obj;
    obj.display();
    ros::spin();
    return 0;
}