CmakeLists.txt

cmake_minimum_required(VERSION 2.8.3)
project(test_issue)

find_package(catkin REQUIRED COMPONENTS
  roscpp
  std_msgs
)

catkin_package()

include_directories(
  ${catkin_INCLUDE_DIRS}
)

add_executable(${PROJECT_NAME}_node src/test.cpp)

target_link_libraries(${PROJECT_NAME}_node
  ${catkin_LIBRARIES}
)

test.cpp

#include "ros/ros.h"
#include "std_msgs/String.h"

#include <sstream>

class A
{
  public:
    A(ros::NodeHandle& nh) 
    {   
        sub = nh.subscribe("cmd", 10, &A::cmd_callback, this);
        cmd_received = "none";
    }   

    void cmd_callback(const std_msgs::String::ConstPtr& msg)
    {   
        cmd_received = msg->data;
        std::cout<< "cmd received in callback: " << cmd_received << std::endl;
    }   

    void update()
    {   
        ROS_INFO_STREAM_THROTTLE(2.0, "cmd received: " << cmd_received);
    }   

  private:
    std::string cmd_received;
    ros::Subscriber sub;
};


int main(int argc, char **argv)
{
    ros::init(argc, argv, "test");

    ros::NodeHandle n;

    A a(n);
    ros::Rate loop_rate(500);

    while (ros::ok())
    {   
        a.update();
        ros::spinOnce();

        loop_rate.sleep();
    }   


    return 0;
}