How to return values from callback function

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Hi All,

I am using below function in future to know the asynchronus result of the thread but the thing is this callback will have no return type as ros::spin needs to be installed here. I would like to know how to design this function so that return type can be given to below function including ros::spin.

void subscriber :: handlecallback( )
{
ros::Subscriber sub = n.subscribe("chatter", 1000, &Listener::callback, &listener);

ros::Subscriber sub1 = n.subscribe("ABC", 1000, &Listener::callback, &listener);

ros::Subscriber sub1 = n.subscribe("CBA", 1000, &Listener::callback, &listener);

ros::spin();
}


std::future<void> result = std::async(&subscriber::handlecallback, &abc, rand()%20);


        try {
            std::cout << "Task returned " << result.get() << "\n";// result.get can't have void here.
        } catch (const std::exception &ex) {
            std::cout << "Task exited with exception: " << ex.what() << "\n";
            std::cerr << "Thread exited with exception: " << ex.what() << "\n";
        }