How do I publish exactly one message?

edit

In addition to what @Lorenz said, latching the connection might be useful. Also, I'm not sure what happens when your node exits right after sending the message, so waiting for some time (or for some subscribers) might be needed even when using latched connections.

Based on the comments from Lorenz, the problem seems to be that the publisher is too fast; that is, it publishes before the subscriber has opened a direct connection to the publisher. Therefore, the goal is to make the publisher "wait" until all subscribers have established a connection. I don't think that the problem stems from closing the publisher too soon.

Here is an idea that I have not tested, but that should work with multiple subscribers. It is not 100% robust, but assuming that no subscribers comes or leaves exactly when the publisher is launched, it should not have problems.

EDIT: This method would not work because a functionality that I expected available from ROS does not seem to be there. See comments in the code.

int numSubscribersConnected = 0;

void connectCallback(const ros::SingleSubscriberPublisher& pub)
{
    numSubscribersConnected++;
}

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

    ros::NodeHandle nh("~");

    ros::Publisher pub = nh.advertise<foo::MyMessage>("/myTopic", 100, connectCallback);

    ros::Rate poll_rate(100);

    ros::Time maxTime(ros::Time::now()+1000); // Will wait at most 1000 ms

    // This is wrong. It returns only the number of subscribers that have already established their direct connections to this publisher
    //int numExistingSubscribers = pub.getNumSubscribers();

    // This method does not exist, as far as I know. In the comments below, I suggested having a similar functionality. This method would return the number of total subscribers to this topic over the ROS network
    int numExistingSubscribers = ros::getNumSubscribers("/myTopic");

    while(numExistingSubscribers!=numSubscribersConnected && ros::Time::now()<maxTime)
    {
        poll_rate.sleep();
    }

    // Now the publisher is ready

    foo::MyMessage msg;
    pub.publish(msg);

    return 0;
}