How to get the number of subscribers on a topic?

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Never used it myself, but perhaps the TopicManager would help?

I'm not sure why you're implying you can't use rostopic list from your node. Nodes can call command line tools. Unfortunately, there doesn't seem to be a an easier way to find this info through the C++ or Python API. I just scrape the output of rostopic list <topic> --verbose in my custom nodes.

It will output something like:

Published topics:
 * <topic> [std_msgs/String] 1 publisher

Subscribed topics:
 * <topic> [std_msgs/String] 1 subscriber

The number before publisher/subscriber is the number of nodes publishing to or subscribing to that topic respectively. You can easily use regexp or simple string parsing to extract that number.

#include <ros/topic_manager.h>
ros::TopicManager::instance()->getNumSubscribers("topic")

This returns the number of subscribing nodes, not single Subscriber objects...