Subscribers¶

Creating a Subscriber Node¶

Basic steps:

Create a node.
Create a subscription on a topic with a callback.
Spin the node to process received messages.

Example Subscriber (C++)¶

#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"

class MinimalSubscriber : public rclcpp::Node
{
public:
  MinimalSubscriber()
    : Node("minimal_subscriber")
  {
    subscription_ = this->create_subscription<std_msgs::msg::String>(
      "chatter",
      10,
      std::bind(&MinimalSubscriber::topic_callback, this, std::placeholders::_1));
  }

private:
  void topic_callback(const std_msgs::msg::String::SharedPtr msg)
  {
    RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg->data.c_str());
  }

  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
};

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<MinimalSubscriber>());
  rclcpp::shutdown();
  return 0;
}

Matching Topic Name and Type¶

The subscriber must use the same topic name as the publisher.
The subscriber must use the same message type as the publisher.

If either of these does not match:

No messages will be delivered.
Tools like ros2 topic info and ros2 node info can help you debug.

Testing Publisher and Subscriber Together¶

Run the publisher:
```
ros2 run my_package minimal_publisher
```
In another terminal, run the subscriber:
```
ros2 run my_package minimal_subscriber
```

You should see the messages printed by the subscriber.