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Polling Subscriber#

Agnocast provides agnocast::PollingSubscriber, a pull-based subscription that lets you fetch the latest message on demand rather than receiving it via a callback. This corresponds to the subscription->take() pattern in rclcpp, but returns a zero-copy shared pointer from shared memory instead of copying data into a provided message.

PollingSubscriber is available at both Stage 1 (rclcpp::Node) and Stage 2 (agnocast::Node).

Migration from rclcpp take()#

Before (rclcpp)#

```cpp class MyPollingNode : public rclcpp::Node { rclcpp::Subscription::SharedPtr sub_; rclcpp::TimerBase::SharedPtr timer_;

void timer_callback() { std_msgs::msg::String msg; rclcpp::MessageInfo msg_info; if (sub_->take(msg, msg_info)) { RCLCPP_INFO(get_logger(), "Polled: %s", msg.data.c_str()); } }

public: MyPollingNode() : Node("my_polling_node") { sub_ = create_subscription( "/topic", rclcpp::QoS(rclcpp::KeepLast(1)), {}); // dummy callback required by rclcpp

timer_ = create_wall_timer(1s,
  std::bind(&MyPollingNode::timer_callback, this));

} }; ```

After (Agnocast Stage 1)#

```cpp class MyPollingNode : public rclcpp::Node { agnocast::PollingSubscriber::SharedPtr sub_; // (1) rclcpp::TimerBase::SharedPtr timer_;

void timer_callback() { auto msg = sub_->take_data(); // (2) if (msg) { RCLCPP_INFO(get_logger(), "Polled: %s", msg->data.c_str()); } }

public: MyPollingNode() : Node("my_polling_node") { sub_ = agnocast::create_subscription( // (3) this, "/topic", rclcpp::QoS(rclcpp::KeepLast(1)));

timer_ = create_wall_timer(1s,
  std::bind(&MyPollingNode::timer_callback, this));

} }; ```

Key changes:

  1. rclcpp::Subscription → agnocast::PollingSubscriber (no dummy callback needed)
  2. sub_->take(msg, msg_info) → sub_->take_data() which returns an agnocast::ipc_shared_ptr<const T> (zero-copy)
  3. Use agnocast::create_subscription free function (no callback argument)

After (Agnocast Stage 2)#

```cpp class MyPollingNode : public agnocast::Node // (1) { agnocast::PollingSubscriber::SharedPtr sub_; agnocast::TimerBase::SharedPtr timer_; // (2)

void timer_callback() { auto msg = sub_->take_data(); if (msg) { RCLCPP_INFO(get_logger(), "Polled: %s", msg->data.c_str()); } }

public: MyPollingNode() : Node("my_polling_node") { sub_ = this->create_subscription( // (3) "/topic", rclcpp::QoS(rclcpp::KeepLast(1)));

timer_ = this->create_wall_timer(1s,
  std::bind(&MyPollingNode::timer_callback, this));

} }; ```

Additional changes from Stage 1:

  1. Base class changes to agnocast::Node
  2. Timer type changes to agnocast::TimerBase::SharedPtr
  3. Subscription creation uses member function instead of free function