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With image transport, you don't actually have to manage all the publishers yourself. The ImageTransport object will do that for you. As for your question though... You can create a shared pointer with an empty deleter function to be able to pass to Image Transport:

class CamPublisher_ : public rclcpp::Node
{
protected:
    rclcpp::Node::SharedPtr node_handle_;
    image_transport::ImageTransport image_transport_;
    image_transport::Publisher image_publisher_;
    ...
public:
    CamPublisher_() :
            Node("camera"),
            // create the shared_ptr node handle for this node.
            node_handle_(std::shared_ptr<CamPublisher_>(this, [](auto *) {})),
            image_transport_(node_handle_),
            image_publisher_(image_transport.advertise("image", 10)
    {
        ....
    }
    void convert_and_publish(const cv::Mat& frame)
    {
        sensor_msgs::msg::Image msg;
        ....
        image_publisher_->publish(msg);
    }
}