Publishing unbound array in ROS2 topic

edit

Hello,

I have a custom msg with one element uint8[] a. In my publisher (example from the tutorial tutorial_interfaces, I am trying to populate that unbounded array but I am getting an error. Here is the code:

#include <chrono>
#include <memory>

#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/msg/num.hpp"     // CHANGE

using namespace std::chrono_literals;

class MinimalPublisher : public rclcpp::Node
{
public:
  MinimalPublisher()
  : Node("minimal_publisher"), count_(0)
  {
    publisher_ = this->create_publisher<tutorial_interfaces::msg::Num>("topic", 10);    // CHANGE
    timer_ = this->create_wall_timer(
      500ms, std::bind(&MinimalPublisher::timer_callback, this));
  }

private:
  void timer_callback()
  {
    auto message = tutorial_interfaces::msg::Num();                               // CHANGE
    //message.a = 170;//this->count_++;                                        // CHANGE
    //message.b = 171;this->count_++;                                        // CHANGE
    message.a.push_back(170);
    message.a.push_back(171);
    //RCLCPP_INFO(this->get_logger(), "Publishing: '%d'", message.a);    // CHANGE
    publisher_->publish(message);
  }
  rclcpp::TimerBase::SharedPtr timer_;
  rclcpp::Publisher<tutorial_interfaces::msg::Num>::SharedPtr publisher_;         // CHANGE
  size_t count_;
};

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

And the error I get is:

root@4f18f7e74ccb:/home/wd_dev# colcon build --packages-select tutorial_interfaces
Starting >>> tutorial_interfaces
[0.331s] WARNING:colcon.colcon_core.prefix_path.colcon:The path '/home/ws_dev/install' in the environment variable COLCON_PREFIX_PATH doesn't exist
[0.331s] WARNING:colcon.colcon_ros.prefix_path.ament:The path '/home/ws_dev/install/tutorial_interfaces' in the environment variable AMENT_PREFIX_PATH doesn't exist
[0.331s] WARNING:colcon.colcon_ros.prefix_path.ament:The path '/home/ws_dev/install/cpp_pubsub' in the environment variable AMENT_PREFIX_PATH doesn't exist
[0.331s] WARNING:colcon.colcon_ros.prefix_path.catkin:The path '/home/ws_dev/install/tutorial_interfaces' in the environment variable CMAKE_PREFIX_PATH doesn't exist
[0.331s] WARNING:colcon.colcon_ros.prefix_path.catkin:The path '/home/ws_dev/install/cpp_pubsub' in the environment variable CMAKE_PREFIX_PATH doesn't exist
--- stderr: tutorial_interfaces                             
/home/wd_dev/src/tutorial_interfaces/src/publisher.cpp: In member function ‘void MinimalPublisher::timer_callback()’:
/home/wd_dev/src/tutorial_interfaces/src/publisher.cpp:26:15: error: request for member ‘push_back’ in ‘message.tutorial_interfaces::msg::Num_<std::allocator<void> >::a’, which is of non-class type ‘tutorial_interfaces::msg::Num_<std::allocator<void> >::_a_type’ {aka ‘unsigned char’}
   26 |     message.a.push_back(170);
      |               ^~~~~~~~~
/home/wd_dev/src/tutorial_interfaces/src/publisher.cpp:27:15: error: request for member ‘push_back’ in ‘message.tutorial_interfaces::msg::Num_<std::allocator<void> >::a’, which is of non-class type ‘tutorial_interfaces::msg::Num_<std::allocator<void> >::_a_type’ {aka ‘unsigned char’}
   27 |     message.a.push_back(171);
      |               ^~~~~~~~~
make[2]: *** [CMakeFiles/talker.dir/build.make:63: CMakeFiles/talker.dir/src/publisher.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:123: CMakeFiles/talker.dir/all] Error 2
make: *** [Makefile:141: all] Error 2
---
Failed   <<< tutorial_interfaces [3.14s, exited with code 2]

Summary: 0 packages finished [3.35s]
  1 package failed: tutorial_interfaces
  1 package had stderr output: tutorial_interfaces

That same line of code used to work with ROS1. Has anything changed? Thanks for the help.