In order to accommodate an encoded string that might have a terminal character, I'm trying to pass the data as a custom message with a char array rather than a standard string. Everything in the publisher seems to be fine, but when I try to read the array back using an iterator the error seems to tell me that the array entry is no longer the std::vector<char> that I expected, but now a std::vector<unsigned char, std::allocator<unsigned char> >::iterator that I can't assign to the iterator type I want to use.

This is my custom message, EncodedString.msg:

char[] data
int32 length

and this is the subscriber code where I try to read it back out:

#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/msg/encoded_string.hpp"

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

private:
  void topic_callback(const tutorial_interfaces::msg::EncodedString::SharedPtr msg) const
  {
    int char_length = msg->length;

    std::string encoded_bytes = "";
    std::vector<char>::iterator char_it;
    for( char_it = msg->data.begin(); msg->data.end(); char_it++ )
    {
      encoded_bytes.append( msg->data[char_it] );
    }
  }
  rclcpp::Subscription<tutorial_interfaces::msg::EncodedString>::SharedPtr subscription_;
};

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

If it looks familiar, it's because I'm using the standard publisher/subscriber to test the concept before I do anything more complicated, so hopefully this is a proper minimal example. The problem arises when I go to build the package, and the first error that I get is:

Starting >>> cpp_pubsub
--- stderr: cpp_pubsub                             
/home/teddybouch/workspace/ros2_ws/src/cpp_pubsub/src/subscriber_member_function.cpp: In member function ‘void MinimalSubscriber::topic_callback(tutorial_interfaces::msg::EncodedString_<std::allocator<void> >::SharedPtr) const’:
/home/teddybouch/workspace/ros2_ws/src/cpp_pubsub/src/subscriber_member_function.cpp:45:36: error: no match for ‘operator=’ (operand types are ‘std::vector<char>::iterator’ {aka ‘__gnu_cxx::__normal_iterator<char*, std::vector<char> >’} and ‘std::vector<unsigned char, std::allocator<unsigned char> >::iterator’ {aka ‘__gnu_cxx::__normal_iterator<unsigned char*, std::vector<unsigned char, std::allocator<unsigned char> > >’})
   45 |     for( char_it = msg->data.begin(); msg->data.end(); char_it++ )
      |                                    ^
In file included from /usr/include/c++/9/bits/stl_algobase.h:67,
                 from /usr/include/c++/9/memory:62,
                 from /home/teddybouch/workspace/ros2_ws/src/cpp_pubsub/src/subscriber_member_function.cpp:15:
/usr/include/c++/9/bits/stl_iterator.h:784:11: note: candidate: ‘constexpr __gnu_cxx::__normal_iterator<char*, std::vector<char> >& __gnu_cxx::__normal_iterator<char*, std::vector<char> >::operator=(const __gnu_cxx::__normal_iterator<char*, std::vector<char> >&)’
  784 |     class __normal_iterator
      |           ^~~~~~~~~~~~~~~~~
/usr/include/c++/9/bits/stl_iterator.h:784:11: note:   no known conversion for argument 1 from ‘std::vector<unsigned char, std::allocator<unsigned char> >::iterator’ {aka ‘__gnu_cxx::__normal_iterator<unsigned char*, std::vector<unsigned char, std::allocator<unsigned char> > >’} to ‘const __gnu_cxx::__normal_iterator<char*, std::vector<char> >&’
/usr/include/c++/9/bits/stl_iterator.h:784:11: note: candidate: ‘constexpr __gnu_cxx::__normal_iterator<char*, std::vector<char> >& __gnu_cxx::__normal_iterator<char*, std::vector<char> >::operator=(__gnu_cxx::__normal_iterator<char*, std::vector<char> >&&)’
/usr/include/c++/9/bits/stl_iterator.h:784:11: note:   no known conversion for argument 1 from ‘std::vector<unsigned char, std::allocator<unsigned char> >::iterator’ {aka ‘__gnu_cxx::__normal_iterator<unsigned char*, std::vector<unsigned char, std::allocator<unsigned char> > >’} to ‘__gnu_cxx::__normal_iterator<char*, std::vector<char> >&&’

Can someone please clarify what is going on here for me?