I think you should think about migrating the code with OOP, but overall - this migration guide can help you with change from ROS1 to ROS2. There is a tutorial from ROS-Industrial too.

E.g. this example code:

#include <sstream>
// #include "ros/ros.h"
#include "rclcpp/rclcpp.hpp"
// #include "std_msgs/String.h"
#include "std_msgs/msg/string.hpp"
int main(int argc, char **argv)
{
//  ros::init(argc, argv, "talker");
//  ros::NodeHandle n;
  rclcpp::init(argc, argv);
  auto node = rclcpp::Node::make_shared("talker");
//  ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);
//  ros::Rate loop_rate(10);
  auto chatter_pub = node->create_publisher<std_msgs::msg::String>("chatter", 1000);
  rclcpp::Rate loop_rate(10);
  int count = 0;
//  std_msgs::String msg;
  std_msgs::msg::String msg;
//  while (ros::ok())
  while (rclcpp::ok())
  {
    std::stringstream ss;
    ss << "hello world " << count++;
    msg.data = ss.str();
//    ROS_INFO("%s", msg.data.c_str());
    RCLCPP_INFO(node->get_logger(), "%s\n", msg.data.c_str());
    chatter_pub->publish(msg);
//    ros::spinOnce();
    rclcpp::spin_some(node);
    loop_rate.sleep();
  }
  return 0;
}

is from the first migration guide. It's with a simple publisher in C++, but subscribing is a bit more complicated, I guess.

EDIT: I found this example on GitHub (there: https://github.com/ros2/examples/tree/foxy/rclcpp/topics/minimal_subscriber) in not_composable.cpp:

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

rclcpp::Node::SharedPtr g_node = nullptr;

/* We do not recommend this style anymore, because composition of multiple
 * nodes in the same executable is not possible. Please see one of the subclass
 * examples for the "new" recommended styles. This example is only included
 * for completeness because it is similar to "classic" standalone ROS nodes. */

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

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  g_node = rclcpp::Node::make_shared("minimal_subscriber");
  auto subscription =
    g_node->create_subscription<std_msgs::msg::String>("topic", 10, topic_callback);
  rclcpp::spin(g_node);
  rclcpp::shutdown();
  // TODO(clalancette): It would be better to remove both of these nullptr
  // assignments and let the destructors handle it, but we can't because of
  // https://github.com/eProsima/Fast-RTPS/issues/235 .  Once that is fixed
  // we should probably look at removing these two assignments.
  subscription = nullptr;
  g_node = nullptr;
  return 0;
}