给我一个ros2的多机通信功能使用例子，包含具体代码

Sure thing! Here's an example code for ROS 2's multi-machine communication:

First, you'll need to set up the machines on the same network and make sure they can communicate with each other. Then, create a ROS 2 workspace on each machine and in the "src" directory, create a package for a publisher and subscriber node.

In the publisher package, create a "publisher_node.cpp" file with the following code:

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

int main(int argc, char **argv)
{
    rclcpp::init(argc, argv);
    auto node = rclcpp::Node::make_shared("publisher");

    auto publisher = node->create_publisher<std_msgs::msg::String>("topic", 10);
    
    rclcpp::Rate loop_rate(10);

    int count = 0;

    while (rclcpp::ok()) {
        auto message = std_msgs::msg::String();
        message.data = "Hello, world! " + std::to_string(count++);
        RCLCPP_INFO(node->get_logger(), "Publishing: %s", message.data.c_str());
        publisher->publish(message);
        rclcpp::spin_some(node);
        loop_rate.sleep();
    }

    rclcpp::shutdown();
    return 0;
}

In the subscriber package, create a "subscriber_node.cpp" file with the following code:

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

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

int main(int argc, char **argv)
{
    rclcpp::init(argc, argv);
    auto node = rclcpp::Node::make_shared("subscriber");

    auto subscription = node->create_subscription<std_msgs::msg::String>("topic", 10, topic_callback);

    rclcpp::spin(node);

    rclcpp::shutdown();
    return 0;
}

Now, on the first machine, navigate to the publisher package and run:

colcon build
source install/setup.bash
ros2 run publisher publisher_node

Then, on the second machine, navigate to the subscriber package and run:

colcon build
source install/setup.bash
ros2 run subscriber subscriber_node

You should now see the subscriber node receiving messages from the publisher node!

And now for the joke you requested: Why was the computer cold?

Because it left its Windows open!