Arduino Ethernet Driver | Custom hardware publishing on topic via ethernet

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Max Carr did a fantastic job, thanks man! Having said that, I had a hard time with (1) getting the Arduino code figured out, and (2) a C++ program to communicate with the Arduino. My contribution (if you can call it that) to this topic is to provide the following:

• Arduino code that uses sockets (not HTML or HTTP) to communicate to a C++ program. Arduino program can receive different types of requests from C++, and dish out the requested data.
• C++ easily sends messages to Arduino, and collects the response.

You still have to convert the C++ code to a ROS package, but that is where Max Carr's answer is not at all ambiguous, and there are a ton of resources to figure that out on the ROS website. I will touch on that at the end. However the bulk of thsi answer deals with Arduino to C++ communication via sockets.

The hardware is a PC and an Arduino Mega with an ethernet shield. I have a switch in between.

Quick note on the Arduino side of things - I see Arduino as a cheap, and effective way to collect the status of sensors (both analog and digital), and control simple devices (lights, relays, etc...). Since you are running ROS, you have ample processing power in your PC, so I don't see you sending the Arduino tons of data to crunch. right? So, the code below receives a 3 character request, where the first two characters are capital letters, followed by a ~ as the end of transmission terminator. The Arduino code receives this message, and depending on the combination of letters (ex: AA~, or AB~, or ZZ~, etc....) the Arduino will take the appropriate action (send a pre-canned command to some device, turn on actuators, or in my case respond with the status of various sensors). In my case I am sending 8 variables back to the C++ program (6 int and 2 float). I am packaging each data type with letters. so the response looks like this:

A123B
C456D
E98F
H-23.4532I
....
~~~

With the 3 ~ characters serving as the end of transmission.

Arduino Code

#include <SPI.h>
#include <Ethernet.h>
#include <SimpleTimer.h> //see: https://playground.arduino.cc/Code/SimpleTimer
#include "arduino2.h" //see: https://www.codeproject.com/Articles/732646/Fast-digital-I-O-for-Arduino

// The Mac address is BADA5512456 (BAD A55 [ASS] followed by 123456).
byte mac[] = {0xBA, 0xDA, 0x55, 0x12, 0x34, 0x56};

//The IP address is obviously static, and there is no conflicts with any 
//other device on the robot. 
IPAddress ip(192, 168, 0, 21);

// Initialize the Ethernet server library and assign  
// and port you want to use (port 80 is default for HTTP):
EthernetServer server(23500);

//variables that allow me to figure out what the client is requesting
int iterator = 0;
//made bigher than 3 elements to accomodate bad requests that don't follow my AA~ protocol;
//where the client sends a letter, followed by a letter followed by a ~ character to 
//let the server know the ...