main.cpp

#include <iostream>
using namespace std;

#include "cf_serial/cf_serial_simple.h"

int main(int argc, char **argv)
{
   try {
      cf_serial_simple serial("COM8");

      string prompt="ping";
      while(true) {

         // send prompt to Arduino
         serial.write_string(prompt);
         cout << prompt << " -> ";

         // receive reply
         string reply;
         size_t timeout_ms = 1050;
         if(serial.read(reply,timeout_ms)) {
            cout <<  reply << endl;

            // send the reply back as new prompt
            prompt = reply;
         }
         else {
            cout << "timeout" << endl;
         }

      }
   }
   catch(std::exception& ex) {
      cout << "Error: " << ex.what() << endl;
   }

   return 0;
}

ping_pong.ino

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
   if(Serial.available() > 0) {
      String input = Serial.readString();
      if(input=="ping") Serial.write("pong");
      else if(input=="pong") Serial.write("ping");
      else Serial.write(" huh?");
   }
}