Using Repl.It

In order to help the Kin and Queen, we will develop a program, in a programming language called C++. With that program we will be able to input a message, and the program will modify it to make it secret.


To be able to run our C++ programs we will use a tool called that will allow us to build and run C++ code in an interactive window like the one below. We will use that tool throughout the entire tutorial.

Before getting any further let’s understand how these windows works. The file main.cpp contains the code, or instructions that the computer will understand and run. Clicking on Run button will command the computer to execute the instructions in that file. The results can be seen in the console tab, in the lower area of

Next, let’s explain the code that we have in main.cpp.

Libraries and “#include”

Go to the beginning of main.cpp. You will find there the following line:

#include <iostream>

This directive or instruction refers to the iostream library. A library is a common place where a set of C++ functions, typically related to a particular common problem or topic, are grouped together. There are libraries to draw, to create and manage internet connections, to perform mathematical operations and for several many other purposes. iostream is a very basic library that allows us to receive or print out messages to and from the console.


The grayed out line(s) in main.cpp starts with //. These are known as comments. Comments are useful in our C++ programs as a way to document them better, and explain in more detail what the goal of the instructions is. Comments are not instructions and are actually ignored by the computer.

Namespace std

using namespace std;

This specific line allows us to access a few already reserved C++ words. A reserved word is a word that has a specific meaning in a given programming language; due to that, it cannot be reused for any other purpose. Here, we are referencing the reserved word std to use the code from the already well-designed and developed C++ standard. using namespace std means we want to make reference to the standard namespace.

Main Function

int main(){
return 0;


main is a mandatory function in any C++ program. It is the entry point of our program. Within the open and closing braces { … } we will place all the instructions that we wish the computer to execute. After clicking Run the computer will execute those instructions. return 0; indicates the program has finished successfully its execution. If instructions were placed after that line, the computer would not execute them.