Learn C++

#include <iostream> /* Includes different resources to be used in the program */
#include <string>

using namespace std; /* Removes the std:: so it can just be cout not std::cout */

void update() { /* A function */
	system("CLS"); /* Clears console */
}

int main(int agrv, char** argc) { /* The main function */
	update(); /* Calls the update function */

	string newUsername; /* Strings */
	string newPassword;
	string username;
	string password;

	std::cout << "New Username: "; /* cout = console output. << is going out so it's output*/
	cin >> newUsername; /* cin = console input. >> is going in so it's input */

	std::cout << "New Password: ";
	cin >> newPassword;

	if (newUsername.length() < 3) { /* If the length of the username is less than 3 do the following */
		update();

		std::cout << "The length of the username cannot be less than 5 characters!" << endl << endl;
		std::cout << "[Press Any Key To Continue]";
		system("PAUSE >NULL");

		int main(); /* Calls the main function */
		main();
	}
	else if (newUsername.length() > 20) { /* If the username is above 20 characters, do the following */
		std::cout << "The length of the username cannot be above 20 characters!" << endl << endl;
		std::cout << "[Press Any Key To Continue]";
		system("PAUSE >NULL");
	}
	else if (newUsername == newPassword || newPassword == newUsername) { /* If newPassword is equal to newUsername, do the following. == is equal to */
		int main();
		main();
	}
	else if (newPassword.length() < 5) {
		update();

		std::cout << "The length of the password cannot be less than 5 characters! " << endl;
		std::cout << "[Press Any Key To Continue]";
		system("PAUSE >NULL");

		int main();
		main();
	}

	update();

	std::cout << "Plase Login" << endl << endl;

	std::cout << "Username: ";
	cin >> username;

	std::cout << "Password: ";
	cin >> password;

	if (username != newUsername) { /* If username is not equal to newUsername, != is not equal */
		update();

		std::cout << "Username or password do not math!" << endl;
		std::cout << "[Press Any Key To Continue]";
		system("PAUSE >NULL");
	}
	else if (password != newPassword) {
		update();

		std::cout << "Username or password do not math!" << endl;
		std::cout << "[Press Any Key To Continue]";
		system("PAUSE >NULL");
	}

	update();

	std::cout << "Successfully logged in as " << username << "!" << endl;
	std::cout << "[Press Any Key To Continue]";
	system("PAUSE >NULL"); /* Pause the console, >NULL just pauses it without it printing 'Press any key to continue' */

	return 0; /* Returns a value, 0 as true / no errors, passed :) 1 as it has an error, say if you're checking for a file and it doesn't exist, you'll do 'return 1' */
}