C++ Login Program: How to add Forget Password function

Question

I recently created a simple C++ Login Program (check code below). However, I am not fully convinced unless I can solve the following issues I have in mind. I really need somebody's help with the following:

1. If I run the program for the first time, obviously I must not login successfully because there's no existing account, what can I do so that if I choose login and enter a username and password the program will output "Account does not Exist"

2. I also want the program to detect if I entered a wrong username or password to an existing account

3. How can I add forget password function?

Thank you so much.

#include<iostream>
#include<string>
#include<cstdlib>
#include<fstream>

using namespace std;

int choice;
bool cinfail;
int confirmation;
string username, password, password2;

void MainMenu();

void writetofile(string username){
     ofstream writefile;
     string file = username+".txt";
     writefile.open(file.c_str());
     writefile << password;
     writefile.close();
     MainMenu();
}

void login(){
     system("cls");
     cout<<"Username: "<<endl;
     cin>>username;
     cout<<"Password: "<<endl;
     cin>>password;

     if((username == username) && (password == password2)){
          cout<<"SUCCESSFUL LOGIN!";
     }
     else{
          cout<<"INVALID USERNAME OR PASSWORD!"<<endl;
     }
}

void RegisterPassword(){
     cout<<"Please enter the password: "<<endl;
     cin>>password;
     cout<<"Please reenter your password: "<<endl;
     cin>>password2;

     if(password == password2){
         cin.clear();
         cin.ignore(10000, '\n');
         writetofile(username);
         exit(1);
     }
     else{
          cout<<"Sorry, invalid password. Try again."<<endl;
          RegisterPassword();
     }
     system("cls");
}

void registerme(){
     system("cls");
     cout<<"REGISTER ACCOUNT"<<endl;
     cout<<"Please enter your username: "<<endl;
     getline(cin, username);
     cout<<"\nUsername - \"" <<username<< "\"\nConfirm? \n\n[1] Yes\n[2] No"<<endl;
     cin>>confirmation;

     if(confirmation == 1){
         RegisterPassword();
     }
     else{
         cout<<"Sorry, invalid input. Try again"<<endl;
         cin.clear();
         cin.ignore(10000, '\n');
         registerme();
     }
}

void exit(){
     exit(0);
}

void MainMenu(){
     cout<<"SIMPLE LOGIN PROGRAM by RZAM\n[1] Login\n[2] Register\n[3] Exit"<<endl;
     cin>>choice;

     do{
          cinfail = cin.fail();
          cin.clear();
          cin.getline(10000,'\n');

          }while(cinfail == true);{
               switch(choice){
                      case 1:
                           login();
                           break;

                      case 2:
                           registerme();
                           break;

                      case 3:
                           exit();
               }
          }
}

main(){
MainMenu();
}

Answer 1

For any forgotten passwords we would usually send an email asking if it's a legitimate request. You can't do that, maybe save the users date of birth when they login or a secret question. They can use this to enter a new password.

For detecting the wrong password to an existing account:

1. Read all usernames into an array
2. Check the entered username against all of them
3. If the username exists, and the password is wrong you can alert the user.