Password to a program - c++

Question

I've made an app that solves some computations with matrices, and I want to authenticate users in order to grant them access when the program starts.
I will show you what I've done already.
int main()
{
const string USERNAME = "claudiu";
const string PASSWORD = "123456";
string usr, pass;
cout << "Enter Username : ";
cin >> usr;
if(usr.length() < 4)
{
cout << "Username length must be atleast 4 characters long.";
}
else
{
cout << "Enter Password : ";
cin >> pass;
if(pass.length() < 6)
{
cout << "Password length must be atleast 6 characters long";
}
else
{
if(usr == USERNAME && pass == PASSWORD)
{
cout << "\n\nSuccessfully granted access" << endl;
}
else
{
cout << "Invalid login details" << endl;
}
}
}
This is how my code looks like. All I want to do is that when I enter a wrong username or a wrong password the program shows the message I wrote and then let me introduce another username and password and when I introduce them correctly, the program starts.

Answer 1

I would make a logged_in variable, then set it to true when the condition is passed and run the whole login process in a while loop:
#include <iostream>
#include <string>
using namespace std;
int main()
{
const string USERNAME = "claudiu";
const string PASSWORD = "123456";
string usr, pass;
bool logged_in = false;
while (!logged_in)
{
cout << "Enter Username : ";
cin >> usr;
if (usr.length() < 4)
{
cout << "Username length must be atleast 4 characters long.";
}
else
{
cout << "Enter Password : ";
cin >> pass;
if (pass.length() < 6)
{
cout << "Password length must be atleast 6 characters long";
}
else
{
if (usr == USERNAME && pass == PASSWORD)
{
cout << "\n\nSuccessfully granted access" << endl;
logged_in = true;
}
else
{
cout << "Invalid login details" << endl;
}
}
}
}
cout << "Passed login!\n";
}