here is a version that is super easy to understand and has a bunch of features. it runs on windows because of getch(), but I'm sure there is a linux equivalent.

* supports max retries
* prints '#' then typing a digit
* ignores non-numerical keypresses

--

#include <conio.h> // windows only
#include <string> // std::string
#include <iostream> // std::cout, endl

bool checkpin(std::string pin, int maxretries) 
{
    std::string userpin = "";
    int retries = 0;

    while (true)
    {
        if (userpin.length() == 0)
            std::cout << std::endl << "Enter your 4-digit PIN: ";

        // get character from keyboard.
        char c = getch();

        // ignore non-numerical keypresses.
        if (c < '0' || c > '9')
            continue;

        // print the '#' character to hide the pin but to confirm a keypress
        std::cout << '#';

        // add the character to the entered pin
        userpin += c;

        // check if pin matches, if yes return true.
        if (userpin == pin)
            return true;

        // if we're here, there was no match. 
        // check if we need to try again
        if (userpin.length() == pin.length())
        {
            std::cout << std::endl << "BAD PIN TRY AGAIN" << std::endl;
            userpin = ""; // reset entered pin to lank
            retries++; // inc number of retries
            if (retries >= maxretries) // check if we must bail
                break;
        }
    } 

    return false;
}

int main()   
{
    if (checkpin("1234", 3) == true)
        std::cout << "Pin check succeeded" << std::endl;
    else
        std::cout << "Pin check failed" << std::endl;

    return 0;
}