I don't think I need to go into detail about the program I'm trying to make, but just ask if you need to know more about.

-In a certain part of my program, the user is asked to enter a three-digit number.
-The number that that user enters is then stored in a file called "used_codes.log".
-The three-digit number that the user enters is given to them in advance and corresponds to an algorithm with another three-digit number.
-I created another program to generate these two corresponding numbers so I don't have to do the math every time.
-The generator saves the numbers to a file called "issued_codes.log"
-What I want to be able to do, is make the generator, before printing a number, to check "issued_codes.log" and never give the same number twice.
-I also want my program to compare the number submitted by the user to the codes in "used_codes.log", and not accept any codes that have been used before

I was thinking of a code somewhere along the lines of this:
Code:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int maint()

{
   promptA:
   int usrCode;
   string usedCodes;
   cout << "Please enter your code: " << endl;
   cin >> usrCode;
   ifstream used_codes ("used_codes.log");
   while (! used_codes.eof())
   {
      getline (used_codes, usedCodes);
   }
   used_codes.close();
   if (usrCode   !=  "???")   // This is the problem, I don't know how to tell it that I don't want it to equal anything in the file.
                       //Also, I don't know how to make C++ recognize the individual numbers;
                       //I've only ever retrieved information from a file as a single string variable.
                       //I want to be able to retrieve the individual numbers as int */ )
   {
      ...
   }
   else
   {
      cerr << "You have entered an incorrect code. Please try again." << endl;
      goto promptA;
   }
   ............................
   ............................
   ............................
   system("PAUSE");
   return 0;
}
BTW, I've tried to convert string data types to int, float, double, and long data types before, but my program shows me a ☺, instead of what I had intended.