manipulating file using functions - c++

Question

Hi I am trying to write program that a program that allows the user to transfer an amount from an account (the account being text file "shop" which simply contains the value 100).
I want to have it so that the user can make as many transfers as they want, without overdrawing the account. The file also needs to be updated after each transaction. Can anyone help me with where I'm going wrong?
int read_balance(void);
void write_balance(int balance);
#include <limits>
#include "stdafx.h"
#include <iostream>
#include <fstream>
using namespace std;
int _tmain(int argc, _TCHAR * argv[])
{
std::cout << "You have choosen to transfer an amount" << std::endl;
std::cout << "How much do you wish to transfer from the shop account?" << std::endl;
int amount = 0;
if (std::cin >> amount)
{
std::cout << "DEBUG: amount:" << amount << "\n";
int balance = read_balance();
if (amount <= 0)
{
std::cout << "Amount must be positive\n";
}
else if (balance < amount)
{
std::cout << "Insufficient funds\n";
}
else
{
int new_balance = balance - amount;
write_balance(new_balance);
std::cout << "New account balance: " << new_balance << std::endl;
}
}
system("pause");
return 0;
}
int read_balance(void)
{
std::ifstream f;
f.exceptions(std::ios::failbit | std::ios::badbit);
f.open("shop.txt");
int balance;
f >> balance;
f.close();
return balance;
}
void write_balance(int balance)
{
std::ofstream f;
f.exceptions(std::ios::failbit | std::ios::badbit);
f.open("shop.txt");
f << balance;
f.close();
}

Answer 1

As the compiler warns you when using precompiled headers for stdafx.h the #include "stdafx.h" must be the first line of code. So it's better to start with
#include "stdafx.h"
#include <limits>
#include <iostream>
#include <fstream>
using namespace std;
int read_balance(void);
void write_balance(int balance);