간단한 코드의 링크에러의 이유를 모르겠습니다.

다음의 코드가 컴파일은 되나 링크시 다음과 같은 에러가 발생합니다.

그런데 도대체 모르겠군요 ㅠ,ㅠ

코드 부분

#include <iostream>

using namespace std;

class BankAccount
{
public:
  void set(int dollers, int cents, double rate);
  void set(int dollers, double rate);
  void update();
  double get_balance();
  double get_rate();
  void output(ostream& outs);
  double difference(BankAccount cn1, BankAccount cn2);

private:
  double balance;
  double interest_rate;
  double fraction(double percent);
};


int main()
{
  BankAccount account1, account2;
  cout << "Start of Test:\n";

  account1.set(123, 99, 3.0);
  cout << "account 1 initial statement:\n";
  account1.output(cout);

  account1.set(100, 5.0);
  cout << "account1 with new setup:\n";
  account1.output(cout);

  account1.update();
  cout << "account1 after update:\n";
  account1.output(cout);

  account2 = account1;
  cout << "account2:\n";
  account2.output(cout);

  cout << account1.difference(account1, account2) << endl;

  return 0;
}

double difference(BankAccount cn1, BankAccount cn2)
{
  return (cn1.get_balance() - cn2.get_balance());
}

void BankAccount::set(int dollers, int cents, double rate)
{
  if(dollers < 0 || cents < 0 || rate < 0)
    {
      cout << "Illegal values for money or interest rate.\n";
      exit(1);
    }

  balance = dollers + 0.01 * cents;
  interest_rate = rate;
}

void BankAccount::set(int dollers, double rate)
{
  if(dollers < 0 || rate < 0)
    {
      cout << "Illegal values for money or interest rate.\n";
      exit(1);
    }
  balance  = dollers;
  interest_rate = rate;
}

void BankAccount::update()
{
  balance = balance + fraction(interest_rate) * balance;
}

double BankAccount::fraction(double percent_value)
{
  return (percent_value/100.0);
}

double BankAccount::get_balance()
{
  return balance;
}

double BankAccount::get_rate()
{
  return interest_rate;
}

void BankAccount::output(ostream& outs)
{
  outs.setf(ios::fixed);
  outs.setf(ios::showpoint);
  outs.precision(2);
  outs << "Account balance $" << balance << endl;
  outs << "Interest rate " << interest_rate << "%" << endl;
}

에러부분

VC 6.0에서 컴파일 했습니다.