Derived class problem

Hey everyone, I'm trying to complete a question out of my C++ texbook but having some problems. Basically, i have a BankAccount class. I then have to write a derived class for a cheque account which has an additional attribute of an overdraft charge.Any withdrawal that will leave the balance with a negative value will have a charge made. This charge is subtracted from the balance (i.e. balance becomes more negative)  each time a withdrawal results in a negative balance.

The ChequeAccount will have function to get the overdraft charge, and the constructor will take 3 parameters:  one to initialise accNum, one to initialise balance and the third to initialise the overdraft charge. i will lateralter BankAccount::withdraw to accommodate the extra charge.

But so far im having enough problems implementing the derived class and sorting out its member functions, can anyone give me some help please?

#include <iostream>
using namespace std;

class BankAccount{

 public:
      BankAccount( int num, double amount);
      BankAccount();

      void deposit(double amount);
      void withdraw( double amount);

      int getAccNum() const;
      double getBalance() const;  
      
    void printDetails()const;
   
protected:
   double balance;
   int accNum;
};


BankAccount::BankAccount (int num, double amount)
{
     balance = amount;
     accNum = num;
}




BankAccount::BankAccount()
{  
     balance = 0;
     accNum = 100;
}  


void BankAccount::deposit (double amount)
{
   balance+= amount;
}

void BankAccount::withdraw (double amount)
{
      if (balance -=amount < 0)
      cout << "test" << endl;
}

      
double BankAccount::getBalance() const  
{
      return balance;
}


int BankAccount::getAccNum() const
{
      return accNum;
}


void BankAccount::printDetails() const
{
   cout<<" Account number "<< accNum <<" has a balance of " << balance << endl;
}
   

class ChequeAccount: public BankAccount{
public:
      ChequeAccount(int num, int amount, int charge);
      double GetOverdraft();
      int getCharge();
private:
      double overdraft;
};


double ChequeAccount::GetOverdraft()
{
      return overdraft;
}

ChequeAccount::ChequeAccount(int num, double amount, double charge)
{
      balance = amount;
    accNum = num;
      overdraft = charge;

}

int ChequeAccount::getCharge()
{
      return overdraft;
}
int main()
{
      ChequeAccount chq  (1234, 50, 5);

      chq.printDetails();
      
      chq.withdraw(55);

      return 0;
}

Thanks to anyone who may be able to help

David