Question

I’m trying to redefine two member functions from their parent’s definition.
I don’t know if I have it right or not, but something in my code has errors attached and I can’t find out what.

some of the header:

class Account 
{
public:
   Account(double);
   void creditBalance(double);
   void debitBalance(double);
   double getBalance() const;
protected:
   double balance;      
};


class CheckingAccount : public Account
{
public:
   CheckingAccount(double, double);
   void feeCreditBalance(double);
   void feeDebitBalance(double);

private:
   double fee = 10;

};

relevant cpp file part:

void Account::creditBalance(double plus)
{
   if(plus > 0)
      balance += plus;
   else
      cout << "Cannot credit negative.";
}

void Account::debitBalance(double minus)
{
   if(minus <= balance)
      balance -= minus;
   else
      cout << "Debit amount exceeded account balance.";
}

void CheckingAccount::feeCreditBalance(double plus)
{
   if(plus > 0){
      balance += plus;
      balance -= fee;
   }
   else
      cout << "Cannot credit negative.";
}

void CheckingAccount::feeDebitBalance(double minus)
{
   if(minus <= balance){
      balance -= minus;
      balance -= fee;
   }
   else
      cout << "Debit amount exceeded account balance.";
}

UPDATE:

I added this:

class Account 
{
public:
   Account(double);
   virtual void creditBalance(double);
   virtual void debitBalance(double);
   double getBalance() const;
protected:
   double balance;      
};

Now I get error: virtual outside class declaration

I could use an example of how to properly initialize fee correctly.

EDIT 2:

I have tried changing the constructor line to this:

CheckingAccount::CheckingAccount(double initBal, double phi) :  Account(initBal), fee(phi)
{
   if(initBal < 0)
      initBal = 0;
   balance = initBal;
   cerr << "Initial balance was invalid.";

   if(phi < 0)
      phi = 0;
   fee = phi;
}

not working, I’m going to work around with changing syntax on the fee(phi) part. I don’t know if anyone will respond to this.

Answer 1

If the intention is that for instances of CheckingAccount accounts the versions which use a fee is called, then you want to use virtual methods.

A virtual method is a method decalared (at least in the base class) as “virtual”, and has the same name and signiture in any derived classes. When a virtual method is called, it will call the “most derived” version for the instance.

To do this just declare “void creditBalance(double);” and “void debitBalance(double);” virtual (ie “virtual void creditBalance(double);” and “virtual void debitBalance(double);”. Then in CheckingAccount rename “feeCreditBalance” and “feeDebitBalance” to “creditBalance” and “debitBalance”.

EDIT: Simple example.

Header

class Base
{
public:
    Base(int x);
    //declare virtual so that derived classes may overide it
    virtual void sayHello();
protected:
    int x;
};

class Derived : public Base
{
public:
    Derived(int x, int y);
    //overide Base::sayHello(). the virtual keyword here is optional.
    virtual void sayHello();
protected:
    int y;
};

Cpp

Base::Base(int x)
    :x(x)
{}
Derived::Devired(int x, int y)
    :Base(x), y(y)
{}
void Base::sayHello()
{
    std::cout << "Hello from Base!" << std::endl;
    std::cout << "X = " << x << std::endl;
}
void Derived::sayHello()
{
    std::cout << "Hello from Derived!" << std::endl;
    std::cout << "X = " << x << "  Y = " << y << std::endl;
}

int main()
{
    Base a(5);
    a.sayHello();//"Hello from Base!..."

    Derived b(10, -20);
    b.sayHello();//"Hello from Derived!..."

    Base *c = &b;//take pointer to b, reference would also do
    c->sayHello();//"Hello from Derived!..." because its a Derived instance, eventhough its a Base variable.
}

You can then derive from Derive again (class DerivedAgain : public Derived) and also overload the functions again.

You can also derive multiple subclasses from Base, which can each have their own overrides for the virtual methods, just like I did for the Derived class.

EDIT2: Added variables to example and how to use initialiser list to initialise the base class and member variables.