Applying the ABC Concept
You’d probably like to see a complete example of an ABC, so let’s apply the concept to representing the Brass and BrassPlus accounts, starting with an ABC called AcctABC. This class should contain all methods and data members that are common to both the Brass and the BrassPlus classes. The methods that are to work differently for the BrassPlus class than they do for the Brass class should be declared as virtual functions. At least one virtual function should be a pure virtual function in order to make the AcctABC class abstract.
Listing 13.11 is a header file that declares the AcctABC class (an ABC) and the Brass and BrassPlus classes (both concrete classes). To facilitate derived class access to base class data, AcctABC provides some protected methods. Recall that protected methods are methods that derived-class methods can call but that are not part of the public interface for derived-class objects. AcctABC also provides a protected member function to handle the formatting previously handled by nonmember functions. Also the AcctABC class has two pure virtual functions, so it is, indeed, an abstract class.
// acctabc.h -- bank account classes
#ifndef ACCTABC_H_
#define ACCTABC_H_
#include <iostream>
#include <string>
// Abstract Base Class
class AcctABC
{
private:
std::string fullName;
long acctNum;
double balance;
protected:
struct Formatting
{
std::ios_base::fmtflags flag;
std::streamsize pr;
};
const std::string & FullName() const {return fullName;}
long AcctNum() const {return acctNum;}
Formatting SetFormat() const;
void Restore(Formatting & f) const;
public:
AcctABC(const std::string & s = "Nullbody", long an = -1,
double bal = 0.0);
void Deposit(double amt) ;
virtual void Withdraw(double amt) = 0; // pure virtual function
double Balance() const {return balance;};
virtual void ViewAcct() const = 0; // pure virtual function
virtual ~AcctABC() {}
};
// Brass Account Class
class Brass :public AcctABC
{
public:
Brass(const std::string & s = "Nullbody", long an = -1,
double bal = 0.0) : AcctABC(s, an, bal) { }
virtual void Withdraw(double amt);
virtual void ViewAcct() const;
virtual ~Brass() {}
};
//Brass Plus Account Class
class BrassPlus : public AcctABC
{
private:
double maxLoan;
double rate;
double owesBank;
public:
BrassPlus(const std::string & s = "Nullbody", long an = -1,
double bal = 0.0, double ml = 500,
double r = 0.10);
BrassPlus(const Brass & ba, double ml = 500, double r = 0.1);
virtual void ViewAcct()const;
virtual void Withdraw(double amt);
void ResetMax(double m) { maxLoan = m; }
void ResetRate(double r) { rate = r; };
void ResetOwes() { owesBank = 0; }
};
#endif
The next step is to implement the methods that don’t already have inline definitions. Listing 13.12 does that.
// acctabc.cpp -- bank account class methods
#include <iostream>
#include "acctabc.h"
using std::cout;
using std::ios_base;
using std::endl;
using std::string;
// Abstract Base Class
AcctABC::AcctABC(const string & s, long an, double bal)
{
fullName = s;
acctNum = an;
balance = bal;
}
void AcctABC::Deposit(double amt)
{
if (amt < 0)
cout << "Negative deposit not allowed; "
<< "deposit is cancelled.
";
else
balance += amt;
}
void AcctABC::Withdraw(double amt)
{
balance -= amt;
}
// protected methods for formatting
AcctABC::Formatting AcctABC::SetFormat() const
{
// set up ###.## format
Formatting f;
f.flag =
cout.setf(ios_base::fixed, ios_base::floatfield);
f.pr = cout.precision(2);
return f;
}
void AcctABC::Restore(Formatting & f) const
{
cout.setf(f.flag, ios_base::floatfield);
cout.precision(f.pr);
}
// Brass methods
void Brass::Withdraw(double amt)
{
if (amt < 0)
cout << "Withdrawal amount must be positive; "
<< "withdrawal canceled.
";
else if (amt <= Balance())
AcctABC::Withdraw(amt);
else
cout << "Withdrawal amount of $" << amt
<< " exceeds your balance.
"
<< "Withdrawal canceled.
";
}
void Brass::ViewAcct() const
{
Formatting f = SetFormat();
cout << "Brass Client: " << FullName() << endl;
cout << "Account Number: " << AcctNum() << endl;
cout << "Balance: $" << Balance() << endl;
Restore(f);
}
// BrassPlus Methods
BrassPlus::BrassPlus(const string & s, long an, double bal,
double ml, double r) : AcctABC(s, an, bal)
{
maxLoan = ml;
owesBank = 0.0;
rate = r;
}
BrassPlus::BrassPlus(const Brass & ba, double ml, double r)
: AcctABC(ba) // uses implicit copy constructor
{
maxLoan = ml;
owesBank = 0.0;
rate = r;
}
void BrassPlus::ViewAcct() const
{
Formatting f = SetFormat();
cout << "BrassPlus Client: " << FullName() << endl;
cout << "Account Number: " << AcctNum() << endl;
cout << "Balance: $" << Balance() << endl;
cout << "Maximum loan: $" << maxLoan << endl;
cout << "Owed to bank: $" << owesBank << endl;
cout.precision(3);
cout << "Loan Rate: " << 100 * rate << "%
";
Restore(f);
}
void BrassPlus::Withdraw(double amt)
{
Formatting f = SetFormat();
double bal = Balance();
if (amt <= bal)
AcctABC::Withdraw(amt);
else if ( amt <= bal + maxLoan - owesBank)
{
double advance = amt - bal;
owesBank += advance * (1.0 + rate);
cout << "Bank advance: $" << advance << endl;
cout << "Finance charge: $" << advance * rate << endl;
Deposit(advance);
AcctABC::Withdraw(amt);
}
else
cout << "Credit limit exceeded. Transaction cancelled.
";
Restore(f);
}
The FullName() and AcctNum() protected methods provide read-only access to the fullName and acctNum data members and make it possible to customize ViewAcct() a little more individually for each derived class.
This version makes a couple of improvements in the implementation of the formatting. The previous version used two function calls to set the formatting and one to restore it:
format initialState = setFormat();
precis prec = cout.precision(2);
...
restore(initialState, prec); // restore original format
The new version defines a structure to hold the two formatting values and uses that structure to set and restore formats with just two calls:
struct Formatting
{
std::ios_base::fmtfglas flag;
std::streamsize pr;
};
...
Formatting f = SetFormat();
...
Restore(f);
It’s a neater look.
A problem with the older version was that the original setFormat() and restore() were standalone functions, so those function names would conflict with client-defined functions of the same name. There are several ways to solve that problem. One is to declare both functions static, making them private to the brass.cpp file or to its successor, acctabc.cpp. A second is to place both functions and the struct Formatting definition into a namespace. But one of the topics for this example is protected access, so this example places the structure definition and the functions in the protected part of the class definition. This makes them available to the base class and the derived class while hiding them from the outside world.
This new implementation of the Brass and BrassPlus accounts can be used in the same manner as the old one because the class methods have the same names and interfaces as before. For example, to convert Listing 13.10 to use the new implementation, you just need to take these steps to convert usebrass2.cpp to a usebrass3.cpp file:
• Link usebrass2.cpp with acctabc.cpp instead of with brass.cpp.
• Include acctabc.h instead of brass.h.
• Replace
Brass * p_clients[CLIENTS];
with
AcctABC * p_clients[CLIENTS];
Listing 13.13 shows the resulting file, renamed usebrass3.cpp.
// usebrass3.cpp -- polymorphic example using an abstract base class
// compile with acctacb.cpp
#include <iostream>
#include <string>
#include "acctabc.h"
const int CLIENTS = 4;
int main()
{
using std::cin;
using std::cout;
using std::endl;
AcctABC * p_clients[CLIENTS];
std::string temp;
long tempnum;
double tempbal;
char kind;
for (int i = 0; i < CLIENTS; i++)
{
cout << "Enter client's name: ";
getline(cin,temp);
cout << "Enter client's account number: ";
cin >> tempnum;
cout << "Enter opening balance: $";
cin >> tempbal;
cout << "Enter 1 for Brass Account or "
<< "2 for BrassPlus Account: ";
while (cin >> kind && (kind != '1' && kind != '2'))
cout <<"Enter either 1 or 2: ";
if (kind == '1')
p_clients[i] = new Brass(temp, tempnum, tempbal);
else
{
double tmax, trate;
cout << "Enter the overdraft limit: $";
cin >> tmax;
cout << "Enter the interest rate "
<< "as a decimal fraction: ";
cin >> trate;
p_clients[i] = new BrassPlus(temp, tempnum, tempbal,
tmax, trate);
}
while (cin.get() != '
')
continue;
}
cout << endl;
for (int i = 0; i < CLIENTS; i++)
{
p_clients[i]->ViewAcct();
cout << endl;
}
for (int i = 0; i < CLIENTS; i++)
{
delete p_clients[i]; // free memory
}
cout << "Done.
";
return 0;
}
The program itself behaves the same as the non-abstract base class version, so given the same input as for Listing 13.10, the output would be the same.