Implementing the ATM System Design Incorporating Inheritance

We now modify our implementation to incorporate inheritance, using class Withdrawal as an example.

1. If a class A is a generalization of class B, then class B is derived from (and is a specialization of) class A. For example, abstract base class Transaction is a generalization of class Withdrawal. Thus, class Withdrawal is derived from (and is a specialization of) class Transaction. Figure 23.12 contains a portion of class Withdrawal’s header file, in which the class definition indicates the inheritance relationship between Withdrawal and Transaction (line 9).

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 1   // Fig. 23.12: Withdrawal.h
 2   // Definition of class Withdrawal that represents a withdrawal transaction
 3   #ifndef WITHDRAWAL_H
 4   #define WITHDRAWAL_H
 5
 6   #include "Transaction.h" // Transaction class definition
 7
 8   // class Withdrawal derives from base class Transaction
 9   class Withdrawal : public Transaction                  
10   {
11   }; // end class Withdrawal
12
13   #endif // WITHDRAWAL_H

Fig. 23.12. Withdrawal class definition that derives from Transaction.

2. If class A is an abstract class and class B is derived from class A, then class B must implement the pure virtual functions of class A if class B is to be a concrete class. For example, class Transaction contains pure virtual function execute, so class Withdrawal must implement this member function if we want to instantiate a Withdrawal object. Figure 23.13 contains the C++ header file for class Withdrawal from Fig. 23.10 and Fig. 23.11. Class Withdrawal inherits data member accountNumber from base class Transaction, so Withdrawal does not declare this data member. Class Withdrawal also inherits references to the Screen and the BankDatabase from its base class Transaction, so we do not include these references in our code. Figure 23.11 specifies attribute amount and operation execute for class Withdrawal. Line 19 of Fig. 23.13 declares a data member for attribute amount. Line 16 contains the function prototype for operation execute. Recall that, to be a concrete class, derived class Withdrawal must provide a concrete implementation of the pure virtual function execute in base class Transaction. The prototype in line 16 signals your intent to override the base class pure virtual function. You must provide this prototype if you’ll provide an implementation in the .cpp file. We present this implementation in Section 23.4. The keypad and cashDispenser references (lines 20–21) are data members derived from Withdrawal’s associations in Fig. 23.10. In the implementation of this class in Section 23.4, a constructor initializes these references to actual objects. Once again, to be able to compile the declarations of the references in lines 20–21, we include the forward declarations in lines 8–9.

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 1   // Fig. 23.13: Withdrawal.h
 2   // Definition of class Withdrawal that represents a withdrawal transaction
 3   #ifndef WITHDRAWAL_H
 4   #define WITHDRAWAL_H
 5
 6   #include "Transaction.h" // Transaction class definition
 7
 8   class Keypad; // forward declaration of class Keypad
 9   class CashDispenser; // forward declaration of class CashDispenser
10
11   // class Withdrawal derives from base class Transaction
12   class Withdrawal : public Transaction
13   {
14   public:
15      // member function overriding execute in base class Transaction
16      virtual void execute(); // perform the transaction
17   private:
18      // attributes
19      double amount; // amount to withdraw
20      Keypad &keypad; // reference to ATM's keypad
21      CashDispenser &cashDispenser; // reference to ATM's cash dispenser
22   }; // end class Withdrawal
23
24   #endif // WITHDRAWAL_H

Fig. 23.13. Withdrawal class header file based on Figs. 23.10 and 23.11.

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