23.4.8. Class Transaction

Class Transaction (Figs. 23.28–23.29) is an abstract base class that represents the notion of an ATM transaction. It contains the common features of derived classes BalanceInquiry, Withdrawal and Deposit. Figure 23.28 expands upon the Transaction header file first developed in Section 23.3. Lines 13, 17–19 and 22 contain function prototypes for the class’s constructor and four member functions, which we discuss shortly. Line 15 defines a virtual destructor with an empty body—this makes all derived-class destructors virtual (even those defined implicitly by the compiler) and ensures that dynamically allocated derived-class objects get destroyed properly when they are deleted via a base-class pointer. Lines 24–26 declare the class’s private data members. Recall from the class diagram of Fig. 23.11 that class Transaction contains an attribute accountNumber (implemented in line 24) that indicates the account involved in the Transaction. We derive data members screen (line 25) and bankDatabase (line 26) from class Transaction’s associations modeled in Fig. 23.10—all transactions require access to the ATM’s screen and the bank’s database, so we include references to a Screen and a BankDatabase as data members of class Transaction. As you’ll soon see, Transaction’s constructor initializes these references. The forward declarations in lines 6–7 signify that the header file contains references to objects of classes Screen and BankDatabase, but that the definitions of these classes lie outside the header file.

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Fig. 23.28. Transaction class definition.

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Fig. 23.29. Transaction class member-function definitions.

Class Transaction has a constructor (declared in line 13 of Fig. 23.28 and defined in lines 8–15 of Fig. 23.29) that takes the current user’s account number and references to the ATM’s screen and the bank’s database as arguments. Because Transaction is an abstract class, this constructor will never be called directly to instantiate Transaction objects. Instead, the constructors of the Transaction derived classes will use base-class initializer syntax to invoke this constructor.

Class Transaction has three public get functions—getAccountNumber (declared in line 17 of Fig. 23.28 and defined in lines 18–21 of Fig. 23.29), getScreen (declared in line 18 of Fig. 23.28 and defined in lines 24–27 of Fig. 23.29) and getBankDatabase (declared in line 19 of Fig. 23.28 and defined in lines 30–33 of Fig. 23.29). Transaction derived classes inherit these member functions from Transaction and use them to gain access to class Transaction’s private data members.

Class Transaction also declares a pure virtual function execute (line 22 of Fig. 23.28). It does not make sense to provide an implementation for this member function, because a generic transaction cannot be executed. Thus, we declare this member function to be a pure virtual function and force each Transaction derived class to provide its own concrete implementation that executes that particular type of transaction.