G.5. Class CashDispenser
Class CashDispenser (Figs. G.7G.8) represents the cash dispenser of the ATM. The class definition (Fig. G.7) contains the function prototype for a default constructor (line 9). Class CashDispenser declares two additional public member functionsdispenseCash (line 12) and isSufficientCashAvailable (line 15). The class trusts that a client (i.e., Withdrawal) calls dispenseCash only after establishing that sufficient cash is available by calling isSufficientCashAvailable. Thus, dispenseCash simply simulates dispensing the requested amount without checking whether sufficient cash is available. Line 17 declares private constant INITIAL_COUNT, which indicates the initial count of bills in the cash dispenser when the ATM starts (i.e., 500). Line 18 implements attribute count (modeled in Fig. 13.29), which keeps track of the number of bills remaining in the CashDispenser at any time.
Figure G.7. CashDispenser class definition.
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1 // CashDispenser.h 2 // CashDispenser class definition. Represents the ATM's cash dispenser. 3 #ifndef CASH_DISPENSER_H 4 #define CASH_DISPENSER_H 5 6 class CashDispenser 7 { 8 public: 9 CashDispenser(); // constructor initializes bill count to 500 10 11 // simulates dispensing of specified amount of cash 12 void dispenseCash( int ); 13 14 // indicates whether cash dispenser can dispense desired amount 15 bool isSufficientCashAvailable( int ) const; 16 private: 17 const static int INITIAL_COUNT = 500; 18 int count; // number of $20 bills remaining 19 }; // end class CashDispenser 20 21 #endif // CASH_DISPENSER_H |
Figure G.8. CashDispenser class member-function definitions.
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1 // CashDispenser.cpp 2 // Member-function definitions for class CashDispenser. 3 #include "CashDispenser.h" // CashDispenser class definition 4 5 // CashDispenser default constructor initializes count to default 6 CashDispenser::CashDispenser() 7 { 8 count = INITIAL_COUNT; // set count attribute to default 9 } // end CashDispenser default constructor 10 11 // simulates dispensing of specified amount of cash; assumes enough cash 12 // is available (previous call to isSufficientCashAvailable returned true) 13 void CashDispenser::dispenseCash( int amount ) 14 { 15 int billsRequired = amount / 20; // number of $20 bills required 16 count -= billsRequired; // update the count of bills 17 } // end function dispenseCash 18 19 // indicates whether cash dispenser can dispense desired amount 20 bool CashDispenser::isSufficientCashAvailable( int amount ) const 21 { 22 int billsRequired = amount / 20; // number of $20 bills required 23 24 if ( count >= billsRequired ) 25 return true; // enough bills are available 26 else 27 return false; // not enough bills are available 28 } // end function isSufficientCashAvailable |
CashDispenser Class Member-Function Definitions
Figure G.8 contains the definitions of class CashDispenser's member functions. The constructor (lines 69) sets count to the initial count (i.e., 500). Member function dispenseCash (lines 1317) simulates cash dispensing. If our system were hooked up to a real hardware cash dispenser, this member function would interact with the hardware device to physically dispense cash. Our simulated version of the member function simply decreases the count of bills remaining by the number required to dispense the specified amount (line 16). Note that line 15 calculates the number of $20 bills required to dispense the specified amount. The ATM allows the user to choose only withdrawal amounts that are multiples of $20, so we divide amount by 20 to obtain the number of billsRequired. Also note that it is the responsibility of the client of the class (i.e., Withdrawal) to inform the user that cash has been dispensedCashDispenser cannot interact directly with Screen.
Member function isSufficientCashAvailable (lines 2028) has a parameter amount that specifies the amount of cash in question. Lines 2427 return TRue if the CashDispenser's count is greater than or equal to billsRequired (i.e., enough bills are available) and false otherwise (i.e., not enough bills). For example, if a user wishes to withdraw $80 (i.e., billsRequired is 4), but only three bills remain (i.e., count is 3), the member function returns false.