Object-oriented Software in Ada 95

154 Inheritance
The abstract class can then be used to derived specific types of bank account. In the following case it has been
used to derive a simple bank account.
with Class_Abstract_Account;
use Class_Abstract_Account;
package Class_Account is
type Account is new Abstract_Account with private;
subtype Money is Class_Abstract_Account.Money;
subtype Pmoney is Class_Abstract_Account.Pmoney;
procedure Deposit ( The:in out Account; Amount:in Pmoney );
procedure Withdraw ( The:in out Account; Amount:in Pmoney;
Get:out Pmoney );
function Balance ( The:in Account ) return Money;
private
type Account is new Abstract_Account with record
Balance_Of : Money := 0.00;
end record;
end Class_Account;
--Amount in account
Note:
Subtype has been used to make the subtypes Money and PMoney visible to clients of the class. If this
had not been done, users of the class would in most cases have to with and use the package
Class_abstract_account.
The implementation of which would be the same as the class Account shown in Section 6.3.5.
Once a class has been derived from an existing class it too may be used as a base class in deriving a new class.
For example, an account that allows a customer to only make three withdrawals in a week can be derived from
Class_Account as follows:
with Class_Account;
use Class_Account;
package Class_Account_Ltd is
type Account_Ltd is new Account with private;
procedure Withdraw ( The:in out Account_Ltd;
Amount:in Pmoney; Get:out Pmoney );
procedure Reset( The:in out Account_Ltd );
private
Withdrawals_In_A_Week : Natural := 3;
type Account_Ltd is new Account with record
Withdrawals : Natural := Withdrawals_In_A_Week;
end record;
end Class_Account_Ltd;
Note:
The derived class overloads the method Withdraw with a new specialized meaning. The method
Reset is used to set the number of withdrawals that may be made in the current week to three.
The implementation for the class is as follows:
package body Class_Account_Ltd is
© M A Smith - May not be reproduced without permission