Object-oriented Software in Ada 95

Inheritance 159
10.7.1 Putting it all together
A program to illustrate the use of the class Named_Account is shown below:
with Class_Named_Account, Statement;
use Class_Named_Account;
procedure Main is
Mike : Named_Account;
Get : Money;
begin
Set ( Mike, "A.N.Other/Brighton/UK" );
Deposit ( Mike, 10.00 );
Statement( Mike );
Withdraw ( Mike, 5.00, Get );
Statement( Mike );
end Main;
which when run, produces the following output:
Statement for : A.N.Other
Mini statement: The amount on deposit is £10.00
Statement for : A.N.Other
Mini statement: The amount on deposit is £ 5.00
10.8 Initialization and finalization
When an instance of a class is created, there can be static initialization of the instance or class attributes in the
object. For example, in the class Account in Section 6.3.4 the initial amount in the account was set to £0.00
when an instance of the class was elaborated. However, this initialization is limited to a simple assignment. In
some cases a more complex initialization is required.
Consider the case of a bank account that records an audit trail of all transactions made on the account. The
audit trail consists of a record written to disk for each transaction made on the account. The audit trail file
descriptor is shared between all instances of the class and is initialized by the first elaboration of an instance of the
class. The file descriptor is closed when the last instance of the class is finalized.
By inheriting from the package Ada.Finalization user defined initialization and finalization can be
defined for a class. This takes the form of two user defined procedures Initialize and Finalize which are
called respectively when an instance of the class is elaborated and when an instance of the class is destroyed. For
example, the point when initialization and finalization take place is annotated on the following fragment of code.
procedure Ex5 is
Mike : Account_At; --Initialization on mike
begin
Deposit( Mike, 100.00 );
declare
Corinna : Account_At; --Initialization on corinna
begin
Deposit( Corinna, 100.00 );
--Finalization on corinna
end;
end Ex5;
--Finalization on mike
© M A Smith - May not be reproduced without permission