Object-oriented Software in Ada 95

Inheritance 155
The specialization of the procedure Withdraw calls the method withdraw in class Account to process the
withdrawal. To avoid infinite recursion, the parameter The is converted to type Account before it is passed as a
parameter to Withdraw. This is termed a view conversion. Overload resolution is then used to determine which
version of Withdraw to call.
procedure Withdraw ( The:in out Account_Ltd;
Amount:in Pmoney; Get:out Pmoney ) is
begin
if The.Withdrawals > 0 then --Not limit
The.Withdrawals := The.Withdrawals - 1;
Withdraw( Account(The), Amount, Get ); --In Account
else
Get := 0.00;
end if;
end Withdraw;
--Sorry
The function reset resets the number of withdrawals that may be made in the current week.
procedure Reset( The:in out Account_Ltd ) is
begin
The.Withdrawals := Withdrawals_In_A_Week;
end Reset;
end Class_Account_Ltd;
10.6.1 Putting it all together
A program to illustrate the use of the class Class_Account_ltd is shown below:
with Class_Account, Class_Account_ltd, Statement;
use Class_Account, Class_Account_ltd;
procedure Main is
Mike : Account_Ltd;
Obtain: Money;
begin
Deposit( Mike, 300.00 ); --In credit
Statement( Account(Mike) );
Withdraw( Mike, 100.00, Obtain ); --Withdraw some money
Withdraw( Mike, 10.00, Obtain ); --Withdraw some money
Withdraw( Mike, 10.00, Obtain ); --Withdraw some money
Withdraw( Mike, 20.00, Obtain ); --Withdraw some money
Statement( Account(Mike) );
end Main;
Note:
The with and use of the package Class_Account so that the subtype Money is directly visible.
The procedure Statement seen earlier is used to print a mini statement.
which when run, produces the following output:
Mini statement: The amount on deposit is £300.00
Mini statement: The amount on deposit is £180.00
Note:
The final withdrawal of £20 is not processed as three withdrawals have already been made this week.
© M A Smith - May not be reproduced without permission