Object-oriented Software in Ada 95

240 Polymorphism
with Ada.Tags;
use Ada.Tags;
procedure Main is
Withdrawals_In_A_Week : constant Natural := 3;
subtype Money is Float;
type Account is tagged record
Balance_Of : Money := 0.00; --Amount in account
end record;
type Account_Ltd is new Account with record
Withdrawals : Natural := Withdrawals_In_A_Week;
end record;
Normal : Account;
Restricted : Account_Ltd;
begin
Normal := ( Balance_Of => 20.0 );
Restricted := ( Normal with 4 );
Restricted := ( Normal with Withdrawals => 4 );
end Main;
Note:
with is used to extend normal, an instance of an Account, into restricted, an instance of an
Account_Ltd.
The components may be named:
restricted := ( normal with withdrawals => 4 );
If there are no additional components, with null record is used to form the extension.
16.9 The observe-observer pattern
A danger in writing any program is that input and output of data values become entangled in the body of the code
of the program. When this happens the program becomes less easy to maintain and will require major changes if
the format of the input or output changes. By separating the input and output from the functionality of the program
allows a cleaner solution to be formulated.
The model-view paradigm in essence consists of:
• An observer: An object that has responsibility for displaying a representation of another
object.
• The observed: An object that has one or more observers who will display the state of the
object.
In the game of noughts and crosses, for example, the observed object would be the object representing the
board. The observer would be an object that has responsibility for displaying a representation of the board onto
the computer screen. There could be several implementations of the observer, depending on how the information
is to be displayed. For example, the observer may be implemented to display the board as a:
• Textual representation: When a console application is written.
• As a graphical image: When an application with a graphical interface is developed.
This separation of responsibility is illustrated in Figure 16.4. In which the observed object oxo is interrogated
by the object oxo_observer so that it can display a graphical representation of the noughts and crosses board
on the computer screen.
© M A Smith - May not be reproduced without permission