Object-oriented Software in Ada 95

19 Persistence
This chapter shows how to create persistent objects. A persistent object will have a life-time beyond the
life-time of the program that created it.
19.1 A persistent indexed collection
The life-time of an object in Ada depends on its declaration, but its life-time will never exist beyond that of the
program. For an object to exist beyond the life-time of an individual execution of a program requires the object’s
state to be saved to disk, allowing the object’s state to be restored in another program. The above process makes
the object persistent. Normally this process is visible to a programmer.
For example, a program to print the IDC (International Dialling Code) for countries selected by a user could
use a persistent object to hold IDC details for individual countries. These details could be amended by the user of
the program and the changes would be retained for subsequent re-running of the program.
A program of this kind could uses the persistent object Tel_List that is an instance of the class Pic
(Persistent Indexed Collection). The class Pic implements a persistent indexed collection of data items. The
index can be an arbitrary value as can the data stored with the index.
The responsibilities of the class PIC are as follows:
Method Responsibility
Initialize Initialize the object.
When the object is initialized with an identity, the state of the
named persistent object is restored into the object.
Finalize If the object has an identity, save the state of the object under
this name.
Add
Add a new data item to the object.
Extract Extract the data associated with an index.
Update Update the data associated with an index.
Set_Name Set the identity of the object.
Get_Name Return the identity of the object.
The package Pack_types contains string definitions for the Country and the IDC.
package Pack_Types is
subtype Country is String(1 .. 12); --Country
subtype Idc is String(1 .. 6); --International Dialling Code
end Pack_Types;
The Class Tel_List is an instantiation of the generic class Pic.
with Class_Pic, Pack_Types;
use Pack_Types;
pragma Elaborate_All( Class_Pic );
package Class_Tel_List is new Class_Pic( Country, Idc, ">" );
Note: The generic class Pic is described in Section 19.2.
© M A Smith - May not be reproduced without permission