Object-oriented Software in Ada 95

Exceptions 205
An efficient implementation is to access the private instance attributes of the class Stack directly. This can be
done by creating a child package of the generic package Class_Stack. However, as the parent class is generic, its
child package must also be generic. The specification of this generic child package is as follows:
generic
package Class_Stack.Additions is
function Top( The:in Stack ) return T;
function Items( The:in Stack ) return Natural;
private
end Class_Stack.Additions;
Note: As the child can see the components of the parent, it can also see any generic types.
The implementation of the class is then:
package body Class_Stack.Additions is
function Top( The:in Stack ) return T is
begin
return The.Elements( The.Tos );
end Top;
function Items( The:in Stack ) return Natural is
begin
return Natural(The.Tos);
end Items;
end Class_Stack.Additions;
A generic child of a package is considered to be declared within the generic parent. Thus, to instantiate an
instance of the parent and child the following code is used:
with Class_Stack;
pragma Elaborate_All( Class_Stack );
package Class_Stack_Pos is new Class_Stack(Positive,10);
with Class_Stack_Pos, Class_Stack.Additions;
pragma Elaborate_All( Class_Stack_Pos, Class_Stack.Additions );
package Class_Stack_Pos_Additions is
new Class_Stack_Pos.Additions;
Note:
The name of the instantiated child package is an Ada identifier.
© M A Smith - May not be reproduced without permission