Object-oriented Software in Ada 95

220 Polymorphism
this program will produce the following results:
push number = 1
push number = 2
push number = 3
push number = 4
Pop number = 4
Pop number = 3
Pop number = 2
Pop number = 1
Pop: Exception Stack_error
This is essentially the same driver code as used on the previous implementation of a stack. This time,
however, the stack is using dynamically allocated storage.
15.5 Hiding the structure of an object (opaque type)
So far, even though a client of a class cannot access the instance attributes of an instance of the class the client can
still see the type and names of the instance attributes used in the object in the specification of the class. The
instance attributes in an object can be hidden by moving the data declarations to the implementation part of the
class. The specification part of the class will now contain an access value to a data structure whose contents are
defined in the implementation part of the class.
If the specification part of the class no longer defines how much storage is to be used then the storage for an
object must be allocated dynamically. The reason for this is that the compiler will not know how much storage to
allocate for an object. Remember that the implementation part may have been separately compiled. The
specification part of the class defines an access value which points to the storage for the object’s instance
attributes. For example, the class for a bank account can now be defined as:
with Ada.Finalization;
use Ada.Finalization;
package Class_Account is
type Account is new Limited_Controlled with private;
subtype Money is Float;
subtype Pmoney is Float range 0.0 .. Float'Last;
procedure Initialize( The:in out Account );
procedure Finalize ( The:in out Account );
procedure Deposit ( The:in out Account; Amount:in Pmoney );
procedure Withdraw ( The:in out Account; Amount:in Pmoney;
Get:out Pmoney );
function Balance ( The:in Account ) return Money;
private
type Actual_Account;
--Details In body
type P_Actual_Account is access all Actual_Account;
type Account is new Limited_Controlled with record
Acc : P_Actual_Account; --Hidden in body
end record;
end Class_Account;
Note:
Apart from the user-defined initialization and finalization, the procedure and function specification is
the same as seen in Section 6.3.4.
The declaration for the type Actual_Account is tentative.
The base type of the class is Limited_Controlled. Thus assignments of instances of Account
are prevented..
© M A Smith - May not be reproduced without permission