Object-oriented Software in Ada 95

210 Polymorphism
The storage for P_People can be visualized as illustrated in Figure 15.2.
P_People
24
People
Figure 15.2 Storage for P_People after it has been assigned the access value of people.
15.1.1 Access to an object via its access value
To access an object via its access value requires the use of .all which de-references the access value. This may
be thought of as an indirection operator. For example, the following program accesses the object people by
using the access value for People stored in the object P_People.
with Ada.Text_Io, Ada.Integer_Text_Io;
use Ada.Text_Io, Ada.Integer_Text_Io;
procedure Main is
type P_Integer is access all Integer;
People : aliased Integer;
P_People : P_Integer;
begin
People := 24;
P_People := People'Access; --Access value for people
Put("The number of people is : "); Put( P_People.all );
New_Line;
End Main;
Note:
In the declaration of P_integer, access all signifies that read and write access may be made to the
object via its access value.
which when run, would produce:
The number of people is : 24
The ideas described above have their origins in low-level assembly language programming where the address
of an item may be freely taken and manipulated. Ada provides a more disciplined way of implementing these
concepts.
15.1.2 Lvalues and rvalues
In working with access values it is convenient to think about the lvalue and rvalue of an object. The lvalue is the
access value of the object, whilst the rvalue is the contents of the object. For example, in the statement:
value := amount;
the object amount will deliver its contents, whilst the object value will deliver its access value so that the
rvalue of amount may be assigned to it. The names lvalue and rvalue are an indication of whether the object is
on the left or the right-hand side of an assignment statement.
© M A Smith - May not be reproduced without permission