Object-oriented Software in Ada 95

Polymorphism 231
16.2.1 Putting it all together
The above classes can be combined into a program which prints details about various rooms or offices in a
building. The program is as follows:
with Ada.Text_Io, Class_room, Class_Office;
use Ada.Text_Io, Class_room, Class_Office;
procedure Main is
W422 : Room;
W414 : Office;
The procedure about can take either an instance of a Room or an Office as a parameter. This is achieved
by describing the parameter as Room'Class. The parameter of type Room'Class will match an instance of
Room plus an instance of any type which is derived directly or indirectly from a Room.
In the procedure about the call to the function Describe( Place ) is not resolvable at compile-time, as
the object place may be either a Room or an Office. At run-time when the type of Place is known to the
system, this call can be resolved. Thus, either Describe in the class Room or Describe in the class Office
will be called.
procedure About( Place:in Room'Class ) is
begin
Put( "The place is" ); New_Line;
Put( " " & Describe( Place ) ) ; --Run time dispatch
New_Line;
end About;
Note:
One way to implement the dynamic binding is for the object to contain information about which
function Describe is to be called. This information can then be interrogated at run-time to allow the
appropriate version of the function Describe to be executed. By careful optimization, the overhead
of dynamic binding can be limited to a few machine cycles.
The body of the test program Main is:
begin
Initialize( W414, 414, "4th Floor west wing", 2 );
Initialize( W422, 422, "4th Floor east wing" );
About( W422 );
About( W414 );
--Call with a room
--Call with an Office
end Main;
Note:
The call to about with an instance of a Room and an Office is possible because the type of the
formal parameter is Room'Class.
© M A Smith - May not be reproduced without permission