Object-oriented Software in Ada 95

246 Polymorphism
16.10.2 An observer for the class Board
The specification for the class Display_Board that will display a representation of the board contains the
single method Update that has responsibility for displaying a representation of the board on a text output
device.
with Class_Observe_Observer, Class_Board, Ada.Text_Io;
use Class_Observe_Observer, Class_Board, Ada.Text_Io;
package Class_Display_Board is
type Display_Board is new Observer with private;
procedure Update( The:in Display_Board; B:in Observable'Class );
private
type Display_Board is new Observer with record
null;
end record;
end Class_Display_Board;
The implementation of the class Display_Board is shown below. The main point of interest is the conversion
of the second parameter B into an instance of a Board. This is required as the object is passed as an instance of
the class Observable. This down conversion will be checked to make sure that this object is an instance of Board.
package body Class_Display_Board is
procedure Update( The:in Display_Board; B: in Observable'Class ) is
begin
for I in 1 .. 9 loop
Put( Cell( Board(B), I ) ); --Its really a Board
case I is
--after printing counter
when 3 | 6 => -- print Row Separator
New_Line; Put("---------"); --
New_Line;
when 9 => -- print new line
New_Line;
when 1 | 2 | 4 | 5 | 7 | 8 => -- print Col separator
Put(" | ");
end case;
end loop;
end Update;
end Class_Display_Board;
16.10.3 The driver code for the program of nought and crosses
© M A Smith - May not be reproduced without permission