Object-oriented Software in Ada 95

Polymorphism 247
The driver program for the game of noughts and crosses follows the same style as seen earlier in Section 8.4.5.
with Class_Board, Class_Display_Board,
Ada.Text_Io, Ada.Integer_Text_Io, Class_Observe_Observer;
use Class_Board, Class_Display_Board,
Ada.Text_Io, Ada.Integer_Text_Io, Class_Observe_Observer;
procedure Main is
Player : Character; --Either 'X' or 'O'
Game : Board; --An instance of Class Board
Move : Integer; --Move from user
begin
Player := 'X';
--Set player
An instance of an observer of the board is passed to the method Add_Observer so that it can be displayed.
Add_Observer( Game, new Display_Board );
Note:
As a pointer to the object is required, a dynamic instance of the object is created for simplicity of code.
Naturally, this could have been done by taking the access value of a non-dynamic instance.
The code for the logic of the game asks each player in turn for a move, when a player has entered a valid
move, the method Notify_Observers is used to request a display of the new state of the board.
while State( Game ) = Playable loop --While playable
Put( Player & " enter move (1-9) : "); -- move
Get( Move ); Skip_Line; -- Get move
if Valid( Game, Move ) then --Valid
Add( Game, Move, Player ); -- Add to board
Notify_Observers( Game );
case State( Game ) is --Game is
when Win =>
Put( Player & " wins");
when Playable =>
case Player is --Next player
when 'X' => Player := 'O'; -- 'X' => 'O'
when 'O' => Player := 'X'; -- 'O' => 'X'
when others => null; --
end case;
when Draw =>
Put( "It's a draw ");
end case;
New_Line;
else
Put("Move invalid"); New_Line; --for board
end if;
end loop;
New_Line(2);
end Main;
16.11 Self-assessment
• What is the difference between static and dynamic binding?
• What is an object’s tag?
• What is a heterogeneous collection of objects? How are heterogeneous collections of objects created
and used in Ada?
• What is a view conversion? Why are view conversions required?
© M A Smith - May not be reproduced without permission