Re: [newbie] simple(?) data structures

[Stephen Leake <stephen_leake@acm.org>]

Roland Illig <roland.illig@gmx.de> writes:

> Many thanks for your detailed answer. Now I know what I forgot to say.
> The thing that makes the definition of the data structure that
> complicated is that I want to choose the size of the Go_Board at
> runtime and I want to pass that Go_Board to other subprograms. Is that
> possible with packages?

For a run-time determined size, the answer in Ada is the same as in
C++; you need to allocate the board object after reading in the size.
However, there are two ways to do this in Ada. The most powerful is
with access types:

package Go_Board is
   type size_Type is range 10 .. 50;
   type Board_Type (size : Size_Type) is tagged limited private;
   type Board_Access_Type is access Board_Type;

   ...
end Go_Board;

with Go_Board;
with Ada.Command_Line;
procedure Go_Main
   Size : constant Go_Board.Size_Type := Go_Board.Size_Type'Value
     (Ada.Command_Line.Argument (1));

   Board : Go_Board.Board_Access_Type := new Go_Board.Board_Type (Size);
begin
   --  play go.
end Go_Main;

This will allow you to change the size again after playing one game;
deallocate Board (using an instance of Ada.Unchecked_Deallocation),
and allocate it again. Then you have to be careful not to save a copy
of the pointer to the board after the board is destroyed.

The other way is to declare the board in a local block:

with Go_Board;
with Ada.Command_Line;
procedure Go_Main
   Size : constant Go_Board.Size_Type := Go_Board.Size_Type'Value
     (Ada.Command_Line.Argument (1));

begin
   declare
       Board : Go_Board. Go_Board.Board_Type (Size);
   begin   
       --  play go.
   end;
end Go_Main;

The second way doesn't need the access type; the board is "allocated"
on the stack. But Board is only visible in the local block, which can
be a problem for some applications. And, you can't change the size
after playing one game. This might be a good way to start; learn about
access types later.

-- 
-- Stephe