Re: Data Structures of limited private types

[Matthew Heaney <matthew_heaney@acm.org>]

rajagopalasrinivasan@my-dejanews.com writes:

> I have a need to build a stack / linked_list of
> "Text_Io.File_Type"s. This being a limited private type, I am not able
> to perform "assignments" of a variable of this type.
> 
> Can anyone give me some pointers or an example of how this can be
> done?

Below is a stack that accepts limited private types as the stack item.  

You have to use functions that return a pointer to the item on the
stack, and manipulate the object indirectly, through a reference.  Like
this

  Op (Get_Top (Stack'Access).all);

If the syntax bothers you, then use a renames to make it more familiar:

declare
   O : LT renames Get_Top (Stack'Access).all;
begin
   Op (O);
end;

The idiosyncratic thing here is that the Push operation doesn't take an
item as a parameter.  You have to push the stack to open up the top
slot, and then operate on the top object, which is owned by the stack.

Ada95 is a huge improvement over Ada83 in this area.  In Ada83 you could
only use this technique if 

o the function was exported by a state machine package (rather than as
an operation on an instance of a type); or

o the data structure items were put on the heap.


I don't think this is a technique many Ada programmers are familiar
with, so I wanted to write a long-ish post to get the word out.

Hope this helps some,
Matt

--STX
generic

   type Stack_Item is limited private;

   Max_Depth : in Positive;

package Limited_Stacks is

   type Stack_Type is limited private;

   type Stack_Item_Access is access all Stack_Item;

   procedure Push (Stack : in out Stack_Type);

   procedure Pop (Stack : in out Stack_Type);

   function Get_Top
     (Stack : access Stack_Type) return Stack_Item_Access;

   function Get_Depth
     (Stack : Stack_Type) return Natural;

   generic
      with procedure Process
        (Item : in out Stack_Item;
         Quit : in out Boolean);
   procedure For_Every_Item (Stack : in out Stack_Type);

private

   type Stack_Item_Array is
      array (Positive range 1 .. Max_Depth) of aliased Stack_Item;

   type Stack_Type is
      limited record
         Items : Stack_Item_Array;
         Top   : Natural := 0;
      end record;

end Limited_Stacks;




package body Limited_Stacks is

   procedure Push (Stack : in out Stack_Type) is
   begin
      Stack.Top := Stack.Top + 1;
   end;

   procedure Pop (Stack : in out Stack_Type) is
   begin
      Stack.Top := Stack.Top - 1;
   end;

   function Get_Top
     (Stack : access Stack_Type) return Stack_Item_Access is
   begin
      return Stack.Items (Stack.Top)'Access;
   end;

   function Get_Depth
     (Stack : Stack_Type) return Natural is
   begin
      return Stack.Top;
   end;

   procedure For_Every_Item (Stack : in out Stack_Type) is

      subtype Index_Range is Positive range 1 .. Stack.Top;

      Quit : Boolean := False;

   begin

      for Index in reverse Index_Range loop

         Process (Stack.Items (Index), Quit);

         exit when Quit;

      end loop;

   end For_Every_Item;

end Limited_Stacks;




with Limited_Stacks;

with Ada.Text_IO; use Ada.Text_IO;

procedure Test_Stacks is

   package File_Stacks is new Limited_Stacks (File_Type, 10);
   use File_Stacks;

   Stack : aliased Stack_Type;

   procedure Display_File
     (File : in out File_Type;
      Quit : in out Boolean) is

      Line : String (1 .. 80);
      Last : Natural;

   begin

      Put ("Contents of file ");
      Put (Name (File));
      New_Line;

      Reset (File, In_File);

      while not End_Of_File (File) loop
         Get_Line (File, Line, Last);
         Put_Line (Line (1 .. Last));
      end loop;

      New_Line (2);

   end Display_File;

   procedure Display_Files is
     new For_Every_Item (Display_File);

begin

   Push (Stack);

   declare
      File : File_Type renames Get_Top (Stack'Access).all;
   begin
      Create (File, Out_File, "first_file.txt");
      Put_Line (File, "this is the first file pushed on the stack");
   end;

   Push (Stack);

   declare
      File : File_Type renames Get_Top (Stack'Access).all;
   begin
      Create (File, Out_File, "second_file.txt");
      Put_Line (File, "this is the second file pushed on the stack");
   end;

   Display_Files (Stack);

end Test_Stacks;