File : adagio-safe_access.adb


------------------------------------------------------------------------------

--                         ADAGIO - ADALID - AENEA.                         --

--                                                                          --

--                            Copyright (C) 2003                            --

--                                 A. Mosteo.                               --

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--  Authors: A. Mosteo. (adagio@mosteo.com)                                 --

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--  $Id: adagio-safe_access.adb,v 1.3 2004/01/21 21:05:41 Jano Exp $


-- This package provides facilities to have safe access pointers.

-- It offers two flavors: a unconstrained type which can be ini-

-- tialized with an access as constraint, or a regular type.


with Adagio.Monitor;
with Adagio.Trace;

with Text_io; use text_io;

with Ada.Unchecked_deallocation;

package body Adagio.Safe_access is

   Debug : Boolean := Item_id /= "Anonymous";

   -- Global monitor for each operation.

   -- This means that there is only *one* mutex of every instantiation.

   Mutex : aliased Adagio.Monitor.Semaphore;

   -- Delete item:

   procedure Free is new 
      Unchecked_deallocation (Item, Item_access);

   procedure Free is new 
      Unchecked_deallocation (Tracker_type, Tracker_access);

   -- Helper decrementing function:

   procedure Discount (this : in out Tracker_access) is
   begin
      if this /= null then
         this.Count := this.Count - 1;
         if this.Count = 0 then
            Free (this.Data);
            Free (this);
         end if;
      end if;
   end Discount;

   -- Helper decrement function:

   procedure Addcount (this : in out Tracker_access) is
   begin
      if this /= null then
         this.Count := this.Count + 1;
      end if;
   end Addcount;

   -- Is null?

   function Is_null (this : in Unconstrained_object) return Boolean is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      return this.Tracker = null or else this.Tracker.Data = null;
   end Is_null;

   -- Is null?

   function Is_null (this : in Object) return Boolean is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      return this.Tracker = null or else this.Tracker.Data = null;
   end Is_null;

   -- Association:

   procedure Bind (this : in out Object; Data : in Item_access) is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      Discount (this.Tracker);
      this.Tracker := new Tracker_type'(Data, 1);
   end Bind;

   -- Unbinding:

   -- The value is no longer controlled

   procedure Unbind (this : in out Object) is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      if this.Tracker /= null then
         if this.Tracker.Count = 1 then
            Free (this.Tracker);
         else
            raise Constraint_error;
         end if;
      end if;
   end;

   -- Get value

   -- Of course, if the access value is copied outside, error can occur.

   function Value (this : in Object) return Item is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      return this.Tracker.Data.all;
   end;

   function Value (this : in Object) return Item_access is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      return this.Tracker.Data;
   end;

   procedure Initialize (this : in out Unconstrained_object) is 
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      this.Tracker := new Tracker_type'(this.Data, 1);
   end Initialize;

   procedure Adjust   (this : in out Unconstrained_object) is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      Addcount (this.Tracker);
   end Adjust;

   procedure Finalize (this : in out Unconstrained_object) is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      Discount (this.Tracker);
   end Finalize;

   procedure Adjust   (this : in out Object) is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      Addcount (this.Tracker);
   end Adjust;

   procedure Finalize (this : in out Object) is
      M : Adagio.Monitor.Object (Mutex'Access);
   begin
      Discount (this.Tracker);
   end Finalize;

end Adagio.Safe_access;