Re: Generic and Access problem

Re: Generic and Access problem

[Grein, Christoph]

At least here, the attachment looks like:

> 
> begin 600 generic_pool.ads
> M9V5N97)I8PT*("`@+2T@5&AI<R!I<R!T:&4@='EP92!F;W(@=&AE(&]B:F5C
> M=',@=&AA="!C;VUP<FES97,@=&AE('!O;VP-"B`@('1Y<&4@271E;5]4>7!E

Rest omitted.

Re: Generic and Access problem

[Sebastian Madunic]

[-- Attachment #1: Type: text/plain, Size: 446 bytes --]

Strange I didn't have any problems opening it, anyway here is another try...

/S

"Grein, Christoph" <christoph.grein@eurocopter.com> wrote in message
news:<mailman.1042107482.19914.comp.lang.ada@ada.eu.org>...
> At least here, the attachment looks like:
> 
> > 
> > begin 600 generic_pool.ads
> > M9V5N97)I8PT*("`@+2T@5&AI<R!I<R!T:&4@='EP92!F;W(@=&AE(&]B:F5C
> > M=',@=&AA="!C;VUP<FES97,@=&AE('!O;VP-"B`@('1Y<&4@271E;5]4>7!E
> 
> Rest omitted.


[-- Attachment #2: generic_pool.adb --]
[-- Type: application/octet-stream, Size: 878 bytes --]

--with System.Address_To_Access_Conversions;

package body Generic_Pool is
------------------------------------------------------------------------
   -- Public types and objects
   Dummy_Item : aliased Item_Type;


   -- create an access-to-variable view of the pool:
--   package Access_Conversions is new
--      System.Address_To_Access_Conversions (Id_Type);

------------------------------------------------------------------------
   -- Returns a reference to the indexed item. 

   function Item (P : Id_Type; Ix : Index_Type) return Reference_Type is
      -- create an access-to-variable view of the pool:
--      P_Ref : constant Access_Conversions.Object_Pointer :=
--         Access_Conversions.To_Pointer(P'Address);
   begin
         return P.Buf(Ix)'access;
   end Item;

------------------------------------------------------------------------
end Generic_Pool;

[-- Attachment #3: generic_pool.ads --]
[-- Type: application/octet-stream, Size: 1000 bytes --]

generic
   -- This is the type for the objects that comprises the pool
   type Item_Type is private;

   -- This is the type for the index that are used to access the objects
   -- of the pool. 
   type Index_Type is (<>);

package Generic_Pool is
------------------------------------------------------------------------
   -- Public types and objects

   type Id_Type is private;
   type Reference_Type is access all Item_Type;

------------------------------------------------------------------------
   -- Returns a reference to the indexed item. 
   function Item (
      P : Id_Type;
      Ix : Index_Type)
      return Reference_Type;

------------------------------------------------------------------------
   -- Private types
private
   type Buffer_Type is array (Index_Type) of aliased Item_Type;

   type Id_Type is
   record
      Buf       : Buffer_Type;
      None_Allocated_Yet : Boolean := True;
      Last_Used   : Index_Type := Index_Type'First;
   end record;
   
end Generic_Pool;

Re: Generic and Access problem

[Grein, Christoph]

generic

   type Item_Type is private;
   type Index_Type is (<>);

package Generic_Pool is

   type Id_Type is private;
   type Reference_Type is access all Item_Type;

   -- There is no operation by which an object of type Id_Type
   -- can get a value.
   -- So how is an object of Item_Type supposed to be stored in Buf?
   -- If you've solved this problem, perhaps your problem with pointer
   -- lifetime is also solved.

   function Item (P: Id_Type; Ix: Index_Type) return Reference_Type;

private

   type Buffer_Type is array (Index_Type) of aliased Item_Type;

   type Id_Type is
   record
      Buf               : Buffer_Type;
      None_Allocated_Yet: Boolean    := True;
      Last_Used         : Index_Type := Index_Type'First;
   end record;
   
end Generic_Pool;

Why don't you you use System.Storage_Pools RM 13.11?

Re: Generic and Access problem

[David C. Hoos]

----- Original Message -----
From: "Sebastian" <sebastian.madunic@avionics.saab.se>
Newsgroups: comp.lang.ada
To: <comp.lang.ada@ada.eu.org>
Sent: Thursday, January 09, 2003 3:55 AM
Subject: Generic and Access problem


> Hi,
>
> I have a generic pool (attachment) that returns a pointer to an object.
The
> compiler doesn't agree with me
> it thinks that the object is an constant? I found a slolution using
> Access_To_Address_Conversion but
> I would prefere not to use it, is there another way to solve this
> problem....

The reason for the "access-to-variable designates constant" message
is that in - mode parameters are always considered constants -- i.e.,
subprograms are not allowed to modify them.

Returning an access to such a parameter implicitly grants the right to
modify the item designated by the access.

>
> Greatful for any suggestions....
The correct spelling of the first word of the above sentence fragment
is "Grateful"
>
> /S
>
>
>
>
>


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