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From: Marek <ficorax@gmail.com>
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Subject: Re: Messing with access types...
Date: Mon, 28 Dec 2020 12:43:15 +0100
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On 28.12.2020 11:14, Dmitry A. Kazakov wrote:
> 
> What are you trying to achieve? Because the code does not make much
> sense to me.
> 
> P.S. If you are trying to do some C bindings flat arrays are easier
> choice than Interfaces.C.Pointers. C has no arrays, but pointers. Ada's
> arrays are never compatible with any pointers, except when flat.
> Therefore using access to String would be wrong in most, if not all, cases.
> 

Code is a fragment some plugin framework for host written in C. When
plugin is initialized it receives pointer to host features. Then I can
scan them to find concrete feature (Id) and retrieve data.
Problem is with instantiation generic function Get_Data, more precisely
in line:

   return T_Access (ATA.To_Pointer (F.Data));

OK, some changes to code to clear view:

with System;

package Util is

   type Feature is record
      Id  : Integer;
      Data : System.Address;
   end record;

   type Feature_Access is access all Feature;

   Null_Feature : constant Feature := (0, System.Null_Address);

   type Feature_Array is array (Natural range <>) of aliased Feature;

   generic
      type T is private;
      type T_Access is access all T;
   function Get_Data (H : access Feature; Id : Integer) return T_Access;

   generic
      type T is private;
      type T_Access is access all T;
   function Get_Query
     (H        : access Feature; Id : Integer; Data : in out T_Access;
      Required : Boolean) return Integer;

end Util;

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

pragma Ada_2012;

with Interfaces.C.Pointers;

with System.Address_To_Access_Conversions;

package body Util is

   package Ptr is new Interfaces.C.Pointers
     (Index              => Natural,
      Element            => Feature,
      Element_Array      => Feature_Array,
      Default_Terminator => Null_Feature);

   use Ptr;

   --------------
   -- Get_Data --
   --------------

   function Get_Data (H : access Feature; Id : Integer) return T_Access
   is
      Pointer : Ptr.Pointer := Ptr.Pointer (H);

      package ATA is new System.Address_To_Access_Conversions (T);

   begin
      if H /= null then
         loop
            declare
               F : access Feature := Pointer;
            begin
               if Id = F.Id then
                  return T_Access (ATA.To_Pointer (F.Data));
               end if;
            end;

            Ptr.Increment (Pointer);

            exit when Pointer = null;

         end loop;
      end if;

      return null;
   end Get_Data;

   ---------------
   -- Get_Query --
   ---------------

   function Get_Query
     (H        : access Feature; Id : Integer; Data : in out T_Access;
      Required : Boolean) return Integer
   is
      function Get is new Get_Data (T, T_Access);
   begin

      Data := Get (H, Id);

      if Required and (Data /= null) then
         return Id;
      end if;

      return 0;
   end Get_Query;

end Util;