Re: Ada Alternatives to Unrestricted_Access

[Jere <jhb.chat@gmail.com>]

On Thursday, March 8, 2018 at 7:43:31 PM UTC-5, Jere wrote:
> On Thursday, March 8, 2018 at 3:27:02 AM UTC-5, Dmitry A. Kazakov wrote:
> > On 07/03/2018 23:29, Jere wrote:
> > > On Wednesday, March 7, 2018 at 3:38:13 PM UTC-5, Dmitry A. Kazakov wrote:
> > >> On 2018-03-07 21:11, Jere wrote:
> > 
> > >>> Since the Container
> > >>> implementation is unknown in the generic, I wasn't able to get a version
> > >>> of the Rosen technique to work as an alternative (same constant vs
> > >>> variable access problem).
> > >>
> > >> I am not sure I understand the problem. Why don't you pass a function
> > >> Self along with the container type:
> > >>
> > >>      generic
> > >>         type Container is tagged whatever;
> > >>         function Self (C : Container'Class)
> > >>            return not null access Container'Class;
> > >>      package Iterative_Mess is
> > > 
> > > Because the package is already in use in other code, so
> > > changing the specification would be a very time consuming
> > > change.
> > 
> > You can default Self when not used.
> > 
> > -- 
> > Regards,
> > Dmitry A. Kazakov
> > http://www.dmitry-kazakov.de
> 
> So I tried playing with this, but I still ran into the same
> problem when trying to implement the Self function.  Inside
> I needed to "un-const" the container, so I came back to this
> same problem.

Example:
with Ada.Text_IO; use Ada.Text_IO;
with System.Address_To_Access_Conversions;

procedure jdoodle is
    type Container is tagged null record;
    
    package A2A is new System.Address_To_Access_Conversions(Container);
    
    type Container_Access is access Container;
    type Container_Constant_Access is access constant Container;
    
    function Self 
        (C : Container'Class)
         return not null access Container'Class
    is begin
        --return C'Unchecked_Access;  --ERROR: access-to-variable designates constant
        --return C'Unrestricted_Access;  -- Works, but not portable, GNAT specific
        return A2A.To_Pointer(C'Address);  -- Also works, unsure if OK in portable Ada
    end Self;
     
begin
    null;
end jdoodle;