How to pass around access types to interfaces?

[Sven <sven.weidauer@gmail.com>]

I'm learning Ada and I've run into a big problem with passing around
access types to interfaces. I guess I just don't know all the
subtleties of access types, I'm used to C/C++ pointers. I didn't find
anything helpful on the net or in the  Ada Reference Manual.

I'm trying to get this code to run:

in some package:
  type My_Interface is interface;
  type My_Interface_Access is access all My_Interface'Class;

  procedure Do_Something( Item : in out Some_Object; With :
My_Interface_Access );

in main program:
  type Concrete_Type is new My_Interface with ...;


I need to pass an object of type Concrete_Type to the With parameter
of the Do_Something procedure. But the Compiler won't let me, no
matter what I do.


   My_Object : access Concrete_Type := new Concrete_Type;
   Do_Something( Item, My_Interface_Access( My_Object ) );

gives the compiler error "cannot convert local pointer to non-local
access type".

   My_Object : My_Interface_Access := new Concrete_Type;

gives the compiler error "type in allocator has deeper level than
designated class-wide type".

I found a way to pass my paramter by using this function:

  function Convert( Item : not null access Concrete_Type ) return
My_Interface_Access is
  begin
    return My_Interface_Access( Item );
  end;

But I don't this is such a good idea.

What is the right way to do this? I can't imagine that what I'm trying
to do is not supported in Ada.