Re: Generic Embedded List Nodes

[Warren <ve3wwg@gmail.com>]

On Monday, 20 June 2016 15:33:16 UTC-4, Niklas Holsti  wrote:
> On 16-06-20 15:26 , Warren wrote:
> 
> > Anyway folks- thanks for your help but I now have a working solution. I'm signing off this thread.
> 
> Before signing off, do please describe your solution.

I thought I had the problem licked using the following generic Object_Of function, but when I whipped up an example, the compile problem returned (or there was pilot error):

   function Object_Of(
      Node: access Emb_Node; 
      Member: Natural
   ) return Object_Type is
      use System.Storage_Elements;
   
      A:    constant System.Address := Node.all'Address;
      B:    constant System.Address := A - Storage_Offset(Member);
      R:    Object_Type;
      for R'Address use B;
      pragma Import(Convention => Ada, Entity => R);
   begin
      return R;
   end Object_Of;

The compiler is complaining with:

warning: controlled object "R" must not be overlaid.

The test record looks like this:

   type My_Recd is
      record
         ID:         Natural;
         Node:       aliased Emb_Node;
         Name:       Unbounded_String;  
      end record;

and the Emb_Node looks like this:

   type Emb_Node is
      record
         Next:    access Emb_Node;
         Prev:    access Emb_Node;
      end record;

The Node member is one culprit because it has two Access members (which count as members needing initialization).   Unbounded_String may be another factor.

What is particularly galling about this is that the record already exists, and does NOT need initialization at this point. All I need the compiler do is return it to me based upon its address.  Arg!

To accomplish this, I may have to resort to invoking a C function. But I don't think that will work either. Not by System.Address at least. More head scratching to follow.

Warren